Esters and Fats

Esters R-COO-R', formed when water is removed from a carboxylic acid and an alcohol, are major components of fragrances, solvents, and lipids. Animal and plant fats and oils are esters called triglycerides.

Basic Knowledge

Esterification(reversible): R-COOH + R'-OH ⇄ R-COO-R' + H2O(concentrated H₂SO₄ catalyst, heating). The reverse reaction, hydrolysis, proceeds with dilute acid or a base(saponification).
Main esters: ethyl acetate CH₃COOC₂H₅(solvent in nail polish and adhesives, fruity odor), isoamyl acetate(banana scent), methyl salicylate(analgesic patches and Salomethyl scent), benzyl acetate(jasmine scent), and ethyl benzoate(heliotrope scent). They are central to fragrances and artificial flavors.
Fats and oils: triesters of higher fatty acids and glycerol(triacylglycerols, TGs).
Saturated fatty acids(solids at room temperature, “fats”): palmitic acid C₁₅H₃₁COOH and stearic acid C₁₇H₃₅COOH.
Unsaturated fatty acids(liquids at room temperature, “oils”): oleic acid C₁₇H₃₃COOH(one C=C), linoleic acid C₁₇H₃₁COOH(two C=C), and linolenic acid C₁₇H₂₉COOH(three C=C).
Iodine value: The number of grams of iodine added to 100 g of fat or oil. It indicates the degree of unsaturation(drying, semidrying, and nondrying oils).
Saponification value: The number of milligrams of KOH required to saponify 1 g of fat or oil. It indicates the average molecular mass of the fatty acids(inversely proportional).
Saponification: fat or oil+NaOH → glycerol+sodium salts of fatty acids(soap).

Further Exploration (Principles and Applications)

Soap: Sodium salts of fatty acids. It is a surfactant with a hydrophilic group -COO^- and a hydrophobic group(a hydrocarbon chain) at opposite ends. In water, it forms micelles(spherical aggregates with hydrophobic groups facing inward), surrounds and disperses oily dirt, and cleans it.
In hard water, it forms insoluble salts(soap scum) with Ca²⁺ and Mg²⁺, reducing its effectiveness. Synthetic detergents(sodium sulfonates such as LAS and AES) work even in hard water, but biodegradability and environmental effects are issues.
Trans fatty acids: During partial hydrogenation in margarine production, cis→trans isomerization occurs, adversely affecting health(cardiovascular disease risk). The WHO recommends limiting intake, and the U.S. FDA decided in 2018 not to permit partially hydrogenated oils as food additives.
Lipid metabolism: Fats and oils are broken down by lipase into fatty acids+monoacylglycerol→absorbed→β-oxidation(broken down into acetyl-CoA in two-carbon units)→citric acid cycle→ATP production. They provide 9 kcal per 1 g, more energy than carbohydrates(4 kcal).

Points to Note(Common Confusions and Frequent Mistakes)

① Esterification is reversible(the equilibrium shifts back in the presence of water), while saponification is irreversible(soap is stable). ② In the mechanism of esterification, the acid's OH group leaves(an isotope¹⁸O experiment confirms this), while the alcohol's H leaves to form H₂O. ③ Soap solutions are weakly basic(sodium salts of fatty acids undergo hydrolysis). ④ Synthetic detergents are neutral to weakly acidic and work even in hard water. ⑤ Saponification of one molecule of fat or oil requires 3 mol of NaOH/KOH(because there are three ester bonds).

Practice

1. Write the equation for the formation of ethyl acetate from acetic acid and ethanol.
2. Write the saponification equation for a fat consisting of three stearic acid units and glycerol with NaOH.
3. Compare the characteristics of fats and oils with high and low iodine values.