Fatty Acids and Triglycerides

Fatty acids are carboxylic acids with long hydrocarbon chains, and triglycerides (fats and oils) are esters of glycerol with three fatty acids; they are the main form of long-term energy storage in animals and provide about 37 kJ per gram, more than twice as much as carbohydrates or proteins. In brief: tripalmitin is formed from glycerol and three molecules of palmitic acid with loss of three water molecules (M = 807.3 g/mol), and fats from saturated fatty acids are solid at room temperature, whereas oils from unsaturated fatty acids are liquid.

Lipids are a diverse group of biological molecules that are mostly insoluble in water. Fats and oils are the most abundant of them in diets, and their composition determines their physical properties and effects on health. This article describes the structure of fatty acids with a data table, how triglycerides are built, why some are solid and others liquid, the role of fats as an energy store, and the chemistry of trans fats, saponification and iodine values, with examples.

What are fatty acids?

A fatty acid is a carboxylic acid with an unbranched hydrocarbon chain, usually with an even number of carbon atoms (typically 12 to 22), because they are built from two-carbon units in the cell. The carboxyl group is the polar head, and the long chain is the nonpolar tail, which makes them insoluble in water.

• Saturated fatty acids have only single C–C bonds. The chain is straight and can pack closely with neighbors.
• Monounsaturated acids have one C=C bond, and polyunsaturated acids have two or more. Natural double bonds are almost always cis, which creates a bend (kink) of about 30° in the chain.

The shorthand C18:1 means 18 carbons and one double bond. The omega (ω) number gives the position of the first double bond from the methyl end: in an omega-3 acid it is on the third carbon, in an omega-6 acid on the sixth. The body cannot make linoleic acid (ω-6) and α-linolenic acid (ω-3), so they are essential fatty acids that must be taken in with the diet.

Common fatty acids

What are triglycerides?

A triglyceride (triacylglycerol) is formed by the esterification of the three –OH groups of glycerol (propane-1,2,3-triol) with three fatty acids, losing three molecules of water:

C3H5(OH)3 + 3 RCOOH → C3H5(OOCR)3 + 3 H2O

The three fatty acids may be the same or different. Natural fats are mixtures of many triglycerides. Triglycerides have no charged groups and are completely nonpolar, so they do not dissolve in water and are stored in cells as droplets. Fats that are solid at room temperature (butter, lard) are rich in saturated fatty acids, and those that are liquid (olive oil, sunflower oil) are rich in unsaturated ones. In the digestive tract, lipase enzymes hydrolyze triglycerides to fatty acids and monoglycerides, which are absorbed and reassembled.

Why are some fats solid and some oils liquid?

Straight saturated chains pack tightly together and attract each other strongly by dispersion forces (van der Waals forces), so a lot of thermal energy is needed to melt them: stearic acid melts at 70 °C. The cis double bonds put kinks in the chain that prevent close packing, so the intermolecular forces are weaker and the melting points much lower: oleic acid (one cis double bond) melts at 13 °C, linoleic acid (two) at −5 °C. The melting point of a fat is therefore determined by the average chain length (longer chains melt higher) and the degree of unsaturation (more double bonds melt lower).

Why are fats such a good energy store?

The carbon in fats is in a highly reduced state (many C–H bonds and few C–O bonds), so the complete oxidation releases a lot of energy: about 37–38 kJ/g (9 kcal/g), compared with about 17 kJ/g (4 kcal/g) for carbohydrates and proteins. Additionally, fats are stored without water (anhydrous), whereas glycogen binds about 2–3 g of water per gram, so that, per gram of tissue, fat stores about 6 times more energy than glycogen. A typical adult stores enough fat for several weeks of energy needs but only about a day of glycogen. Fat also insulates the body and protects the organs.

What are trans fats and omega-3 acids?

Trans fats contain double bonds with a trans configuration. They are produced mainly by the partial hydrogenation of vegetable oils (which converts some cis double bonds to trans), and occur in small amounts in the meat and milk of ruminants. Trans fatty acids have straight chains like saturated ones (elaidic acid, the trans isomer of oleic acid, melts at 45 °C, against 13 °C for oleic acid), and they are linked with a higher risk of cardiovascular disease, because they raise LDL and lower HDL cholesterol. Their use has been restricted or banned in many countries.

Omega-3 fatty acids such as EPA (C20:5) and DHA (C22:6), found in oily fish, are used in cell membranes (especially in the brain and retina) and as precursors of signaling molecules, and diets rich in them have been linked with benefits for the heart.

What are the typical reactions?

• Hydrolysis and saponification: the triglyceride with NaOH gives glycerol and soap (sodium salts of the fatty acids).
• Hydrogenation: H₂ with a nickel catalyst adds to the C=C bonds, hardening liquid oils to give semisolid fats such as margarine.
• Oxidation (rancidity): oxygen attacks the double bonds of unsaturated fats and gives aldehydes and acids with unpleasant smells. Antioxidants such as vitamin E and BHT slow this down.
• Transesterification: with methanol and a base the triglycerides give fatty acid methyl esters (biodiesel) and glycerol.

Worked examples

Example 1: the mass of tripalmitin

Glycerol: 92.09 g/mol. Palmitic acid (C₁₆H₃₂O₂): 256.42 g/mol. M(tripalmitin) = 92.09 + 3 × 256.42 − 3 × 18.02 = 92.09 + 769.26 − 54.06 = 807.29 g/mol, which is C₅₁H₉₈O₆.

Example 2: iodine value of triolein

Triolein (glycerol with three oleic acids, C₅₇H₁₀₄O₆, M = 885.4 g/mol) has 3 C=C bonds, each of which adds one I₂ (253.8 g/mol). The iodine value is the mass of I₂ in grams taken up by 100 g of fat: 3 × 253.8 / 885.4 × 100 = 86.0. A higher iodine value shows more unsaturation. Olive oil has about 80–85, and sunflower oil about 125–140, consistent with its greater content of linoleic acid.

Example 3: energy from fat

A food contains 30 g of fat. The energy is 30 × 37 = 1110 kJ, or about 265 kcal. If 30 g of carbohydrate had been eaten instead, it would provide only 30 × 17 = 510 kJ (about 120 kcal).

Example 4: mass of NaOH for saponification

To saponify 100 g of tripalmitin (807.29 g/mol): n = 100 / 807.29 = 0.1239 mol, and 3 mol of NaOH per mole are needed: 0.3716 mol × 40.00 g/mol = 14.9 g of NaOH, and 0.1239 mol of glycerol (11.4 g) is released with 0.3716 mol of sodium palmitate (103.3 g).

Example 5: how many different triglycerides?

With 3 different fatty acids available, and three positions on the glycerol, there are 3³ = 27 possible triglycerides (counting the optical isomers resulting from the 2-position separately, there are more). The variety explains why natural fats are complex mixtures.

How was the chemistry of fats discovered?

Michel Eugène Chevreul showed in the early 19th century, in a series of studies culminating in his book of 1823, that fats are compounds of glycerol and fatty acids, and that saponification splits them. He discovered stearic and oleic acid, and received many honors. Marcellin Berthelot made synthetic triglycerides from glycerol and fatty acids in 1854. The hydrogenation of oils was developed by Wilhelm Normann in 1901, and margarine was invented in 1869 by Hippolyte Mège-Mouriès, in response to a call from Napoleon III for a cheap substitute for butter. The essential nature of some fatty acids was discovered by George and Mildred Burr in 1929.

Which mistakes are common?

• Thinking that all unsaturated fats are bad: natural cis unsaturated fats are beneficial; trans fats are the problem.
• Mixing up fats and oils: they differ in state (solid or liquid), not in chemical class.
• Forgetting the three waters in triglyceride formation: three molecules are lost.
• Assuming that more double bonds raise the melting point: they lower it.
• Confusing the omega number with the carbon position from the carboxyl end: it counts from the methyl end.