Alcohols

Alcohols contain the hydroxyl (–OH) functional group and have the general formula CₙH₂ₙ₊₁OH. This lesson covers the classification of alcohols (primary, secondary, tertiary), their oxidation reactions with acidified potassium dichromate, the importance of distillation vs reflux, dehydration to alkenes, and esterification. Understanding alcohol chemistry is crucial for organic synthesis pathways.

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Classification of Alcohols

Alcohols are classified by the number of carbon atoms bonded to the carbon bearing the –OH group.

Classification	Carbon bearing –OH is bonded to...	Example	Oxidation product
Primary (1°)	0 or 1 other carbon	CH₃CH₂OH (ethanol)	Aldehyde → carboxylic acid
Secondary (2°)	2 other carbons	CH₃CH(OH)CH₃ (propan-2-ol)	Ketone
Tertiary (3°)	3 other carbons	(CH₃)₃COH (2-methylpropan-2-ol)	Resistant to oxidation

Key Definition: The carbon bearing the –OH group determines the classification, not the total number of carbons in the molecule. Count how many other C atoms are directly bonded to that specific carbon.

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Physical Properties of Alcohols

• Alcohols have higher boiling points than alkanes of similar molecular mass because of hydrogen bonding between the O–H groups.
• Short-chain alcohols (methanol, ethanol, propan-1-ol) are miscible with water because they can form hydrogen bonds with water molecules.
• As chain length increases, the non-polar hydrocarbon chain dominates and solubility in water decreases.
• Alcohols are good solvents for both polar and non-polar substances (the –OH is polar; the carbon chain is non-polar).

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Oxidation of Alcohols

The oxidising agent used at A-Level is acidified potassium dichromate — K₂Cr₂O₇ with dilute sulfuric acid (H₂SO₄). In equations, the oxidising agent is represented as [O] for simplicity.

The dichromate ion (Cr₂O₇²⁻) is orange and is reduced to Cr³⁺ ions, which are green. This colour change (orange → green) is the visual indicator that oxidation has occurred.

Primary Alcohol Oxidation

A primary alcohol can be oxidised to an aldehyde (partial oxidation) or further to a carboxylic acid (full oxidation). The choice depends on the experimental technique.

To an aldehyde — use distillation:

CH₃CH₂OH + [O] → CH₃CHO + H₂O

(ethanol → ethanal)

Method: Heat the primary alcohol with acidified K₂Cr₂O₇ and distil immediately. The aldehyde (lower boiling point) distils off as it forms, preventing further oxidation.

To a carboxylic acid — use reflux:

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

(ethanol → ethanoic acid)

Method: Heat the primary alcohol with excess acidified K₂Cr₂O₇ under reflux. Reflux keeps the reaction mixture at its boiling point while returning vapours to the flask, ensuring complete oxidation.

Secondary Alcohol Oxidation

A secondary alcohol is oxidised to a ketone only. Ketones resist further oxidation because there is no H atom on the carbonyl carbon to remove.

CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O

(propan-2-ol → propanone)

Method: Heat with acidified K₂Cr₂O₇ under reflux (or distillation — the ketone cannot be oxidised further in either case).

Tertiary Alcohol Oxidation

Tertiary alcohols are not oxidised by acidified dichromate. There is no hydrogen atom on the carbon bonded to –OH, so the C–H bond that must break during oxidation does not exist.

The dichromate solution stays orange — no colour change occurs.

Alcohol type	Product with mild oxidation	Product with strong oxidation	Dichromate colour change
Primary	Aldehyde (distillation)	Carboxylic acid (reflux)	Orange → green
Secondary	Ketone	Ketone (no further oxidation)	Orange → green
Tertiary	No reaction	No reaction	Stays orange

Exam Tip: Distillation vs reflux is a favourite exam question. Distillation collects the product as it forms (removes aldehyde before further oxidation). Reflux keeps everything in the flask (ensures complete oxidation to carboxylic acid). Always state the technique and explain why it gives the desired product.