11.2: Ester Formation and Waxes
- Understand how esters are made.
- Identify the molecules used to make waxes.
- Describe the primary biological function of waxes.
Esterification
Esters can be prepared by esterification, a reaction in which a carboxylic acid and an alcohol, heated in the presence of an acid catalyst, form an ester and water through a dehydration followed by condensation reaction.
The reaction is reversible. As a specific example of an esterification reaction, butyl acetate can be made from acetic acid and 1-butanol.
Waxes
Lipid waxes are esters of one long-chain fatty acid with one unbranched long-chain alcohol containing 14 to 30 C′s per chain. For example, m yricyl palmitate found in beeswax is the ester of palmitic acid and myricyl alcohol.
Figure \(\PageIndex{1}\): palmitic acid and myricyl alcohol gives myricyl palmitate in beeswax
Figure \(\PageIndex{2}\): A wax ester - Myricyl palmitate
Waxes form a protective and waterproof coating on leaves, fruits, and stems of plants and on the skin, fur, and feathers of animals. Plant waxes on the surfaces of leaves, stems, flowers, and fruits protect the plant from dehydration and invasion by harmful microorganisms. In animals the waxes serve as protective coatings, keeping the surfaces of feathers, skin, and hair pliable, water repellent and buoyant. In fact, if the waxy coating on the feathers of a water bird is dissolved as a result of the bird swimming in an oil slick, the feathers become wet and heavy, and the bird, unable to maintain its buoyancy, drowns. Some microorganisms use waxes as energy storage molecules.
Some of the most common lipid waxes are listed in Table 11.2.1. These include beeswax obtained from the honeycomb, carnauba wax from palm trees, jojoba wax from jojoba bushes, and spermaceti obtained from the head and cavity of sperm whales. Beeswax and carnauba wax are used as protective coating on cars, furniture, floors, etc. Jojoba wax and spermaceti are used to make candles and cosmetics. A mixture of waxes obtained from wool is used in lotions for face and skin softening.
| Wax name | Condensed formula | Source |
|---|---|---|
| Beeswax |
|
Honeycomb |
| Carnauba wax |
|
Palm tree |
| Jojoba wax |
|
Jojoba bush |
| Spermaceti |
|
Sperm whale |
Petroleum Waxes and Earwax
Petroleum-derived waxes or paraffin are also called waxes but they are not esters, they are mixtures of long-chain hydrocarbons. Earwax is a mixture of esters of glycerol, esters of cholesterol, phospholipids, etc. from dead skin and from secretions of cerumen glands. Like other lipid waxes, it protects the ear canal against bacteria, fungi, and water. Excess earwax can cause blockage in the ear canal and hearing loss.
Summary
Esters are made by the reaction of a carboxylic acid with an alcohol, a process that is called esterification. Waxes are formed from long chain fatty acids and long chain alcohols.