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CraftExam
medium difficulty5 min

Preparation of Alcohols and Phenols

Alcohols can be prepared by hydration of alkenes and reduction of aldehydes or ketones, while phenol is industrially prepared by the cumene process.

In one line: Alkene adds water; carbonyl gains hydrogen; cumene gives phenol.

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Understand it

In acid-catalysed hydration, water adds across a carbon-carbon double bond to form an alcohol, usually following Markovnikov orientation for unsymmetrical alkenes. Aldehydes reduce to primary alcohols and ketones reduce to secondary alcohols using reducing agents such as sodium borohydride or lithium aluminium hydride. In the cumene process, benzene is converted into cumene, which forms cumene hydroperoxide on oxidation and gives phenol along with acetone on acidic cleavage.

Example

Ethene gives ethanol on acid-catalysed hydration. Propanone gives propan-2-ol on reduction.

Rule to remember

Reaction patterns: alkene + H2O/H+ gives alcohol; RCHO + reducing agent gives RCH2OH; R2CO + reducing agent gives R2CHOH; cumene oxidation followed by acid cleavage gives phenol and acetone.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1Prepare propan-2-ol from propene.
  2. 2Use acid-catalysed hydration; H and OH add across the double bond, and OH attaches to the more substituted carbon, giving CH3CHOHCH3.

Method you can reuse

Look for the functional group in the reactant, choose addition or reduction, apply orientation if an unsymmetrical alkene is present, then check whether the product class matches the starting material.

Visual tip

A reaction-flow diagram for the cumene process can help connect cumene, cumene hydroperoxide, phenol and acetone.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What alcohol is obtained by reduction of ethanal?

Think first, then show answer
Ethanol, because aldehydes reduce to primary alcohols.

Common wrong answer

Writing propan-1-ol from normal acid hydration of propene is usually wrong because Markovnikov addition gives propan-2-ol.

Don't mix this up

A frequent error is writing a tertiary alcohol as the product of ketone reduction; simple ketones reduce to secondary alcohols.

Exam tip

In preparation questions, identify the starting functional group first: alkene suggests addition of water, aldehyde or ketone suggests reduction, and cumene points to phenol.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

It is commonly asked as product prediction, conversion, reagent selection, or a short note on the cumene process.
  1. 1

    Enough for 1 mark

    Alcohols can be prepared by hydration of alkenes and reduction of aldehydes or ketones, while phenol is industrially prepared by the cumene process.

  2. 2

    Add for 2 marks

    Example: Ethene gives ethanol on acid-catalysed hydration. Propanone gives propan-2-ol on reduction.

  3. 3

    Add for 3 marks

    In acid-catalysed hydration, water adds across a carbon-carbon double bond to form an alcohol, usually following Markovnikov orientation for unsymmetrical alkenes.

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