Esters are functional group derivatives of carboxylic acids where the acidic hydrogen of the \( -\text{COOH} \) group has been replaced by an alkyl or aryl carbon chain, resulting in the functional group \( -\text{COOR} \). In this lesson, we explore the formation, hydrolysis pathways, and commercial uses of esters, alongside their biological occurrence as fats, oils, and their transesterification into biodiesel.
🔑 Key Principle
The ester linkage is vulnerable to nucleophilic attack at the polar carbonyl carbon atom. Adding water under different pH conditions causes the ester to undergo hydrolysis, producing different sets of products depending on whether the catalyst is acidic or basic.
Ester Hydrolysis Pathways
Hydrolysis is the chemical breakdown of a molecule using water. For esters, this process can be catalysed by either a dilute aqueous acid or a dilute aqueous base, typically carried out under reflux.
A chemical reaction in which a bond is cleaved by the addition of a water molecule, often facilitated by an acid or base catalyst.
Pathway A: Acid-Catalysed Hydrolysis
Acid hydrolysis is a reversible process. The ester is heated under reflux with a dilute aqueous mineral acid (such as dilute \( \text{HCl} \) or dilute \( \text{H}_2\text{SO}_4 \)):
Because the reaction is in dynamic equilibrium, it does not go to completion, resulting in a mixture of reactants and products. To maximise the yield of carboxylic acid or alcohol, water must be added in large excess.
Pathway B: Base-Catalysed Hydrolysis (Saponification)
Base hydrolysis is irreversible. The ester is heated under reflux with a dilute aqueous alkali (such as dilute \( \text{NaOH} \) or \( \text{KOH} \)):
This reaction goes to completion because the carboxylic acid formed reacts immediately with the hydroxide ions to form a stable carboxylate salt (\( \text{RCOO}^- \)):
The negatively charged carboxylate ion is resistant to nucleophilic attack by the alcohol because of electrostatic repulsion, which prevents the reverse reaction from occurring. To obtain the free carboxylic acid, the mixture must be acidified with a strong acid after the reaction has finished:
The base-catalysed hydrolysis of an ester or triester to form an alcohol and a carboxylate salt. Historically used for making soap from animal fats.
When asked for the products of base hydrolysis, you must draw the salt of the carboxylic acid (such as sodium propanoate) rather than the parent carboxylic acid. Drawing the acid in alkaline conditions will cost you marks, as acids and bases cannot coexist without reacting.
a) The acid-catalysed hydrolysis of methyl propanoate.
b) The base-catalysed hydrolysis of ethyl benzoate using aqueous sodium hydroxide.
Solution:
a) Acid Hydrolysis of methyl propanoate:
Acid hydrolysis is reversible and yields the free carboxylic acid and alcohol. Splitting the ester link \( \text{-COO-} \):
- The acid part has 3 carbons: propanoic acid (\( \text{CH}_3\text{CH}_2\text{COOH} \)).
- The alcohol part has 1 carbon: methanol (\( \text{CH}_3\text{OH} \)).
b) Base Hydrolysis of ethyl benzoate using NaOH:
Base hydrolysis yields the carboxylate salt and alcohol. Splitting the ester link:
- The acid part contains a benzene ring (benzoate): sodium benzoate (\( \text{C}_6\text{H}_5\text{COONa} \)).
- The alcohol part has 2 carbons (ethyl): ethanol (\( \text{CH}_3\text{CH}_2\text{OH} \)).
Commercial Uses of Esters
Esters are widely exploited across chemical and manufacturing sectors due to their physical and chemical properties:
| Application | Property Exploited | Typical Examples |
|---|---|---|
| Solvents | Polar molecules, relatively low boiling points (evaporate easily), dissolve organic solute species. | Ethyl ethanoate (used in glues, paints, and nail polish removers). |
| Perfumes and Flavourings | Sweet, fruity, and distinctive odours; high volatility allows easy detection. | Methyl butanoate (apple scent), pentyl ethanoate (banana scent). |
| Plasticisers | Get between polymer chains, reducing intermolecular forces and making the plastic more flexible. | Phthalate esters added to polyvinyl chloride (PVC) to make flexible tubing. |
Vegetable Oils and Fats
Vegetable oils and animal fats are naturally occurring triesters. They are formed from one molecule of propane-1,2,3-triol (commonly known as glycerol) reacting with three molecules of long-chain carboxylic acids (fatty acids).
A triester formed when glycerol (propane-1,2,3-triol) undergoes condensation with three fatty acid molecules.
The difference between fats (solid at room temperature) and oils (liquid) lies in the structure of the fatty acid chains:
- Fats (saturated): Contain hydrocarbon chains with only \( \text{C-C} \) single bonds. The chains are straight, packing together closely and establishing strong van der Waals forces, leading to higher melting points.
- Oils (unsaturated): Contain one or more \( \text{C}=\text{C} \) double bonds, causing kinks in the hydrocarbon chains. This prevents close packing, weakening intermolecular forces and lowering melting points.
Saponification of a triglyceride with aqueous sodium hydroxide yields glycerol and three moles of sodium salts of long-chain fatty acids (which are used as soap):
Transesterification and Biodiesel
Biodiesel is a fuel made from renewable biological sources (such as waste cooking oil or rapeseed oil). It consists of a mixture of methyl esters of long-chain fatty acids.
A reaction where the alkoxy group of an ester is swapped for another alcohol group, converting one ester into another in the presence of an acid or base catalyst.
To produce biodiesel, triglycerides from vegetable oil are reacted with methanol in the presence of a strong base catalyst, typically sodium hydroxide (\( \text{NaOH} \)) or potassium hydroxide (\( \text{KOH} \)):
This reaction splits the triglyceride, producing one molecule of glycerol (which is separated and used in cosmetics or pharmaceuticals) and three molecules of methyl esters (biodiesel). Biodiesel is a cleaner alternative to fossil diesel, releasing carbon dioxide that was recently absorbed by the crops during photosynthesis, making it approximately carbon-neutral.
Solution:
1. Represent the triglyceride structure:
\[ \text{CH}_2(\text{OOC-C}_{17}\text{H}_{35})\text{-CH}(\text{OOC-C}_{17}\text{H}_{35})\text{-CH}_2(\text{OOC-C}_{17}\text{H}_{35}) \]2. Set up the transesterification equation with 3 moles of methanol:
\[ \text{C}_3\text{H}_5(\text{OOC-C}_{17}\text{H}_{35})_3 + 3\text{CH}_3\text{OH} \rightarrow \text{CH}_2\text{OH-CHOH-CH}_2\text{OH} + 3\text{C}_{17}\text{H}_{35}\text{COOCH}_3 \]3. Identify the products: The biodiesel component is methyl stearate (methyl octadecanoate). The co-product is propane-1,2,3-triol (glycerol).
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