2022 H2 Chem P1 Q17 — Introduction to Organic Solution

The answer is A (2 and 3).

A carbocation forms when a bond to carbon breaks heterolytically with both bonding electrons leaving on the other atom, and nothing forms a new bond to that carbon.
• Step 1 does not form a carbocation: the two half-headed arrows show two radicals, \((\text{CH}_3)_3\text{C}\cdot\) and \(\text{Cl}\cdot\), combining to form a C-Cl bond, giving the neutral \((\text{CH}_3)_3\text{CCl}\).
• Step 2 forms a carbocation: the C-Cl bond breaks heterolytically with both electrons going to Cl, giving \((\text{CH}_3)_3\text{C}^+\) and \(\text{Cl}^-\).
• Step 3 forms a carbocation: the C-O bond breaks heterolytically with both electrons going to O, giving \((\text{CH}_3)_3\text{C}^+\) and \(\text{H}_2\text{O}\).
• Step 4 does not form a carbocation: as the C-Cl bond breaks, the lone pair on \(\text{O}^-\) forms a C=O bond to the same carbon, giving the neutral ketone \((\text{CH}_3)_2\text{C=O}\) and \(\text{Cl}^-\).

• B is wrong: step 4 gives a ketone, not a carbocation, and step 3 is left out.
• C is wrong: step 1 is radical combination, which forms a bond and no ion.
• D is wrong: step 4 gives a ketone, and steps 2 and 3, which do form carbocations, are left out.