2018 H2 Chem P1 Q21 — Introduction to Organic Solution

The answer is C.
A carbocation is a positive ion in which the positive charge sits on a carbon atom.
• \([\text{CH}_3\text{CO}]^+\) (the acylium ion, \(\text{CH}_3\text{-}\overset{+}{\text{C}}\text{=O}\)): the positive charge is on the carbonyl carbon, so it is a carbocation.
• \([(\text{CH}_3)_3\text{C}]^+\): the positive charge is on the central carbon bonded to three methyl groups (a tertiary carbocation).
• \([\text{CH}_3\text{OH}_2]^+\): oxygen has formed a third bond (to an extra \(\text{H}^+\)), so the positive charge is on O, not C.
• \([(\text{CH}_3)_4\text{N}]^+\): nitrogen has formed four bonds, so the positive charge is on N (a quaternary ammonium ion), not C.
• A is wrong: \([\text{CH}_3\text{OH}_2]^+\) has its positive charge on oxygen.
• B is wrong: \([(\text{CH}_3)_4\text{N}]^+\) has its positive charge on nitrogen.
• D is wrong: neither species is a carbocation; the charges are on N and O.