What you'll learn

• How ethanol is made industrially from ethene, and by fermentation.
• How alcohols are classified, dehydrated and oxidised.
• How to test for alcohols and carboxylic acids.
• How carboxylic acids form salts and esters, and how esters can be separated by distillation.

1. The key functional groups

Organic chemistry is much easier when you focus on the functional group: the part of a molecule that controls its typical reactions.

For this section:

• Alcohols contain the hydroxyl group, –OH. Example: ethanol, CH₃CH₂OH.
• Carboxylic acids contain the carboxyl group, –COOH. Example: ethanoic acid, CH₃COOH.
• Esters contain the ester link, –COO–. Example: ethyl ethanoate, CH₃COOCH₂CH₃.

The main reactions in C3.4 fit together like this.

2. Making ethanol

Industrial preparation from ethene

Ethanol can be made by hydration of ethene. Hydration means adding water across a carbon-carbon double bond.

Ethene reacts with steam:

C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g)

Typical conditions:

• phosphoric(V) acid catalyst, H₃PO₄, on a solid support
• about 300 °C
• high pressure, around 60–70 atm
• unreacted ethene and steam are recycled

This is a fast, continuous industrial process and gives relatively pure ethanol. Its main disadvantage is that ethene usually comes from crude oil, so it is not renewable.

Curly-arrow mechanism: acid-catalysed hydration of ethene

In mechanism terms, this is electrophilic addition.

1. A curly arrow goes from the alkene π bond to H⁺, forming a carbocation.
2. A curly arrow goes from a lone pair on the oxygen of water to the positively charged carbon, forming protonated ethanol.
3. A base removes H⁺; the O–H bond electrons return to oxygen, producing ethanol and regenerating the acid catalyst.

Fermentation

Ethanol can also be made by fermentation, where enzymes in yeast convert glucose into ethanol and carbon dioxide under anaerobic conditions.

C₆H₁₂O₆(aq) → 2CH₃CH₂OH(aq) + 2CO₂(g)

Conditions are mild: warm, around 25–40 °C, and no oxygen. The ethanol solution is dilute because ethanol eventually kills the yeast, so distillation is needed to concentrate it.

Other alcohols, such as propan-1-ol or butan-1-ol, can be produced by fermentation using different microorganisms or feedstocks. The mixture is then separated by distillation or fractional distillation.

3. Classifying alcohols

Alcohols are classified by looking at the carbon atom directly bonded to the –OH group.

• A primary alcohol has that carbon attached to one other carbon, or none in methanol. General form: RCH₂OH.
• A secondary alcohol has that carbon attached to two other carbons. General form: R₂CHOH.
• A tertiary alcohol has that carbon attached to three other carbons. General form: R₃COH.

4. Dehydration of alcohols

Dehydration means removal of water from a molecule. Alcohols can be dehydrated to form alkenes.

For ethanol:

CH₃CH₂OH(l) → CH₂=CH₂(g) + H₂O(l)

Typical conditions:

• concentrated sulfuric acid or phosphoric acid catalyst
• heat

Curly-arrow mechanism: acid-catalysed dehydration

A simplified mechanism is:

1. A lone pair on the alcohol oxygen attacks H⁺, forming a protonated alcohol, R–OH₂⁺. This turns –OH into water, a better leaving group.
2. The C–O bond electrons move onto oxygen as water leaves.
3. A base removes H⁺ from an adjacent carbon; the C–H bond electrons form the C=C double bond, giving the alkene and regenerating the acid catalyst.

5. Oxidation of alcohols

Alcohols are oxidised using acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄. The observation is orange to green because Cr₂O₇²⁻ is reduced to Cr³⁺.

Primary alcohols can be oxidised in two stages:

• primary alcohol → aldehyde
• aldehyde → carboxylic acid

For ethanol:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

To get the aldehyde, distil it off as it forms. To get the carboxylic acid, heat under reflux with excess oxidising agent.

Secondary alcohols oxidise to ketones:

CH₃CHOHCH₃ + [O] → CH₃COCH₃ + H₂O

Tertiary alcohols are not oxidised under these conditions because the carbon bonded to –OH has no hydrogen atom to remove.

6. Tests for alcohols and carboxylic acids

Dichromate(VI) test for alcohols

Warm the compound with acidified potassium dichromate(VI).

• Primary alcohol: orange to green.
• Secondary alcohol: orange to green.
• Tertiary alcohol: remains orange.

Sodium hydrogencarbonate test for carboxylic acids

Carboxylic acids react with sodium hydrogencarbonate to produce carbon dioxide gas.

CH₃COOH(aq) + NaHCO₃(aq) → CH₃COONa(aq) + CO₂(g) + H₂O(l)

The observation is effervescence. If the gas is bubbled through limewater, it turns milky.

7. Reactions of carboxylic acids with bases, carbonates and hydrogencarbonates

Carboxylic acids are weak acids, but they still neutralise bases to form salts.

With sodium hydroxide:

CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)

With sodium carbonate:

2CH₃COOH(aq) + Na₂CO₃(aq) → 2CH₃COONa(aq) + CO₂(g) + H₂O(l)

With sodium hydrogencarbonate:

CH₃COOH(aq) + NaHCO₃(aq) → CH₃COONa(aq) + CO₂(g) + H₂O(l)

8. Esterification

An esterification reaction occurs when a carboxylic acid reacts with an alcohol to form an ester and water.

General equation:

carboxylic acid + alcohol ⇌ ester + water

Example:

CH₃COOH(l) + CH₃CH₂OH(l) ⇌ CH₃COOCH₂CH₃(l) + H₂O(l)

Conditions:

• concentrated sulfuric acid catalyst
• heat, often under reflux

The reaction is reversible, so the yield may be improved by using an excess of one reactant or removing water.

Curly-arrow mechanism: acid-catalysed esterification

1. A lone pair on the carbonyl oxygen of the carboxylic acid attacks H⁺, making the carbonyl carbon more electrophilic.
2. A lone pair on the alcohol oxygen attacks the carbonyl carbon; the C=O π electrons move onto oxygen.
3. Proton transfers occur so that an –OH group becomes –OH₂⁺, a good leaving group.
4. Water leaves, and a lone pair reforms the C=O bond.
5. Loss of H⁺ gives the ester and regenerates the acid catalyst.

9. Separation by distillation

Distillation separates liquids using differences in boiling point. In this topic, it is used to collect an ester product from a reaction mixture.

The core apparatus includes a heated round-bottom flask, thermometer, Liebig condenser and collecting flask. Cooling water enters at the lower end of the condenser and leaves at the upper end so the condenser jacket stays full.

Specified practical: preparation of an ester and separation by distillation

A typical method is:

1. Add the carboxylic acid and alcohol to a flask.
2. Add a few drops of concentrated sulfuric acid catalyst and anti-bumping granules.
3. Heat gently using an electric heater or water bath.
4. Distil the volatile ester product and collect the fraction boiling near its boiling point.
5. Further purification may involve washing, drying with an anhydrous salt, and redistilling.

Safety points matter: alcohols and esters are flammable, concentrated sulfuric acid is corrosive, and volatile organic vapours should not be inhaled.