Aldehydes, Ketones and Carboxylic Acids · Study notes

CHENomenclature & Preparation

Nomenclature & Classification

This subtopic tests IUPAC naming of aldehydes and ketones, classifying a carbonyl as aldehyde vs ketone, and drawing structures from names. The carbonyl's position and the longest chain containing it set the suffix and the locant.

Aldehydes take the $\text{-}al$ suffix, ketones the $\text{-}one$ suffix; the $\text{-}CHO$ carbon is always C1 in an aldehyde, so it needs no locant.
Worked names: $CH_3CH_2CHO$ = propanal; $CH_3COCH_2CH_3$ = butan-2-one; $C_6H_5COCH_3$ = 1-phenylethan-1-one (acetophenone); $HCHO$ = methanal.
Branched examples: $(CH_3)_2CHCHO$ = 2-methylpropanal (an aldehyde); $CH_3COC(CH_3)_3$ = 3,3-dimethylbutan-2-one (a ketone).
Classification: aldehydes have $\text{-}CHO$ at a chain end (carbonyl bonded to at least one H); ketones have the carbonyl between two carbons ( $\text{C=O}$ with no attached H).
Structures from names: 3-methylbutanal $= (CH_3)_2CHCH_2CHO$ ; pentan-3-one $= CH_3CH_2COCH_2CH_3$ ; phenylacetaldehyde $= C_6H_5CH_2CHO$ .
Number the chain to give the carbonyl carbon the lowest possible locant; the longest chain chosen must include the $\text{C=O}$ carbon.
The carbonyl is the principal characteristic group, so it gets priority in numbering over alkyl branches, halogens and most other substituents - they become locant-prefixed substituents.
Carboxylic acids take the $\text{-}oic\ acid$ suffix with the $\text{-COOH}$ carbon as C1 (ethanoic acid $= CH_3COOH$ , benzoic acid $= C_6H_5COOH$ ); when $\text{-COOH}$ is attached directly to a ring you use $\text{-}carboxylic\ acid$ .
Common (trivial) names worth recognising alongside IUPAC: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), benzaldehyde, acetic acid (ethanoic acid), formic acid (methanoic acid).
Cyclic ketones are named with $\text{cyclo-}$ and $\text{-}one$ (cyclohexanone); the carbonyl carbon is C1 and the ring is numbered to give substituents the lowest locants.
An aldehyde always outranks a ketone in priority, so in a molecule with both the $\text{-}CHO$ is the suffix (named $\text{-}al$ ) and the $\text{C=O}$ of the ketone becomes an $\text{oxo-}$ prefix.
Dialdehydes/diketones take $\text{-}dial$ / $\text{-}dione$ with both locants (e.g. pentane-2,4-dione), keeping the chain that contains both carbonyls.

Where the marks go

Giving the aldehyde carbon a locant - the $\text{-}CHO$ carbon is always C1 in an aldehyde and is written simply as "-al" with no number.
Choosing a longest chain that excludes the carbonyl carbon - the parent chain MUST contain the $\text{C=O}$ carbon even if a longer all-carbon chain exists.
Misclassifying $C_6H_5CHO$ (benzaldehyde, an aldehyde) as a ketone because it is aromatic - the $\text{C=O}$ still bears an H, so it is an aldehyde.
Numbering a ketone from the wrong end so the $\text{C=O}$ gets a higher locant (e.g. naming pentan-2-one as pentan-4-one).
Forgetting to alphabetise and lowest-set the substituent locants on branched carbonyls (e.g. 3,3-dimethylbutan-2-one, not 1,1-dimethyl... numbering).

How the board asks it

Structure / namingthe -al/-one suffix rules and lowest-locant numbering
Give the IUPAC name of $CH_3COC(CH_3)_3$ and of $(CH_3)_2CHCHO$ .
Structure / namingderiving structures from IUPAC names
Draw the structural formula of $3$ -methylbutanal and of pentane- $2,4$ -dione.
Distinguishaldehyde vs ketone classification by the carbonyl environment
Classify each of $C_6H_5CHO$ , $CH_3COCH_2CH_3$ and $HCHO$ as an aldehyde or a ketone, justifying your choice in each case.
Give reasonsprincipal-group priority of aldehyde over ketone
Account for the fact that $CH_3COCH_2CHO$ is named $3$ -oxobutanal rather than as a ketone.
Structure / namingcommon names mapped to systematic names
Write the IUPAC names corresponding to the common names acetone, formaldehyde and acetic acid.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.