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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.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    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

    Then show how to use it

    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.

  3. 3

    Add one concrete example

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

  4. 4

    Avoid this incomplete answer

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

Definition

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.

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.

Memory hook

Aldehyde is at the edge; ketone is kept within the chain.

Examples and method

Worked example

For CH3COCH2CH3, choose the four-carbon chain containing C=O. Number from the end nearer to C=O, so the carbonyl is at carbon 2. The name is butan-2-one.

Method to apply

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.

Diagram support

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.

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

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

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.

Quick check

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

CH3CH2CHO is propanal. It is an aldehyde because the -CHO group is at the end of the chain.

Answer writing and exam use

1-mark answer

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-mark answer

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

3-mark answer

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. 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. For CH3COCH2CH3, choose the four-carbon chain containing C=O. Number from the end nearer to C=O, so the carbonyl is at carbon 2. The name is butan-2-one. Usually appears as structure identification, IUPAC naming, common name matching, or assertion-reason based on polarity of the carbonyl group. Writing butan-3-one instead of butan-2-one, because the chain was numbered from the wrong end.
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