Esters (HSC SSCE Chemistry): Revision Notes

Esters

What are esters?

Esters are an important class of organic compounds found widely in biological systems and used extensively in industry. They possess distinctive fruity aromas and contribute to the flavours and fragrances of many fruits and flowers. Natural esters include solid animal fats and both animal and vegetable oils.

The general structure of an ester features a carbonyl group ($\text{C=O}$) bonded to an oxygen atom, which is attached to an alkyl group. The general formula for an ester is:

$\text{R-C(O)-O-R'}$

where $\text{R}$ and $\text{R'}$ represent alkyl groups.

Industrial and natural importance

Beyond their natural occurrence, esters have significant industrial applications. They can be synthesised easily and are produced synthetically as artificial flavourings and colourings. Several billion kilograms of polyesters are manufactured annually for products including fabrics and plastic bottles. Many esters also serve as solvents in industry - for example, ethyl ethanoate is commonly found in nail polish remover.

Naming esters

The systematic naming of esters follows a two-part system based on their molecular structure:

First part - the alkyl group

The first word identifies the alkyl group directly attached to the oxygen atom of the ester group. This portion originally came from the alcohol molecule. It uses the standard alkyl naming convention:

• One carbon = methyl
• Two carbons = ethyl
• Three carbons = propyl
• Four carbons = butyl
• Six carbons = hexyl

Second part - the carboxylate

The second word derives from the carboxylic acid component. This is the hydrocarbon chain on the other side of the ester link. Remember to include the carbon that forms part of the ester group when counting. The naming changes the acid suffix from -oic acid to -oate.

Worked example

Properties of esters

Physical state and boiling points

The presence of $\text{C-O}$ and $\text{C=O}$ bonds makes the ester group polar. However, esters tend to exist as liquids at room temperature, with boiling points much lower than carboxylic acids of similar molecular mass. This difference occurs because the primary intermolecular forces in esters are dipole-dipole forces, which are weaker than the hydrogen bonding present in carboxylic acids and alcohols.

Name	Molecular Mass (g/mol)	Boiling Point (°C)	Solubility in Water (g/100g water)
Methyl methanoate	60	32	24
Methyl ethanoate	74	57	6.7
Methyl propanoate	88	80	7.3
Ethyl ethanoate	88	77	7.5
Propyl ethanoate	102	102	2

Solubility

Most esters show limited solubility in water due to two factors:

1. Lack of hydrogen bonding - esters cannot form hydrogen bonds as effectively as carboxylic acids or alcohols
2. Large hydrophobic alkyl groups - these non-polar regions repel water molecules

Functional group isomerism

Carboxylic acids and esters are functional group isomers - they share the same molecular formula but possess different functional groups.

Preparing esters

The esterification reaction

Esters form through a reaction between an alcohol and a carboxylic acid:

$\text{R-COOH} + \text{R'-OH} \xrightarrow[\text{H}^+]{\text{Reflux}} \text{R-COO-R'} + \text{H}_2\text{O}$

This reaction produces a water molecule as a by-product. Reactions where functional groups from two molecules join together and release a small molecule are called condensation reactions.

Example reaction

Conditions for preparation

Ester formation is naturally very slow - it can take months or years in nature. Laboratory preparation requires specific conditions to accelerate the reaction:

Catalyst

Adding concentrated sulfuric acid acts as a catalyst, significantly speeding up the reaction and allowing it to occur within a reasonable timeframe.

Heating

The mixture must be heated to increase the reaction rate. However, conventional heating in an open container would cause loss of reactants because organic chemicals used in ester formation are volatile and evaporate easily, even at low temperatures.

Reflux

A process called reflux solves the evaporation problem. The reflux apparatus consists of:

Safety considerations

Excess reagent

Adding one reagent in excess drives the equilibrium reaction towards product formation. According to Le Chatelier's principle, this results in a higher percentage yield of ester.

Purifying esters

After refluxing for approximately 30 minutes, the flask contains a mixture of substances. Because esterification is an equilibrium reaction, both reactants and products remain present, along with the catalyst.

Organic Components	Inorganic Components
Ester (product) - not soluble in water	Water (reaction product and from solutions added)
Carboxylic acid (leftover reactant) - small acids are water soluble, larger ones are not	Sulfuric acid (present as $\text{H}^+$ and $\text{SO}_4^{2-}$ ions) - soluble in water through ion-dipole bonds
Alcohol (leftover reactant) - short-chain alcohols are water soluble, long-chain alcohols are not

Purification process

A separating funnel is the key piece of equipment for purification. It separates immiscible liquids (liquids that don't dissolve in each other) which form distinct layers, with the less dense layer on top. Opening the stopcock at the base allows the denser component to drain into a collection vessel, leaving the less dense component in the funnel.

Step 1: Washing with water

Pour the entire reaction mixture into a separating funnel and add water. After standing, the mixture separates into two layers:

Organic layer

• Contains the ester
• May also contain alcohol and carboxylic acid if they have long chains
• Usually less dense than water, forming the top layer
• Check which layer is organic by adding 1-2 drops of water - water drops pass through the organic layer and dissolve in the aqueous layer

Aqueous layer

• Contains water, sulfuric acid
• Contains any soluble short-chain alcohol and carboxylic acid molecules
• Usually forms the bottom layer

Step 2: Addition of sodium carbonate

Add sodium carbonate ($\text{NaHCO}_3$) or sodium hydrogen carbonate to remove any remaining carboxylic acid. Any insoluble carboxylic acid reacts with carbonate ions to produce a carboxylate ion ($\text{RCOO}^-$) and carbon dioxide gas:

$\text{R-COOH} + \text{NaHCO}_3 \rightarrow \text{R-COO}^-\text{Na}^+ + \text{H}_2\text{O} + \text{CO}_2$

The insoluble carboxylic acid converts to a soluble carboxylate ion. This ion dissolves despite its large hydrocarbon chain because it can form ion-dipole bonds with water molecules.

Add water to the separating funnel. It separates again into aqueous and organic layers. Separate the layers and discard the aqueous layer.

Step 3: Distillation

Different components in the mixture have different boiling points. Distillation exploits these differences to isolate the pure ester. By knowing the boiling points of the carboxylic acid, ester, and alcohol, you can collect the ester at its specific boiling point temperature, producing a pure final product.

Remember!