Investigate the structural formulae, properties, applications, formation by esterification, and hydrolysis (including saponification) of esters

What this dot point is asking

NESA wants you to name esters as alkyl alkanoate, write the esterification equation between an alcohol and a carboxylic acid with concentrated $H_2SO_4$ catalyst, describe the reflux procedure, list common applications of esters, and write both acid and base hydrolysis equations including saponification of long-chain esters to make soap.

The answer

Structure and naming

An ester contains the linkage $-COO-$. The general formula is $R-COO-R'$, where $R$ comes from the carboxylic acid and $R'$ comes from the alcohol.

Esters are named in two words: alkyl (from the alcohol) alkanoate (from the acid, including the carbonyl carbon).

To work backwards from an ester to its parent acid and alcohol: split at the single-bonded $C-O$, add $H$ to the acid oxygen and add $OH$ to the alcohol carbon.

Esterification (Fischer esterification)

An alcohol reacts with a carboxylic acid in the presence of a concentrated $H_2SO_4$ catalyst, heated under reflux, to give an ester and water:

The reaction is reversible and reaches equilibrium, typically with 60 to 70% conversion. Concentrated $H_2SO_4$ has two roles:

1. Catalyst: protonates the carbonyl oxygen of the acid, making the carbonyl carbon more electrophilic and easier for the alcohol to attack.
2. Dehydrating agent: absorbs the water byproduct, shifting equilibrium right by Le Chatelier's principle.

Reflux is essential because the reactants are volatile (boiling points 65 to 120 degrees C). Reflux returns evaporated reactants to the flask, so the mixture stays at temperature for long enough to reach equilibrium without losing material.

Purification. Pour the cooled mixture into saturated sodium hydrogencarbonate to neutralise unreacted acid (effervescence stops when complete). Separate the organic layer, dry over anhydrous $MgSO_4$, then distil at the boiling point of the ester.

Physical properties of esters

Esters have a polar $C=O$ but no $O-H$, so they cannot hydrogen-bond to each other. Boiling points are lower than the parent acid and alcohol of similar molar mass, comparable to ketones. Small esters are volatile liquids with characteristic fruity smells.

Solubility in water decreases sharply with chain length: methyl ethanoate is somewhat soluble, ethyl ethanoate has limited solubility (about 8% w/w), and esters above C6 are essentially insoluble. Esters are good solvents for non-polar organic compounds (paints, varnishes, glues).

Applications

• Flavours and fragrances: short-chain esters give fruits their characteristic smells. Synthetic esters are added to lollies, drinks, perfumes and air fresheners.
• Solvents: ethyl ethanoate is the active solvent in nail polish remover; butyl ethanoate is used in lacquers.
• Biodiesel: methyl esters of long-chain fatty acids ($C_{16}$ to $C_{18}$) are biodiesel, made by transesterification of vegetable oil with methanol and a NaOH catalyst.
• Plasticisers: dialkyl phthalate esters soften PVC.
• Fats and oils: triesters of glycerol with fatty acids (triglycerides) are biological lipids.

Hydrolysis: the reverse reactions

Acid hydrolysis. Reflux the ester with dilute sulfuric acid; the reverse of esterification:

Reversible, reaches equilibrium. Useful to identify an unknown ester by isolating its acid and alcohol fragments.

Base hydrolysis (saponification). Reflux the ester with aqueous NaOH:

Irreversible because the carboxylate anion $R-COO^-$ is unreactive and cannot recombine with the alcohol. The reaction goes to completion, giving the sodium salt of the carboxylic acid and the alcohol.

When the ester is a triglyceride (the natural form of fats and oils), saponification with NaOH gives glycerol plus three sodium fatty-acid salts, which are soap:

This is the chemistry of soap-making: hot fat or oil is stirred with sodium hydroxide solution, the soap is salted out with brine, washed, and pressed into bars. KOH gives softer potassium soaps (liquid soaps).

Mechanism in brief

Fischer esterification is acid-catalysed nucleophilic acyl substitution. The acid is protonated on the carbonyl O, the alcohol O attacks the carbonyl C, proton transfers occur, water leaves, and a proton is lost from the protonated ester. You do not need the full curly-arrow mechanism for HSC, but you do need to know what each reactant contributes.

Examples in context

Example 1. Biodiesel manufacture from tallow at Australian Renewable Fuels Picton. The Picton plant southwest of Sydney converts beef tallow into biodiesel by base-catalysed transesterification: triglyceride plus three methanol gives glycerol plus three fatty acid methyl esters (FAMEs). Each FAME molecule contains the ester linkage $R-COO-CH_3$. The reaction uses NaOH at 60 degrees C and is fast, irreversible and high-yielding. Quality control engineers check ester content by GC-MS against the AS 3572 fuel-grade standard. The HSC esterification mechanism extended to triglyceride substrates explains the entire industrial flowsheet, and students applying mass balance can predict that 1.0 tonne of tallow yields about 1.05 tonnes of biodiesel.

Example 2. NSW HSC depth study synthesis of ethyl ethanoate. A common Stage 6 depth study has students reflux 5 mL of ethanol with 5 mL of glacial ethanoic acid and 1 mL of concentrated sulfuric acid for 20 minutes. After cooling and pouring into sodium carbonate solution, students smell the characteristic pear-drop aroma of ethyl ethanoate: $CH_3CH_2OH + CH_3COOH \rightleftharpoons CH_3COOCH_2CH_3 + H_2O$. Yield rarely exceeds 70 percent because the equilibrium is unfavourable; Le Chatelier reasoning explains why excess alcohol or removal of water by distillation would push yield higher. The synthesis appears almost every year as a 5 to 7 mark Section II question.

Try this

Q1. Name the ester formed when propan-1-ol reacts with ethanoic acid, and write the balanced equation including the catalyst. [3 marks]

• Cue. Propyl ethanoate; $CH_3COOH + CH_3CH_2CH_2OH \rightleftharpoons CH_3COOCH_2CH_2CH_3 + H_2O$ with conc $H_2SO_4$ catalyst.

Q2. A 9.20 g sample of ethanol is reacted with excess butanoic acid to form ethyl butanoate. If the percent yield is 60 percent, calculate the mass of ester produced. [3 marks]

• Cue. $n(\text{ethanol}) = 9.20 / 46.07 = 0.200$ mol; theoretical $n(\text{ester}) = 0.200$ mol; actual $= 0.120$ mol; mass $= 0.120 \times 116.16 = 13.9$ g.

Q3. Sodium hydroxide is used to hydrolyse ethyl ethanoate. (a) Write the balanced equation. (b) State why this reaction is described as saponification when applied to a triglyceride. (c) Contrast this with acid hydrolysis. [2+2+1 marks]

• Cue. (a) $CH_3COOCH_2CH_3 + NaOH \rightarrow CH_3COONa + C_2H_5OH$. (b) Triglyceride saponification gives soap (sodium salts of fatty acids). (c) Acid hydrolysis is reversible; base hydrolysis is irreversible because the carboxylate ion is stable.