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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

    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.

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

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

Memory hook

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

Examples and method

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 to apply

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.

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

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.

Exam tip

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

Quick check

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

Propanone is formed because propan-2-ol is a secondary alcohol.

Answer writing and exam use

1-mark answer

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-mark answer

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

3-mark answer

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. 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. To prepare benzaldehyde from benzoyl chloride, use Rosenmund reduction: C6H5COCl treated with H2 in the presence of poisoned Pd catalyst gives C6H5CHO. Asked as named reaction identification, conversion sequence, reagent completion, or product prediction from alcohols, alkenes, alkynes, acid chlorides, and nitriles. Choosing LiAlH4 for acid chloride to aldehyde conversion; it reduces too far to alcohol instead of stopping at aldehyde under ordinary use.
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