Carbonyl reactions and optical isomerism

What you’ll learn

• Distinguish aldehydes and ketones using reaction evidence.
• Describe nucleophilic addition at a carbonyl group.
• Explain chirality, enantiomers and racemic mixtures.

Understand the idea

Aldehydes contain a terminal carbonyl group and ketones have two carbon groups attached to the carbonyl carbon. Both can accept hydrogen bonds from water but cannot hydrogen-bond to each other through a carbonyl group alone. An aldehyde or ketone gives a precipitate with 2,4-DNPH; its derivative melting point can support identification. Aldehydes reduce Tollens' reagent, whereas ordinary ketones do not. Aliphatic aldehydes also give the characteristic red copper(I) oxide precipitate with Fehling's or Benedict's reagent.

The carbonyl bond is polarised towards oxygen. In nucleophilic addition, a curly arrow runs from the nucleophile's lone pair to the carbonyl carbon, and another from the carbon–oxygen pi bond to oxygen. Protonation then gives an addition product. Reduction converts an aldehyde to a primary alcohol and a ketone to a secondary alcohol. A methyl ketone gives the yellow iodoform-test precipitate; ethanal and compounds oxidisable to the relevant carbonyl structure can also respond.

A tetrahedral carbon bonded to four different groups is a chiral centre. Its two mirror-image arrangements are enantiomers and rotate plane-polarised light equally in opposite directions under identical conditions. An equal mixture is racemic and shows no net rotation. Attack on either face of a planar carbonyl can create a racemate when a new chiral centre forms in an achiral environment; SN1 pathways similarly allow attack on a planar intermediate, whereas SN2 gives inversion at the attacked centre.

Key formulae

$$\mathrm{RCHO+[O]\longrightarrow RCOOH}$$

$$\mathrm{RCHO+2[H]\longrightarrow RCH_2OH}$$

$$\mathrm{RCOR'+2[H]\longrightarrow RCH(OH)R'}$$

$$\mathrm{RCHO+HCN\longrightarrow RCH(OH)CN}\quad\text{(conceptual addition equation)}$$

Worked example

A compound of formula C₃H₆O gives a 2,4-DNPH precipitate and a positive Tollens’ test. Identify it and the organic product of its oxidation.

1. The 2,4-DNPH result supports an aldehyde or ketone carbonyl.
2. The positive Tollens’ result identifies the aldehyde rather than propanone.
3. The three-carbon aldehyde is propanal; oxidation changes its terminal aldehyde group into a carboxylic acid.

Answer: Propanal; oxidation gives propanoic acid (or propanoate in alkaline solution).