Esters: Esterification and Hydrolysis

Chemistry you can smell

Peel a banana, sniff a pineapple or chew wintergreen gum. Much of what you smell is an ester: banana's sweet note comes largely from 3-methylbutyl ethanoate, pineapple's from ethyl butanoate, and wintergreen from methyl salicylate. Esters are also in nail polish, solvents, fats and oils, and the polyester in your clothing.

An ester has the group –COO– between two carbon groups: R–COO–R′. It looks like a carboxylic acid whose acidic H has been replaced by a carbon group. In fact, that is how esters are made: from a carboxylic acid and an alcohol.

Esterification

When a carboxylic acid is heated with an alcohol and a few drops of concentrated sulfuric acid (the catalyst), they join and lose a water molecule:

carboxylic acid + alcohol ⇌ ester + water

CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O

• It is a condensation reaction: two molecules join and a small molecule (water) is released.
• The OH comes from the acid and the H from the alcohol. Chemists proved this by making the alcohol with a heavy oxygen isotope, ¹⁸O: the ¹⁸O ended up in the ester, not in the water.
• The reaction is reversible and reaches equilibrium (you will study this in the unit Chemical Equilibrium). Using excess alcohol, or removing water, increases the yield of ester.
• In the lab, the mixture is often heated under reflux so the volatile reactants and products don't escape.

Naming esters

An ester's name has two words, like the name of a salt:

1. First word: the alcohol part, named as an alkyl group: methyl, ethyl, propyl...
2. Second word: the acid part, with -oic acid changed to -oate: methanoate, ethanoate, propanoate...

To tell the parts apart, find the C=O. The carbon chain that includes the C=O carbon came from the acid. The group bonded to the single-bonded O came from the alcohol.

Watch out: chemists often write the acid part first in the formula, CH₃COOCH₂CH₃, but say the alcohol part first in the name, ethyl ethanoate. The formula can also be written the other way round: CH₃CH₂OOCCH₃ is the same compound.

Common names use the common acid names: ethyl ethanoate is ethyl acetate, a solvent in some nail polish removers and glues.

Properties of esters

• Esters are polar (two C–O bonds and a C=O), but they have no O–H, so they cannot hydrogen-bond to each other. Their boiling points are lower than those of acids and alcohols of similar mass, and higher than those of alkanes: ethyl ethanoate (88 g/mol) boils at 77 °C, while butanoic acid (88 g/mol) boils at 164 °C.
• Their oxygens can accept hydrogen bonds from water, so small esters are slightly soluble: about 8 g of ethyl ethanoate dissolves in 100 mL of water.
• Many small esters are volatile with fruity or flowery smells. Food scientists blend them to make artificial flavours.

Bigger esters you know:

• Fats and oils are triglycerides: esters of glycerol with three long-chain fatty acids.
• ASA (acetylsalicylic acid, sold as aspirin) is an ester made from salicylic acid. Many peoples, including First Nations in North America, have long used willow bark to relieve pain; it contains salicin, which the body converts to salicylic acid. Chemists later made ASA, which is gentler on the stomach.
• Polyesters such as PET (water bottles and fleece) contain thousands of ester links (Lesson 15).

Hydrolysis: splitting esters

Acid hydrolysis is the reverse of esterification. Heating an ester with water and an acid catalyst gives back the acid and the alcohol:

CH₃COOCH₂CH₃ + H₂O ⇌ CH₃COOH + CH₃CH₂OH

Base hydrolysis (saponification) uses a strong base such as NaOH. It goes to completion, because the acid formed is immediately turned into its salt:

CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH

Making soap. Heating a fat with sodium hydroxide hydrolyses all three ester links, giving glycerol and three sodium salts of fatty acids: soap. A soap ion has a long non-polar tail, which dissolves in grease, and an ionic head (–COO⁻), which dissolves in water, so soap lifts grease off your hands and into the rinse water.

Investigation: making esters (school lab only)

Purpose: to prepare several esters and relate their smells to their structures (B2.4).

Procedure outline:

1. In a test tube, mix 10 drops of a carboxylic acid (for example ethanoic acid) with 10 drops of an alcohol (for example 3-methylbutan-1-ol, ethanol or octan-1-ol).
2. Your teacher adds 2 drops of concentrated sulfuric acid.
3. Warm the tube in a hot-water bath (about 60 °C, heated on a hot plate) for 10 to 15 minutes.
4. Pour the mixture into a small beaker of cool sodium hydrogen carbonate solution, which neutralizes the leftover acids and lets the ester float as an oily layer.
5. Waft to identify the smell.

Safety (A1.4): concentrated sulfuric acid is highly corrosive: goggles, gloves and lab coat; your teacher dispenses it. Alcohols and esters are flammable: no open flames; use a hot plate. Work in a well-ventilated room or fume hood. Never taste anything, and dispose of organic waste in the labelled container.

Variables (A1.5): the acid and alcohol (independent); the smell (dependent); volumes, temperature and time (controlled). Smell is a subjective observation, so have several people record it independently.