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Preparation Methods for Aldehydes and Ketones

Aldehydes and ketones can be prepared by controlled oxidation of alcohols, ozonolysis of alkenes, hydration of alkynes, and selective reduction methods such as Rosenmund and Stephen reactions.

In one line: Preparation is reagent matching: alcohols oxidise, acid chlorides gently reduce, alkenes split.

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Understand it

  • Preparation questions depend on matching the starting compound, reagent, and target carbonyl compound.
  • Primary alcohols give aldehydes under controlled oxidation, while secondary alcohols give ketones.
  • Ozonolysis cleaves a C=C bond to give carbonyl compounds.
  • Acid chlorides can be converted to aldehydes by Rosenmund reduction, and nitriles can be reduced to aldehydes under Stephen reaction conditions.

Example

CH3CH2OH on controlled oxidation gives CH3CHO. CH3COCl on Rosenmund reduction gives CH3CHO.

Rule to remember

Primary alcohol + controlled oxidation → aldehyde; secondary alcohol + oxidation → ketone; acid chloride + H2/Pd-BaSO4 → aldehyde; nitrile + SnCl2/HCl followed by hydrolysis → aldehyde; alkene + ozonolysis → carbonyl fragments.

See it worked out

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

Worked example

To prepare benzaldehyde from benzoyl chloride, use Rosenmund reduction: C6H5COCl treated with H2 in the presence of poisoned Pd catalyst gives C6H5CHO.

Method you can reuse

Identify the functional group in the starting compound, compare carbon count with the target, check whether oxidation, reduction, hydration, or cleavage is needed, then choose the reagent with the correct selectivity.

Visual tip

A reaction-route flowchart is useful, but a structural diagram is not essential because the key is reagent selection and product mapping.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Which carbonyl compound is formed by oxidation of propan-2-ol?

Think first, then show answer
Propanone is formed because propan-2-ol is a secondary alcohol.

Common wrong answer

Choosing LiAlH4 for acid chloride to aldehyde conversion; it reduces too far to alcohol instead of stopping at aldehyde under ordinary use.

Don't mix this up

Using strong oxidising conditions for preparing aldehydes from primary alcohols may over-oxidise the aldehyde to a carboxylic acid.

Exam tip

For conversions, decide whether the carbon skeleton must be preserved, cleaved, or reduced. This usually identifies the correct preparation route.

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

Asked as named reaction identification, conversion sequence, reagent completion, or product prediction from alcohols, alkenes, alkynes, acid chlorides, and nitriles.
  1. 1

    Enough for 1 mark

    Aldehydes and ketones can be prepared by controlled oxidation of alcohols, ozonolysis of alkenes, hydration of alkynes, and selective reduction methods such as Rosenmund and Stephen reactions.

  2. 2

    Add for 2 marks

    Example: CH3CH2OH on controlled oxidation gives CH3CHO. CH3COCl on Rosenmund reduction gives CH3CHO.

  3. 3

    Add for 3 marks

    Primary alcohols give aldehydes under controlled oxidation, while secondary alcohols give ketones.

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