Alcohols

Full Notes

Alcohols are organic compounds with an –OH (hydroxyl) group .

They are classified by the number of carbon atoms attached to the carbon bonded to the –OH group:

• Primary alcohol (1°): OH group on a carbon bonded to one other carbon
• Secondary alcohol (2°): OH on a carbon bonded to two other carbons
• Tertiary alcohol (3°): OH on a carbon bonded to three other carbons

This classification is important for predicting reaction outcomes.

Combustion of Alcohols

Alcohols burn in oxygen to produce carbon dioxide and water.

General equation:
C_nH_2n+1OH + O₂ → CO₂ + H₂O

All alcohols release energy on combustion, making them useful fuels. Complete combustion occurs with excess oxygen; incomplete combustion produces CO or C.

Reactions with Halogenating Agents

Alcohols react with halogenating agents to form halogenoalkanes:

These are examples of substitution reactions, replacing the OH group with a halogen atom.

These are the key halogenation reactions you need to know.

With PCl₅

Chloroalkanes formed using PCl₅

With KBr

Bromoalkanes formed using KBr with H₂SO₄ (50% concentration)

With red phosphorus + iodine, I₂

Iodoalkanes formed using Iodine (I₂) and red phosphorus

Oxidation of Alcohols

Primary and Secondary alcohols can be oxidised by oxidising agents.

The products formed depend on the classification of alcohol (see above).

For alcohol oxidation, the oxidising agent is usually acidified potassium dichromate(VI), K₂Cr₂O₇ + H₂SO₄. It changes from orange to green when reduced.

Matt’s exam tip

Remember for organic chemistry, oxidation can be considered a carbon atom gaining a bond to a more electronegative element (usually oxygen). To provide the oxygen needed, we use an oxidising agent and show oxygen from the oxidising agent in equations as [O]. Common oxidising agents include acidified K2Cr2O7 and acidified KMnO4.

For primary alcohols:

Primary alcohols oxidise first to aldehydes, then to carboxylic acids using heat and acidified potassium dichromate (VI) as an oxidising agent.

Observation: Orange Cr₂O₇²⁻ (dichromate) turns green (Cr³⁺ formed).

1. Formation of Aldehyde (Distillation):

Distillation is needed to obtain the aldehyde because the aldehyde has a low boiling point and will easily evaporate once formed, leaving the reaction mixture as a vapour.

Example Ethanol → Ethanal

CH₃CH₂OH + [O] → CH₃CHO + H₂O

2. Formation of Carboxylic Acid (Reflux):

Reflux is needed to obtain the carboxylic acid because the aldehyde must be continually condensed and forced to re-enter the reaction mixture, enabling it to be further oxidised.

Example Ethanal → Ethanoic Acid

CH₃CHO + [O] → CH₃COOH

For secondary alcohols:

Secondary alcohols oxidise to ketones when heated with acidified potassium dichromate(VI)

Observation: Orange Cr₂O₇²⁻ (dichromate) turns green (Cr³⁺ formed).

Example Propan-2-ol → Propanone

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

Elimination Reactions of Alcohols

Alcohols can undergo elimination (dehydration) to form alkenes:

Conditions:

• Catalyst: Concentrated H₂SO₄ or H₃PO₄
• Heated under reflux (~170 °C)

General reaction: C_nH_2n+1OH → C_nH_2n + H₂O

Example Ethanol → Ethene

CH₃CH₂OH → CH₂=CH₂ + H₂O

This is useful for making alkenes for addition polymerisation.

Techniques for Preparation and Purification

When preparing a liquid organic product like an alcohol or an alkene, the following techniques can be used:

• Reflux – continuous heating and condensation to ensure complete reaction
• Separation – use of a separating funnel to isolate organic and aqueous layers
• Distillation – purify product by boiling point
• Drying – using anhydrous salts (e.g. MgSO₄) to remove water
• Boiling point determination – compare measured boiling point with data book values to confirm identity

Summary

• Alcohols contain an OH group and are classified as primary, secondary or tertiary.
• Alcohols undergo combustion, substitution with halogenating agents, oxidation and dehydration.
• Primary oxidise to aldehydes then acids and secondary to ketones. Tertiary do not oxidise.
• Dehydration forms an alkene and water.