Aldehydes and Ketones

1. Introduction

Aldehydes and ketones both contain the carbonyl group $\ce{>C=O}$ and are together called carbonyl compounds.

• In an aldehyde the carbonyl carbon carries at least one hydrogen: general formula $\ce{R-CHO}$ (the $\ce{-CHO}$ group is called the formyl or aldehyde group). The simplest is methanal $\ce{HCHO}$.
• In a ketone the carbonyl carbon carries two carbon groups: general formula $\ce{R-CO-R'}$. The simplest is propanone $\ce{CH3COCH3}$.

Both have the molecular formula $\ce{C_nH_{2n}O}$ and are therefore functional isomers of each other.

The carbonyl group. The carbon is $sp^2$ hybridized and planar (bond angle about $120^\circ$). Because oxygen is more electronegative, the $\ce{C=O}$ bond is strongly polar, $\ce{C^{\delta+}=O^{\delta-}}$. The electron-poor carbon is attacked by nucleophiles, which is why nucleophilic addition is the characteristic reaction.

2. Nomenclature, Isomerism and Classification

2.1 Nomenclature

IUPAC: aldehydes replace the -e of the alkane by -al; ketones replace -e by -one with a locant for the carbonyl carbon.

2.2 Classification and Isomerism

Aldehydes/ketones are aliphatic ($\ce{CH3CHO}$) or aromatic ($\ce{C6H5CHO}$, benzaldehyde). They show:

• Chain and position isomerism within each class.
• Functional isomerism: an aldehyde and a ketone of the same formula (propanal $\ce{CH3CH2CHO}$ vs propanone $\ce{CH3COCH3}$); they are also functionally isomeric with certain unsaturated alcohols/ethers.
• Metamerism in ketones (butan-2-one has no metamer, but pentan-2-one and pentan-3-one/diethyl ketone illustrate different alkyl groups on the carbonyl).