Reactions of Aldehydes and Ketones: Quick Revision

A brief note on Reactions of Aldehydes and Ketones. Read it before a test. Try each question at the end before you open its answer.

Nucleophilic addition

• The nucleophile attacks the carbonyl carbon, which changes from $sp^2$ to $sp^3$. The alkoxide then takes a proton.
• Reactivity: methanal, then other aldehydes, then ketones. Aromatic aldehydes are less reactive than aliphatic ones.
• HCN (trace of base): cyanohydrin, one more carbon. NaHSO$_3$: bisulphite compound, for aldehydes and methyl ketones. ROH and dry HCl: acetal. Z-NH$_2$: C=N-Z; 2,4-DNP gives an orange or red solid.
• Grignard reagent: methanal gives a primary alcohol, other aldehydes secondary, ketones tertiary.

Reduction

• NaBH$_4$, LiAlH$_4$, H$_2$ and a metal: alcohol. Clemmensen (Zn-Hg, conc. HCl) and Wolff-Kishner (NH$_2$NH$_2$, then KOH in glycol): CH$_2$.

Oxidation and tests

• Aldehydes oxidise easily. Ketones need strong oxidants and the chain breaks.
• Tollens: silver mirror, any aldehyde. Fehling: red Cu$_2$O, aliphatic aldehydes only. Iodoform: yellow CHI$_3$, with CH$_3$CO- or CH$_3$CH(OH)-.

Alpha hydrogens

• Aldol: dilute NaOH, an alpha hydrogen, then heat for the unsaturated carbonyl compound. Crossed aldol: four products unless one partner has no alpha hydrogen.
• Cannizzaro: concentrated NaOH, no alpha hydrogen. Half alcohol, half acid salt.
• The carbonyl group is deactivating and meta-directing in the benzene ring.

A worked example

$1.10\ \text{g}$ of ethanal ($44.05\ \text{g/mol}$) is warmed with excess Tollens' reagent. What mass of silver ($107.87\ \text{g/mol}$) forms?

1. Moles. $\dfrac{1.10}{44.05} = 25.0\ \text{mmol}$.
2. Silver. Two atoms for each molecule: $50.0\ \text{mmol}$.
3. Mass. $0.0500\ \text{mol} \times 107.87\ \text{g/mol} = 5.39\ \text{g}$.

Practice with new numbers

A program to try

The program lists which of the four tests each compound passes.

Mistakes to avoid

• Fehling's test for benzaldehyde. It is not given.
• Iodoform for every aldehyde. Only ethanal.
• Aldol with no alpha hydrogen. Use Cannizzaro in concentrated base.
• Clemmensen on an acid-sensitive compound. Use Wolff-Kishner.

Check yourself

1. What does propanone give with NaBH$_4$ and then dilute acid?

2. Which test is given by ethanal but not by propanal?

3. What does benzaldehyde give with concentrated NaOH?

4. Why is 2,4-DNP used as a test for the whole family?

Sources

• OpenStax, Chemistry 2e, Chapter 20, section 20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters.
• NCERT, Chemistry Part II for Class XII, Chapter 8.