Carboxylic acids

Properties of Carboxylic Acids. Structure, Compositions and Properties of Simple and Compound Lipids. Stereoisomerism. Polyfunctional Compounds

 

Carboxylic acids are compounds with the carboxyl functional group

 

             

 

 

The carboxylic group is the functional group of the carboxylic acids. It is kind of a combined carbonyl and hydroxyl group. The carbon atom in the group has sp² hybridization. The carbon atom has a double bond to one oxygen atom a single bond to the other. This other oxygen atom is also bonded to a hydrogen atom. It is this hydrogen atom that gives these compounds their acidic properties and the “acid” part of their names

In carboxylic acids, the bonds to the carboxyl carbon lie in one plane and are separated by about 120°. The carboxylic carbon is less electrophilic than carbonyl carbon because of the possible resonance structure shown below:

Structure of the Carboxyl Group

The most stable conformation of formic acid is an almost planar arrangement of the molecule.

 

The carbon is sp² hybridized , and the O-H bond lies in the plane described by the sp² carbon, eclipsing the C=O double bond.

This unexpected geometric arrangement can be explained by resonance (or conjugation).

 

Classification of Carboxylic Acids

I. Considering the hydrocarbonate radicals nature in the molecule, organic acid are being classified in:

a) Saturated acid is acid, which has only simple bonds in molecule.

For example:

    

acetic acid                                 propionic acid

 

b) Unsaturated acid is acid, which has both as simple and double bounds in molecule.

For example:

      

acrylic acid                                         oleic acid

 

c) aromatic acid is acid, which contain aromatic ring.

 

 

II. Considering the number of carboxyl groups in the molecule, acids can be classified in:

a)    mocarboxylic acids are acids group in molecule.

For example:

 

                 

acetic acid                                                          butanoic acid

 

b)    dicarboxylic acids are acids which has two carboxylic groups in molecule.

For example:

 

     

 

c)    policarboxylic acids are acids which has more than two carboxylic groups in molecule.

For example:

 

tricarboxylic acid – Citric acid

 

Physical Properties

The first three members are colourless liquids and have pungent smell. The next six members are oily liquids with a faint unpleasant odour. Still higher acids are colourless waxy solids.

They have higher boiling points than the corresponding alcohols of comparable molecular masses. Carboxylic acids have higher boiling points due to the presence of intramolecular hydrogen bonding. Due to the hydrogen bonding, carboxylic acids exist as dimers. The first four members of aliphatic carboxylic acids are very soluble in water. The solubility in water decreases gradually with rise in molecular mass. All are soluble in alcohol or ether. Benzoic acid is sparingly soluble in cold water but is soluble in hot water, alcohol and ether.

 

Formation

1. Oxidation of Primary Alcohols and Aldehydes

Carboxylic acids can also be formed by the oxidation of primary alcohols Mild oxidation changes the alcohol into an aldehyde and strong oxidation takes it all the way to carboxylic acid.

 

2. Hydrolysis of esters

Hydrolysis of esters with mineral acids or alkalines gives carboxylic acids

 

 

3. Hydrolysis of nitriles

The nitriles are hydrolysed in dilute aqueous acidic or alkaline medium

 

 

Reactions

Carboxylic acids can dissociate in aqueous solution into carboxylate ions and protons. The equilibrium constant for this process is K_a, and more frequently we talk in terms of pK_a.

 

 

 

Values of pK_a for common alkyl carboxylic acids are around 5 (K_a ~ 10^-5).

 

E.g. ethanoic acid has pK_a = 4.74, (alcohols have pK_a ~ 18, so carboxylic acids are about 10¹³ times more acidic than alcohols).

The reason why carboxylic acids are much more acidic than alcohols is because the carboxylate anion is much more stable than the alkoxide anion.

 

 

Both alcohols are carboxylic acids are acidic since their respective O-H bonds can be broken heterolytically, giving a proton and an oxygen anion.

The difference lies in the fact that the carboxylate anion has the negative charge spread out over two oxygen atoms, whereas the alcohol has the negative charge localized on a single oxygen atom.

 

 

The carboxylate anion can be viewed as a resonance hybrid of the two anionic structures, or as a conjugated system of three interacting p orbitals containing four electrons (like the allylic anion system).

The C and two oxygens are all sp² hybridized, and the remaining p orbitals create the p MO system giving rise to the half p bond between each C and O, and the half negative charge on the end oxygens.

 1. The acidity of carboxylic acid

Carboxylic acids are weak acids that ionized to form carboxylate ion and hydronium ion in water:

 

 Carboxylic acids are weaker acids than the strong acids (HCl, H₂SO₄, HNO₃), but stronger acids than phenols and much stronger than alcohols

2. With metals and metal hydroxides

The carboxylic acids evolve hydrogen with electropositive metals and form salts with alkalies. They react with weaker bases such as carbonates and hydrogencarbonates to evolve carbon dioxide. This reaction is used to detect the presence of carboxyl group in an organic compound.

 

3. Esterification

Esterification is the reaction of a carboxylic acid and alcohol in the presence of an acid catalyst to produce an ester. The main chain of an ester comes from the carboxylic acid, while the alkyl group in an ester comes from the alcohol.

4. Formation of anhydride

Carboxylic acids on heating with mineral acids such as H₂SO₄ or with P₂O₅ give corresponding anhydride.

 

5. Formation of acid chlorides

The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl₅ or PCl₃.

 

 

6. Reaction with ammonia

Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides. For example:

 

7. Reduction

Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride.

 

 

Dicarboxylic acids

Dicarboxylic acids are organic compounds that are substituted with two functional carboxylic acid groups. In molecular formula for dicarboxylic acids, these groups are often written as , where R may be carbohydrate chain.

Examples:

Saturated dicarboxylic acids

Oxalic acid (ethanedioic acid)     

Malonic acid (propanedioic acid)       

Succinic acid (butanedioic acid)                  

 

Unsaturated dicarboxylic acids

 

 

Saturated dicarboxylic acids are crystalline substances. Water solubility decreases proportionally with the substances molecular mass. Acids with an odd number of carbon atoms are more soluble in comparison with those with an eveumber.

Due to the existence of two carboxyl groups in the molecule, those acids are stronger than saturated monocarboxylic acids.

 

 

Chemical properties

In general, dicarboxylic acids show the same chemical behavior and reactivity as monocarboxylic acids.

Specific reactions

1. Oxalic acid when boiled at over 200°C, decomposes in carbon dioxide and formic acid that can be divided in carbon monoxide an water (in the presence of sulphuric acid).

2. Malonic acid, when boiled at 120-150°C loses carbon dioxide and passes in acetic acid.

 

Urea

Urea (also known as carbamide) is a waste product of many living organisms, and is the major organic component of human urine. This is because it is at the end of chain of reactions which break down the amino acids that make up proteins. An adult typically excretes about 25 grams of urea per day.

Urea (NH2CONH2) is of great importance to the agriculture industry as a nitrogen-rich fertilizer. Urea is made from ammonia and carbon dioxide.

 

Aqueous solutions of urea decompose to form CO₂ and NH₃.

 

Biuret is the result of condensation of two molecules of urea.

 

 

The “Biuret Test” is a chemical test used for detecting the presence of peptide bonds, for diagnosis of hyperproteinuria for example. The biuret test is a chemical test used for detecting the presence of peptide bonds. In a positive test, a copper(II) ion is reduced to copper(I), which forms a complex with the nitrogens and carbons of the peptide bonds in an alkaline solution. A violet color indicates the presence of proteins.

Biuret reagent is so-named, not because it contains biuret, but because of its reaction to the peptide-like bonds in the biuret molecule.

 

 

 

Lipids

 

Although lipid analyst tend to have a firm understanding of what is meant by the term “lipid”, there is no widely-accepted definition. General text books usually describe lipids in woolly terms as a group of naturally occurring compounds, which have in common a ready solubility in such organic solvents as hydrocarbons, chloroform, benzene, ethers and alcohols. They include a diverse range of compounds, like fatty acids and their derivatives, carotenoids, terpenes, steroids and bile acids. It should be apparent that many of these compounds have little by way of structure or function to relate them. In fact, a definition of this kind is positively misleading, since many of the substances that are now widely regarded as lipids may be almost as soluble in water as in organic solvents.

The lipids are large group of naturally occurring organic compounds that are related by their solubility in nonpolar organic solvents (e.g. ethet, chloroform, acetone and benzene) and general insolubility in water.

 

 

Classification of Lipids

 

Based on their chemical composition lipids are classified as;

Simple lipids: These are alcohol esters of fatty acids, which include; neutral lipids (glycerides), oils, wax, etc

Neutral lipids: Neutral fat consists mainly of triacylglycerols (triglycerides) which are composed of three fatty acids each in ester linkage with a single glycerol. Most neutral fats such as those in vegetable oils, butter and animal oil are mixture of simple and mixed triacylglycerols. Vegetable oils (corn oil) are liquid at room temperature due to high level of unsaturated fatty acids. These are converted industrially into solid fats by catalytic hydrogenation.

Biological waxes bee wax are esters of long chain saturated and unsaturated fatty acids with long chain alcohols. Bee wax is secreted by the abdominal glands of the worker bees. The wax is used by worker bees for building the hive. Sebum or ear wax is secreted by cutaneous glands for lubricating ear drum. Paraffin wax is obtained from petroleum. It is used in cosmetic and polishes and making candles. Waxes serve as water repellants. Certain skin glands of vertebrates secrete waxes to protect skin. Aquatic birds secrete wax from preen gland to keep their feather water repellant.

Compound lipids. Lipids with additional groups are called compound lipids. The compound lipids includes;

Phospho lipids: These are triglyceride lipids in which one fatty acid is replaced by phosphate group. Some phospholipids also have nitrogenous compound such as choline in lecithin, ethanol amine in cephalin attached to the phosphate group. Phospholipids are amphipathic carrying both hydrophilic (water attracting) and hydrophobic (water repellant) non-polar groups.

Sphingolipids: These are composed of one molecule of the long chain amino alcohol sphingosine. Spingolipids are the large class of membrane lipids, which have a polar head group and two non-polar tails. When the head groups of sphingolipids contain one sugar, instead of phosphate, they are called as cerebrosides. The neural tissues are rich in cerebrosides.

 

 

Role of Lipids

The main purpose of lipids is to store energy.

Other very important function include:

–        structural elements (cell membrane)

–        dissolving fat soluble vitamins

–        transportation of some molecules through the blood

–        emulsifying agent (bile salts)

For many years, lipids were considered to be intractable and uninteresting oily materials with two main functions – to serve as a source of energy and as the building blocks of membranes. They were certainly not considered to be appropriate candidates for such important molecular tasks as intracellular signalling or local hormonal regulation. In 1929, George and Mildred Burr demonstrated that linoleic acid was an essential dietary constituent, but it was many years before the importance of this finding was recognized by biochemists in general. With the discovery by Bergström, Samuelsson and others in 1964 that the essential fatty acid arachidonate was the biosynthetic precursor of the prostaglandins with their effects on inflammation and other disease states, the scientific world in general began to realize that lipids were much more interesting than they had previously thought.

A major milestone was achieved in 1979 with the discovery of the first biologically active phospholipid, platelet-activating factor. At about the same time, there arose an awareness of the distinctive functions of phosphatidylinositol and its metabolites. Since then, virtually every individual lipid class has been found to have some unique biological role that is distinct from its function as a source of energy or as a simple construction unit of a membrane. Indeed it is now recognised that lipids in membranes function also in the trafficking of cellular constituents, the regulation of the activities of membrane proteins and signalling.

All multi-cellular organisms, use chemical messengers to send information between organelles and to other cells and as relatively small hydrophobic molecules, lipids are excellent candidates for signalling purposes. The fatty acid constituents have well-defined structural features, such as cis-double bonds in particular positions, which can carry information by binding selectively to specific receptors. In esterified form, they can infiltrate membranes or be translocated across them to carry signals to other cells. During transport, they are usually bound to proteins so their effective solution concentrations are very low, and they are can be considered to be inactive until they reach the site of action and encounter the appropriate receptor.

Storage lipids, such as triacylglycerols, in their cellular context are inert, and indeed esterification with fatty acids may be a method of de-activating steroidal hormones, for example, until they are actually required. In contrast, polar phospholipids have both hydrophobic and hydrophilic sites that can bind via various mechanisms to membrane proteins and influence their activities. Glycosphingolipids carry complex carbohydrate moieties that have a part to play in the immune system, for example. Lipids have been implicated in a number of human disease states, including cancer and cardiovascular disease, sometimes in a detrimental and sometimes in a beneficial manner. In short, every scientist should now be aware that lipids are just as fascinating as all the other groups of organic compound that make up living systems.

 

Fatty Acids

The fatty acid is a carboxylic acid with a long unbranched aliphatic chain (“tail”), which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an eveumber of carbon atoms, from 4 to 28.

Saturated fatty acids

Saturated fatty acids are long-chain carboxylic acid containing only carbon–carbon single bonds.

 

Unsaturated fatty acids

Unsaturated fatty acids are long-chain carboxylic acid containing one or more carbon–carbon double bonds.

 

Unsaturated fatty acids §       Have one or more double C=C bond §       Typically contain cis double bonds. §       Have low melting points. §       Are liquids at room temperature.

 

 

 

 

 

 

 

 

 

The two carbon atoms in the chain that are bound next to either side of the double bond can occur in a cis or trans configuration. In most naturally occurring unsaturated fatty acids all are cis bonds. Most fatty acids in the trans configuration (trans fats) are not found iature and are the result of human processing (e.g., hydrogenation).

 

 

 

 

 

Essential fatty acids

Fatty acids that are required by the body but cannot be made in sufficient quantity from other substrates, therefore must be obtained from food and are called essential fatty acids. Two fatty acids are essential in humans, linoleic acid and linolenic acid. They are widely distributed in plant oils.

 

Omega-3 and Omega-6 Fatty Acids

Commoames have been also developed; certain fatty acid names have been popularized by the media.  The acids can be named for how far the double bond lays away from the final tail carbon.  Linolenic acid has a double bond, three carbons from the fatty acid’s end.  It is classified as an omega-3 fatty acid, the omega carbon being the terminal carbon and the bond being found on the third carbon from the end.  Linoleic acid is an Ω-6 fatty acid. 

Unsaturated fats such as those in vegetable oils and fish are recognized as more beneficial to health than saturated fats.

Vegetables contain omega-6 acids, meaning the first double bond occur at carbon 6. Examples of omega-6 acids are linoleic and arachidonic acids.

Fish have high levels of omega-3 acids, meaning the first double bond occur at carbon 3. Examples of omega-3 acids include linolenic, eicosapentaenoic, and docosahexaenoic acids.

Cold-water fish are a source of omega-3 fatty acids.

 

            

 

 

Glycerides

 

They a formed by dehydration reactions in which the three OH groups of a glycerol molecule react with the carboxylic acid groups of three different or same fatty acids to form three ester bonds and also three molecules of water.

 

 

These triglycerides (or triacylglycerols) are found in both plants and animals, and compose one of the major food groups of our diet. Tryglicerides that are solid or semisolid at room temperature are classified as fats, and occur predominantly in animals. Those triglycerides that are liquid are called oils and originate chiefly in plants, although triglycerides from fish are also largely oils.

As might be expected from the properties of the fatty acids, fats have a predominance of saturated fatty acids and oils are composed largely of unsaturated acids.

Triglycerides having three identical acyl chains, such as tristearin and triolein are called “simple”, while those composed of different acyl chains are called “mixed” (such as oleopalmitostearin).

 

        

 

Reactions

Hydrolysis Reaction

Just as the fats can be assembled (or synthesized) by means of a dehydration reaction, they can also be dissembled by hydrolysis reactions to reform the glycerol and fatty acids which were present when it was put together. In our bodies, this reaction is generally carried out in the presence of enzymes that catalyze the hydrolysis reaction.

 

 

Saponification Reaction

This reaction can take place outside our bodies. When a fat is heated with a strong base such as sodium hydroxide, saponification of the fat gives glycerol and the sodium salts of the fatty acids, which are soaps. KOH produces a softer, liquid soap. Oils that are polyunsaturated produce softer soaps. Names like “coconut” or “avocado shampoo” tell you the sources of the oil used in the reaction.

 

 

Soap is mainly used for washing, bathing and cleaning.

 

The hydrophobic portion is made up of the long hydrocarbon chain from the fatty acid. In other words, whereas normally oil and water do not mix, the addition of soap allows oils to dissolve in water, allowing them to be rinsed away. Synthetic detergents operate by similar mechanisms to soap.

 

 

Soaps and Detergents

Carboxylic acids and salts having alkyl chains longer than eight carbons exhibit unusual behavior in water due to the presence of both hydrophilic (CO₂) and hydrophobic (alkyl) regions in the same molecule. Such molecules are termed amphiphilic (Gk. amphi = both) or amphipathic. Fatty acids made up of ten or more carbon atoms are nearly insoluble in water, and because of their lower density, float on the surface when mixed with water. Unlike paraffin or other alkanes, which tend to puddle on the waters surface, these fatty acids spread evenly over an extended water surface, eventually forming a monomolecular layer in which the polar carboxyl groups are hydrogen bonded at the water interface, and the hydrocarbon chains are aligned together away from the water. This behavior is illustrated in the diagram on the right. Substances that accumulate at water surfaces and change the surface properties are called surfactants.

 

 

Alkali metal salts of fatty acids are more soluble in water than the acids themselves, and the amphiphilic character of these substances also make them strong surfactants. The most common examples of such compounds are soaps and detergents, four of which are shown below. Note that each of these molecules has a nonpolar hydrocarbon chain, the “tail”, and a polar (often ionic) “head group”. The use of such compounds as cleaning agents is facilitated by their surfactant character, which lowers the surface tension of water, allowing it to penetrate and wet a variety of materials.

 

Very small amounts of these surfactants dissolve in water to give a random dispersion of solute molecules. However, when the concentration is increased an interesting change occurs. The surfactant molecules reversibly assemble into polymolecular aggregates called micelles. By gathering the hydrophobic chains together in the center of the micelle, disruption of the hydrogen bonded structure of liquid water is minimized, and the polar head groups extend into the surrounding water where they participate in hydrogen bonding. These micelles are often spherical in shape, but may also assume cylindrical and branched forms, as illustrated on the right. Here the polar head group is designated by a blue circle, and the nonpolar tail is a zig-zag black line.

An animated display of micelle formation is presented below. Notice the brownish material in the center of the three-dimensional drawing on the left. This illustrates a second important factor contributing to the use of these amphiphiles as cleaning agents. Micelles are able to encapsulate nonpolar substances such as grease within their hydrophobic center, and thus solubilize it so it is removed with the wash water. Since the micelles of anionic amphiphiles have a negatively charged surface, they repel one another and the nonpolar dirt is effectively emulsified. To summarize, the presence of a soap or a detergent in water facilitates the wetting of all parts of the object to be cleaned, and removes water-insoluble dirt by incorporation in micelles.

 

 

The oldest amphiphilic cleaning agent known to humans is soap. Soap is manufactured by the base-catalyzed hydrolysis (saponification) of animal fat (see below). Before sodium hydroxide was commercially available, a boiling solution of potassium carbonate leached from wood ashes was used. Soft potassium soaps were then converted to the harder sodium soaps by washing with salt solution. The importance of soap to human civilization is documented by history, but some problems associated with its use have been recognized. One of these is caused by the weak acidity (pK_a ca. 4.9) of the fatty acids. Solutions of alkali metal soaps are slightly alkaline (pH 8 to 9) due to hydrolysis. If the pH of a soap solution is lowered by acidic contaminants, insoluble fatty acids precipitate and form a scum. A second problem is caused by the presence of calcium and magnesium salts in the water supply (hard water). These divalent cations cause aggregation of the micelles, which then deposit as a dirty scum.

These problems have been alleviated by the development of synthetic amphiphiles called detergents (or syndets). By using a much stronger acid for the polar head group, water solutions of the amphiphile are less sensitive to pH changes. Also the sulfonate functions used for virtually all anionic detergents confer greater solubility on micelles incorporating the alkaline earth cations found in hard water. Variations on the amphiphile theme have led to the development of other classes, such as the cationic and nonionic detergents shown above. Cationic detergents often exhibit germicidal properties, and their ability to change surface pH has made them useful as fabric softeners and hair conditioners. These versatile chemical “tools” have dramatically transformed the household and personal care cleaning product markets over the past fifty years.

 

Hydrogenation Reaction

Hydrogenation of unsaturated fats converts carbon-carbon double bonds to single bonds. The hydrogen gas bubbled through the heated oil the presence of a nickel catalyst (or another transition metal).

 

 

Halogenation

Neutral fats containing unsaturated fatty acids have the ability of adding halogens (e.g., hydrogen or hydrogenation and iodine or iodination) at the double bonds. It is a very important property to determine the degree of unsaturation of the fat or oil that determines its biological value

Oxidation (Rancidty)

This toxic reaction of triglycerides leads to unpleasant odour or taste of oils and fats developing after oxidation by oxygen of air, bacteria, or moisture. Also this is the base of the drying oils after exposure to atmospheric oxygen.

 

Prevention of rancidity is achieved by:

Avoidance of the causes (exposure to light, oxygen, moisture, high temperature and bacteria or fungal contamination). By keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., good storage conditions).

Addition of anti-oxidants to prevent peroxidation in fat (i.e., rancidity). They include phenols, naphthols, tannins and hydroquinones. The most commoatural antioxidant is vitamin E that is important in vitro and in vivo.

 

For describe the fat composition using the following number:

Iodine number (or “iodine value” or “iodine index”) is the mass of iodine in grams that is consumed by 100 grams of fat. Iodine number uses for determination of the amount of unsaturation contained in fatty acids. This unsaturation is in the form of double bonds which react with iodine compounds. The higher the iodine number, the more unsaturated fatty acid bonds are present in a fat.

Saponification number (or “saponification value”) represents the number of milligrams of potassium hydroxide (KOH) or sodium hydroxide (NaOH) required to saponify 1 g of fat under the conditions specified. The long chain fatty acids found in fats have low saponification value because they have a relatively fewer number of carboxylic functional groups per unit mass of the fat as compared to short chain fatty acids. It more moles of base are required to saponify “N” grams of fat then there are more moles of the fat the chain lengths are relatively small.

Acid number (or “acid value”, or “neutralization number”) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize free fatty acids which present in one gram of fat. As oil-fats rancidity, triglycerides are converted into fatty acids and glycerol, causing an increase in acid number.

 

Saturated fat

Everyone should eat less saturated fat. Doctors believe that people who eat saturated fat diets increase their risk of developing life-threatening cholesterol deposits in the lining of their arteries.

Sources:

·                 High-fat dairy products such as full-fat cheese, cream, ice cream, whole milk, 2% milk and sour cream.

·                 High-fat meats like regular ground beef, bologna, hot dogs, sausage, bacon and spareribs

·                 Lard

·                 Butter

·                 Fatback and salt pork

·                 Cream sauces

·                 Gravy made with meat drippings

·                 Chocolate

·                 Palm oil and palm kernel oil

·                 Coconut and coconut oil

·                 Poultry (chicken and turkey) skin

·                 Cream

Try to eat less than 10% of your total calories as saturated fat. For most people this means less than 20 grams of saturated fat per day. For people with high cholesterol, this is less than 15 grams of saturated fat per day. Many adults, especially women or sedentary men, may need less.

Trans fat

Trans fats are produced when liquid oil is made into a solid fat. This process is called hydrogenation. Trans fats act like saturated fats and can raise your cholesterol level. Sources of trans fat include:

·                 Processed foods like snacks (crackers and chips) and baked goods (muffins, cookies and cakes) with hydrogenated oil or partially hydrogenated oil

·                 Stick margarines

·                 Shortening

·                 Some fast food items such as french fries.

Unsaturated fat

Unsaturated fat subdivided into two groups: monounsaturated fat and polyunsaturated fat. These fats, basically, are liquid in the room temperature, but some of them are solid (e.g. margarine).

Monosaturated fat

Monounsaturated fats are called “good or healthy” fats because they can lower your bad (LDL) cholesterol.

Sources:

·                 Avocado

·                 Canola oil

·                 Nuts like almonds, cashews, pecans, and peanuts

·                 Olive oil and olives

·                 Peanut butter and peanut oil

·                 Sesame seeds

ADA recommends eating more monounsaturated fats than saturated or trans fats in your diet. To include more monounsaturated fats, try to substitute olive or canola oil instead of butter, margarine or shortening when cooking. Sprinkling a few nuts or sesame seeds on a salad is an easy way to eat more monounsaturated fats. But be careful! Nuts and oils are high in calories, like all fats. If you are trying to lose or maintain your weight, you want to eat small portions of these foods. For example, 6 almonds or 4 pecan halves have the same number of calories as 1 teaspoon of oil or butter. Work with your dietitian to include healthy fats into your meal plan without increasing your total calories.

Polyunsaturated fat

Polyunsaturated fats are also “healthy” fats. ADA recommends that you include these in your diet as well as monounsaturated fats.

 

 

Sources:

·                 Corn oil

·                 Cottonseed oil

·                 Safflower oil

·                 Soybean oil

·                 Sunflower oil

·                 Walnuts

·                 Pumpkin or sunflower seeds

·                 Soft (tub) margarine

·                 Mayonnaise

·                 Salad dressings

·                 Flax oil.

 

Waxes

Waxes are esters of fatty acids with long chain monohydric alcohols. Waxes are biosynthesized by many plants or animals. They typically consist of several components, including wax esters, wax acids, wax alcohols and hydrocarbons. Wax esters are typically derived from a variety of carboxylic acids and a variety of fatty alcohols.

Animal waxes.

Plant waxes

Especially in warm climates, plants secrete waxes as a way to control evaporation and hydration. Plant waxes provides protection from disease and insects, and helps the plants resist drought.

Waxes are mainly consumed industrially as components of complex formulations, often for coatings.

Waxes and hard fats such as tallow have long been use to make candles.

 

Phospholipids

 

Phospholipids are made from glycerol, two fatty acids, and a phosphate group. Phospholipids are made up of 1 glycerol molecule and only 2 fatty acid molecules (not 3!). This is a big difference because in place of the third fatty acid, phospholipids have a polar group attached to the glycerol molecule.

The phosphate group in phospholipids is attached to the glycerol but has another molecule attached to its other end. The phosphate moiety of the resulting phosphatidic acid is further esterified with ethanolamine, choline or serine in the phospholipid itself.

         

 

Examles of phospholipids:

Lecithin and cephalin are abundant in brain and nerve tissues, found in egg yolk, wheat germ, and yeast.

Phospholipids are the most abundant lipids in cell membranes and play an important role in cellular permeability

The membrane of a cell separates the contents of a cell from the external fluids. It is semipermeable so that nutrients can enter the cell and waste products can leave. There are two rows of phospholipids in a cell membrane, that they are arranged like a sandwich. Their nonpolar tails, which are hydrophobic (water-fearing), move to the center, while their polar heads, which are hydrophilic (water-loving) align on the outer edge of the membrane. This double row arrangement of phospholipids is called a lipid bilayer.

 

 

Most of the phospholipids in the lipid bilayer contain unsaturated fatty acids. Due to the kinks in the carbon chains at the cis double bonds, the phospholipid s do not fit closely together. As a result, the lipid bilayer is not a rigid, fixed structure, but one that is dynamic and fluid -like. In this liquid-like bilayer, there are also proteins, carbohydrates, and cholesterol molecules. For this reason, the model of biological membranes is referred to as the fluid mosaic model of membranes.

 

Sphingolipids

Sphingolipids are similar to phospholipids. Contain sphingosine (a long-chain amino alcohol), a fatty acid, phosphate, and a small amino alcohol. Have polar and nonpolar regions.

Sphingosine is a long-chain unsaturated amino alcohol.

Sphingomyelins are found in large amounts in brain and nerves and in smaller amounts in lung, spleen, kidney, liver and blood

Glycosphingolipids

They are present in cerebral tissue, therefore are called cerebrosides

According to the number and nature of the carbohydrate residue(s) present in the glycosphingolipids the following are

Cerebrosides. They have one galactose molecule (galactosides).

Gangliosides. They have several sugar and sugaramine residues.

 

Cerebrosides occur in myelin sheath of nerves and white matter of the brain tissues and cellular membranes.  They are important for nerve conductance.

 

Gangliosides

Gangliosides are similar to cerebrosides, but contain two or more monosaccharides. They are more complex glycolipids that occur in the gray matter of the brain, ganglion cells. They transfer biogenic amines across the cell membrane and act as a cell membrane receptor for hormones and viruses.

Gangliosides contain sialic acid (N-acetylneuraminic acid), ceramide (sphingosine + fatty acid of 18-24 carbon atom length), 3 molecules of hexoses (1 glucose + 2 galactose) and hexosamine. The most simple type of it the monosialoganglioside. It works as a receptor for cholera toxin in the human intestine.

Steroids

Steroids constitute an important class of biological compounds. Steroids are nonhydrolyzable lipids.

Steroids are usually found in association with fat. They can be separated from fats after saponification since they occur in the unsaponifiable residue.

Steroids are compounds containing the steroid nucleus, which consists of three cyclohexane rings and one cyclopentane ring fused together (no fatty acids). The four rings in the steroid nucleus are designated A, B, C, and D. Numbered carbon atoms beginning in ring A

 

  

Cholesterol• is the most abundant steroid in the body, has methyl groups (carbons 10, 13) an alkyl chain (carbon 17), and an –OH group (carbon 3) attached to the steroid nucleus

 

Cholesterol in the body.

Cholesterol is a component of cellular membranes, myelin sheath, and brain and nerve tissue. It is also found in the liver, bile salts, and skin, where it forms vitamin D. In the adrenal gland, it is used to synthesize steroid hormones. Cholesterol in the body in obtained from eating meats, milk, and eggs, and it is also synthesized by the liver from fats, carbohydrates, and proteins. There is no cholesterol in vegetable and plant products.

If the diet is high in cholesterol, the liver produces less. A typical American daily diet includes 400-500 mg of cholesterol (one of the highest in the world). However, we should consume no more than 300 mg of cholesterol a day.

 

 

Bile Salts

Bile acids are produced from oxidation of cholesterol in the liver producing cholic and chenodeoxycholic acids that are conjugated with glycine or taurine to produce glycocholic, glycochenodeoxycholic, taurocholic and taurochenodeoxycholic acids. They react with sodium or potassium to produce sodium or potassium bile salts.

 

Bile salts are synthesized in the liver and stored in the gallbladder. Have polar and nonpolar regions that act like soaps to make fat soluble in water. Bile salts assist in the digestion of lipids and other non-soluble molecules. Help in absorption of cholesterol. Activation of pancreatic lipase. Help digestion and absorption of fat-soluble vitamins. Intestinal antiseptic that prevent putrefaction

 

 

Steroid hormones

Hormones are chemical messengers that serve as a kind of communication system from one part of the body to another. The steroid hormones, which include the sex hormones are closely related in structure to cholesterol and depend on cholesterol for their synthesis. Two important male sex hormones, testosterone  and  androsterone, promote the growth  of muscle and of facial hair and the maturation of the male sex organs and of sperm. The estrogens, a group of female sex hormones, direct the development of female characteristics: the uterus increases in size, fat is deposited in the breasts, and the pelvis broadens. Progesterone prepares the uterus for the implantation of a fertilized egg.

 

Adrenal corticosteroids are produced by the adrenal glands located on the top of each kidney. Include aldosterone, which regulates electrolytes and water balance by the kidneys. Include cortisone, a glucocorticoid, which increases blood glucose level and stimulates the synthesis of glycogen in the liver.

 

Lipoproteins

Lipids are nonpolar and made more soluble by combining them with glycerophospholipids and proteins to form water-soluble complexes called lipoproteins.

Lipoproteins surround nonpolar lipids with polar lipids and protein for transport to cells. They are soluble in water because the surface consists of polar lipids

Lipoproteins in the body.

Lipids must be transported through the bloodstream to tissues where they are stored, used for energy, or to make hormones. However, most lipids are nonpolar and insoluble in the aqueous environment of blood. They are made more soluble by combining them with phospholipids and proteins to form water-soluble complexes called lipoproteins. 

 

 

In general, lipoproteins are spherical particles with an outer surface of polar proteins and phospholipids that surround hundreds of nonpolar molecules of triacylglycerols and cholesteryl esters (formed by the esterification of the hydroxyl group in cholesterol with a fatty acid).

There are two main categories of lipoproteins distinguished by how compact/dense they are. LDL or low density lipoprotein is the “bad guy,” being associated with deposition of “cholesterol” on the walls of someone’s arteries. HDL or high density lipoprotein is the “good guy,” being associated with carrying “cholesterol” out of the blood system, and is more dense/more compact than LDL.

 

Prostaglandins Thromboxanes & Leukotrienes

The members of this group of structurally related natural hormones have an extraordinary range of biological effects. They can lower gastric secretions, stimulate uterine contractions, lower blood pressure, influence blood clotting and induce asthma-like allergic responses. Because their genesis in body tissues is tied to the metabolism of the essential fatty acid arachadonic acid (5,8,11,14-eicosatetraenoic acid) they are classified as eicosanoids. Many properties of the common drug aspirin result from its effect on the cascade of reactions associated with these hormones.

The metabolic pathways by which arachidonic acid is converted to the various eicosanoids are complex and will not be discussed here. A rough outline of some of the transformations that take place is provided below. It is helpful to view arachadonic acid in the coiled conformation shown in the shaded box.

Leukotriene A is a precursor to other leukotriene derivatives by epoxide opening reactions. The prostaglandins are given systematic names that reflect their structure. The initially formed peroxide PGH₂ is a common intermediate to other prostaglandins, as well as thromboxanes such as TXA₂.

Lipid Soluble Vitamins

The essential dietary substances called vitamins are commonly classified as “water soluble” or “fat soluble”. Water soluble vitamins, such as vitamin C, are rapidly eliminated from the body and their dietary levels need to be relatively high. The recommended daily allotment (RDA) of vitamin C is 100 mg, and amounts as large as 2 to 3 g are taken by many people without adverse effects. The lipid soluble vitamins, shown in the diagram below, are not as easily eliminated and may accumulate to toxic levels if consumed in large quantity. The RDA for these vitamins are:

Vitamin A   800 μg ( upper limit ca. 3000 μg)
Vitamin D   5 to 10 μg ( upper limit ca. 2000 μg)
Vitamin E   15 mg ( upper limit ca. 1 g)
Vitamin K   110 μg ( upper limit not specified)

From this data it is clear that vitamins A and D, while essential to good health in proper amounts, can be very toxic. Vitamin D, for example, is used as a rat poison, and in equal weight is more than 100 times as poisonous as sodium cyanide.

From the structures shown here, it should be clear that these compounds have more than a solubility connection with lipids. Vitamins A is a terpene, and vitamins E and K have long terpene chains attached to an aromatic moiety. The structure of vitamin D can be described as a steroid in which ring B is cut open and the remaining three rings remain unchanged. The precursors of vitamins A and D have been identified as the tetraterpene beta-carotene and the steroid ergosterol, respectively.

 

Polyfunctional compounds

 

Molecules with more than one functional groups, called polyfunctional.

Hydroxy acids, are a class of chemical compounds that consist of carboxylic and hydroxyl groups.

Oxo acid include aldehydo– or keto– group besides carboxyl group.

Example of hydroxyl acids:

 

Example of oxo carboxylic acids

 

Specific Properties of Hydroxy Carboxylic Acid

Alpha-hydroxy acids contain both hydroxyl and carboxyl functional groups, they can undergo self-esterification when heated to form cyclic six-membered diesters. These diesters are sometimes referred to generically as lactides. However, the term “lactide” is used herein to refer only to the cyclic diester of lactic acid.

Alpha hydroxy acids (AHAs) are naturally occurring carboxylic acids with a hydroxyl group  (-OH) on the carbon adjacent to the carboxyl group.

 

When heated, α-hydroxy-acid can dehydrate to form lactide, β- hydroxy-acid can form  α,β-unsaturated carboxylic acid, while  γ-hydroxy-acid can form  γ-lactone, depending on the positions of the hydroxyl group of hydroxyl acid.

 

 

 

Glycolic acid

Glycolic acid (or hydroxyacetic acid) is the smallest α-hydroxy acid. This colorless, odorless and hygroscopic crystalline solid is highly soluble in water.

Due to its excellent capability to penetrate skin, glycolic acid finds applications in skin care products, most often as a chemical peel performed by a dermatologist or plastic surgeon in concentrations of 20 to 70 % or at-home kits in lower concentrations between 10 and 20 %.

Glycolic acid is used to improve the skin’s appearance and texture. It may reduce fine lines wrinkles, acne scarring, hyperpigmentation and improve many other skin conditions, including actinic keratosis, hyperkeratosis, and seborrheic keratosis. Once applied, glycolic acid reacts with the upper layer of the epidermis, weakening the binding properties of the lipids that hold the dead skin cells together. This allows the stratum corneum to be exfoliated, exposing live skin cells.

Glycolic acid is also a useful intermediate for organic synthesis, in a range of reactions including: oxidation-reduction, esterification and long chain polymerization. It is used as a monomer in the preparation of polyglycolic acid and other biocompatible copolymers (e.g. PLGA). Among other uses this compound finds employment in the textile industry as a dyeing and tanning agent, in food processing as a flavoring agent and as a preservative. Glycolic acid is often included into emulsion polymers, solvents and additives for ink and paint in order to improve flow properties and impart gloss.

 

Lactic acid

Lactic acid, also known as milk acid, is chemical compound that plays a role in several biochemical processes.

Lactic acid in the form of its salt (lactate) is prodused in muscle tissue as a result of the anaerobic breakdown of glucose. Excess lactate is the cause of muscle soreness produced after strenuous exercise when the body’s supply of oxygen is reduced.

Lactic acid is found primarily in sour milk products, such as koumiss, leban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor or sourdough breads. This acid is used in beer brewing to lower the pH and increase the body of the beer.

 

Tartaric acid

Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds, and is one of the main acids found in wine. It is added to other foods to give a sour taste and is used as an antioxidant. Salts of tartaric acid are known as tartrates.

Felling’s reagent an aqueous solution of copper sulfate, sodium hydroxide, and potassium sodium tartrate used to test for the presence of sugars and aldehydes in a substance, such as urine.

 

 

Citric acid

        

 

Citric acid is a natural component and common metabolite of plants and animals. It is the most versatile and widely used organic acid in foods, beverages, and pharmaceuticals. Because of its functionality and environmental acceptability, citric acid and its salts (primarily sodium and potassium) are used in many industrial applications for chelation, buffering, pH adjustment, and derivatization. These uses include laundry detergents, shampoos, cosmetics, enhanced oil recovery, and chemical cleaning. Citric acid occurs in the terminal oxidative metabolic system of virtually all organisms. This oxidative metabolic system, variously called the Krebs cycle, the tricarboxylic acid cycle, or the citric acid cycle, is a metabolic cycle involving the conversion of acetate derived from carbohydrates, fats, or proteins to carbon dioxide and water. This cycle releases energy necessary for an organism’s growth, movement, luminescence, chemosynthesis, and reproduction.

Citric acid decomposed in presence of sulphuric acid.

 

 

Pyruvic acid

Pyruvate is an important chemical compound in biochemistry. It is the output of the anaerobic metabolism of glucose known as glycolysis. One molecule of  glucose breaks down into two molecules of pyruvate, which are then used to provide further energy, in one of two ways. Pyruvate is converted into acetyl-coenzyme A, which is the main input for a series of reactions known as the  Krebs cycle. Pyruvate is also converted to  oxaloacetate by an anaplerotic reaction which replenishes Krebs cycle intermediates; alternatively, the oxaloacetate is used for gluconeogenesis.

These reactions are named after Hans Adolf Krebs, the biochemist awarded the 1953 Nobel Prize for physiology, jointly with Fritz Lipmann, for research into metabolic processes. The cycle is also called the citric acid cycle, because citric acid is one of the intermediate compounds formed during the reactions.

If insufficient oxygen is available, the acid is broken down anaerobically, creating lactate in animals and ethanol in plants and microorganisms. Pyruvate from glycolysis is converted by anaerobic respiration to lactate using the enzyme lactate dehydrogenase and the coenzyme NADH in lactate fermentation, or to acetaldehyde  and then to ethanol in alcoholic fermentation.

Pyruvate is a key intersection in the network of metabolic pathways. Pyruvate can be converted into carbohydrates via gluconeogenesis, to fatty acids or energy through acetyl-CoA, to the amino acid alanine and to ethanol. Therefore it unites several key metabolic processes.