Nomenclature & Classification

CHENomenclature & Preparation

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

NextMethods of Preparation

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.