Nonpolar Lipids

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 16:18

A good example of a nonpolarDescribes a molecule with no net permanent dipole; this can occur when there is no separation of centers of positive and negative electrical charge or because there are bond dipoles that cancel each others' effects. A polar molecule will assume certain orientations more than others in an electric field. lipidAn organic compound found in tissue and that is soluble in nonpolar solvents. is the neutral fatA substance that is an ester of glycerol and three fatty acids; a triglyceride. glycerol tristearate. This most-common form of animal fat serves as a storehouse for energyA system's capacity to do work. and as insulation against heatEnergy transferred as a result of a temperature difference; a form of energy stored in the movement of atomic-sized particles. loss. On a molecular level it is constructed from three molecules of stearic acidIn Arrhenius theory, a substance that produces hydrogen ions (hydronium ions) in aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) donor. In Lewis theory, a species that accepts a pair of electrons to form a covalent bond. and one of glycerol:


A great many nonpolar lipids can be made by combining different long- chain acids with glycerol. Because these acids were originally derived from fats, they are collectively referred to as fatty acids.

Notice that for each stearic or other fatty acid molecule that combines with one of the —OH groups of glycerol, a molecule of water is given off, and so the reaction is a condensationThe process in which a liquid forms from gas or vapor of the same substance. A chemical reaction in which two molecules combine to form a very small molecule and a larger molecule than either of the two reactants.. It turns out that a great many important biological molecules are put together by condensation reactions during which water is given off. The reverse of Eq. (1), in which water reacts with a large molecule and splits it into smaller pieces, is called hydrolysisAny reaction in which water (hydro) is split into two parts (lysis). Examples include the reaction of an anion with water to form the conjugate acid and hydroxide ion and hydrolysis of an ester or amide, in which the H from water bonds to form an alcohol or amine and the OH bonds to a carbonyl carbon to form a carboxylic acid.. By carrying out hydrolysis living organisms can break down molecules manufactured by other species. The simple building blocks obtained this way can then be recombined by condensation reactions to form structures appropriate to their new host.

By contrast with the glycerol tristearate found in animals, vegetable fats contain numerous double bonds in their long hydrocarbonA compound containing only the elements carbon and hydrogen. chains. This polyunsaturation introduces “kinks” in the hydrocarbon chains because of the barrier to rotation and the 120° angles associated with the double bonds. Consequently it is more difficult to align the chains side by side (see Fig. 1), and the unsaturatedDescribes 1) a solution that contains less than the equilibrium concentration of a solute, or 2) an organic compound containing double or triple bonds. fats do not pack together as easily in a crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point.. As was true with alkanes, chain length also determines whether a fat is liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container or solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions., and where the melting pointThe temperature at which a solid becomes a liquid. Also called freezing point. occurs.

Figure 1 Caprylic Acid has a shorter carbon chain than palmitic acid and thus melts at a lower temperature. When a double bond leaves part of a fatty acid chain unsaturated, the molecules can not pack in a crystal lattice as tightly, and this lowers the melting point.

Most unsaturated fats (like corn oil) are liquids at ordinary temperatures, while saturated fats (like butter) are solids. Vegetable oils can be converted by hydrogenationAny addition reaction of hydrogen with an organic compound; usually the organic compound has a carbon-carbon double bond or triple bond. to compounds that are solids. This process involves adding H2 catalytically to the double bonds:

Image:Hydrogenation of Unsaturated Hydrocarbons .jpg

Hydrolysis of fats [the reverse of Eq. (1)] is important in the manufacture of soaps. It can be speeded up by the addition of a strong baseA base that dissociates completely or ionizes completely in a particular solvent. like NaOH or KOH, in which case the reaction is called saponificationThe base-catalyzed hydrolysis of an ester; often used in the context of hydrolyzing fats to make soap.. Since saponification requires that the pH of the reaction mixtureA combination of two or more substances in which the substances retain their chemical identity. be high, the fatty acid that is produced will dissociate to its anionA negatively charged ion. An ion that is attracted toward the anode in an electrolytic cell.. When glycerol tristearate is saponified with NaOH for example, sodium stearate, a relatively water-solubleAble to dissolve in a solvent to a significant extent. substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula. and a common soap, is formed.

The ability of soaps to clean grease and oil from soiled surfaces is a result of the dual hydrophobicWater-hating; not attracted to water molecules or polar molecules.-hydrophilicWater-loving; attracted to water molecules and polar molecules. structures of their molecules. The stearate ion, for example, consists of a long nonpolar hydrocarbon chain with a highly polar —COO group at one end.

Image:The Stearate ion, a Soap .jpg

The hydrophobic hydrocarbon chain tries to avoid contact with aqueousDescribing a solution in which the solvent is water. media, while the anionic group readily accommodates the dipoleIn an electrically neutral species, separated, equal positive and negative charges that consitute a positive and a negative pole; such a species tends to assume certain orientations more than others in an electric field. attractions and hydrogen bonds of water molecules.

The two main ways that the hydrophobic portions of stearate ions can avoid water are to cluster together on the surface or to dissolve in a small quantity of oil or grease (see Fig. 2).

Figure 2 Behavior of soap molecules in aqueous solution. Micelles containing grease molecules are made more soluble by the polar ends of soap molecules.

In the latter case the hydrophilic heads of the soap molecules contact the water outside the grease, forming a structure known as a micelle. Since the outsides of the micelles are negatively charged, they repel one another and prevent the grease droplets from recombining. The grease is therefore suspended (emulsified) in the water and can be washed away easily.

Natural soaps, such as sodium stearate, were originally made in the home by heating animal fat with wood ashes, which contained potash, K2CO3. Large quantities are still produced industrially, but to a considerable extent soaps have been replaced by detergents. This is a consequence of the undesirable behavior of soaps in hard waterWater containing high concentrations of cations having charge greater than +1; hardness can be removed by ion exchange.. Calcium, magnesium, and other hard-water cations form insolubleUnable to dissolve appreciably in a solvent. compounds when combined with the anions of fatty acids. This produces scummy precipitates and prevents the soap molecules from emulsifying grease unless a large excess is used.

Detergents such as alkylbenzenesulfonates (ABS) and linear alkylbenzenesulfonates (LAS) have structures very similar to sodium stearate except that the charged group in their hydrophilic heads is —SO3 attached to a benzene ring. The ABS detergents also have methyl (CH3) groups branching off their hydrocarbon chains.

Image:ABS and LAS Detergents .jpg

Such molecules do not precipitate with hard-water cations and therefore are more suitable for machine washing of clothes. The LAS detergents replaced ABS during the mid-nineteen-sixties when it was discovered that the latter were not biodegradable. They were causing rivers and even tap water to become covered with detergent suds and foam. Apparently the enzymes in microorganisms that had evolved to break down the unbranched hydrocarbon chains in natural fats and fatty acids were incapable of digesting the branched chains of ABS molecules. LAS detergents, though manufactured by humans, mimic the structures of naturally occurring molecules and are biodegradable.