Carbonyl & Carboxylic Acid Compounds & Esters

Aldehydes and ketones (carbonyl compounds)

Both contain the carbonyl group C=O. Aldehydes have it at the end ($\text{RCHO}$); ketones have it within the chain ($\text{RCOR'}$). The C=O is polar, so they undergo nucleophilic addition.

Reduction by $\text{NaBH}_4$: aldehyde → primary alcohol; ketone → secondary alcohol.
Nucleophilic addition of HCN (with $\text{KCN}$/trace acid) → a hydroxynitrile (adds a C; the CN⁻ lone pair attacks the $\text{C}^{\delta+}$).