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Carbonyl Group and Naming of Aldehydes and Ketones

The carbonyl group is a carbon-oxygen double bond, C=O. Aldehydes contain the group -CHO at the end of a carbon chain, while ketones contain the group >C=O within the chain.

In one line: Aldehyde is at the edge; ketone is kept within the chain.

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Understand it

  • The C=O bond is polar because oxygen is more electronegative than carbon.
  • This makes the carbonyl carbon electron-deficient and reactive towards nucleophiles.
  • In naming, aldehydes use the suffix -al and the carbonyl carbon is always carbon 1.
  • Ketones use the suffix -one and need the lowest possible position number for the carbonyl carbon.

Example

CH3CHO is ethanal, while CH3COCH3 is propanone. In butan-2-one, the carbonyl group is on carbon 2, so it is a ketone.

Rule to remember

Aldehyde pattern: R-CHO, suffix -al.

Ketone pattern: R-CO-R', suffix -one.

Number the longest chain containing C=O and give the carbonyl carbon the lowest valid locant.

See it worked out

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

Worked example

For CH3COCH2CH3, choose the four-carbon chain containing C=O.

  1. 1Number from the end nearer to C=O, so the carbonyl is at carbon 2.
  2. 2The name is butan-2-one.

Method you can reuse

Locate the C=O group, check whether the carbonyl carbon is terminal or internal, choose the longest chain containing it, number to give the carbonyl lowest priority number, then apply -al or -one.

Visual tip

A simple comparison diagram is useful: R-CHO labelled as aldehyde and R-CO-R' labelled as ketone, with the carbonyl carbon and oxygen polarity marked.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Name CH3CH2CHO using IUPAC nomenclature and state whether it is an aldehyde or ketone.

Think first, then show answer
CH3CH2CHO is propanal. It is an aldehyde because the -CHO group is at the end of the chain.

Common wrong answer

Writing butan-3-one instead of butan-2-one, because the chain was numbered from the wrong end.

Don't mix this up

Students often write ethanone for CH3CHO or forget that an aldehyde carbonyl carbon must be counted as carbon 1.

Exam tip

First identify whether the C=O carbon is terminal or internal. Terminal C=O with H gives aldehyde; internal C=O between two carbon groups gives ketone.

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

Usually appears as structure identification, IUPAC naming, common name matching, or assertion-reason based on polarity of the carbonyl group.
  1. 1

    Enough for 1 mark

    The carbonyl group is a carbon-oxygen double bond, C=O. Aldehydes contain the group -CHO at the end of a carbon chain, while ketones contain the group >C=O within the chain.

  2. 2

    Add for 2 marks

    Example: CH3CHO is ethanal, while CH3COCH3 is propanone. In butan-2-one, the carbonyl group is on carbon 2, so it is a ketone.

  3. 3

    Add for 3 marks

    The C=O bond is polar because oxygen is more electronegative than carbon.

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