Explain why aldehydes and ketones react with a weak acid such as hydrogen cyanide but do not react with strong acids such as hcl or h2so4 (other than being protonated by them).

[tex]\underbrace{\hbox{HCl }}_{\hbox{strong acid}} + {\text{ H$_{2}$O }} \longrightarrow \text{H}_{3}\text{O}^{+}+ \underbrace{\hbox{Cl^{-}}}_{\hbox{weak nucleophile}}[/tex]

Because they are strong acids, their conjugate bases are extremely weak bases/nucleophiles.

Thus, they can protonate the carbonyl oxygen, but the conjugate bases cannot act as nucleophiles.

Weak acids

Weak acids like HCN dissociate only slightly in solution.

[tex]\underbrace{\hbox{HCN}}_{\hbox{weak acid}} + {\text{ H$_{2}$O }} \rightleftharpoons \text{H}_{3}\text{O}^{+}+ \underbrace{\hbox{:CN^{-}}}_{\hbox{strong nucleophile}}[/tex]

Because HCN is a weak acid, its conjugate base is a strong nucleophile.

Thus, it generates relatively few hydronium ions, but the cyanide ion is a strong nucleophile that can attack the partially positive carbon and form the cyanohydrin.

RCH=O + CN⁻ ⟶ RCH(CN)O⁻ ⟶ RCH(CN)OH

As the CN⁻ ions react with the aldehyde or ketone, they are removed from solution.

According to Le Châtelier's Principle, more HCN dissociates to replace the CN⁻ ions, and the reaction goes nearly to completion.