Arrange the following electrophiles in order of increasing reactivity for an Sn1 reaction...?

Mechanism for SN1 reaction :
Step 1 : reactant → carbonium ion + Br⁻
Step 2 : carbonium ion + nucleophile → product

The more stable the carbonium ion (carbocation ion) intermediate, the faster the reaction is.

The allylbromide (the third compound) is the most reactive. This is because the carbonium cation formed is stabilized to a great extent by resonance effect.

The second compound is the least reactive. This is because the oxygen atom in C=O is electron withdrawing by both resonance effect and inductive effect, and thus the carbonium ions is much destabilized as the positive charge is concentrated.

The first compound is a 1° alkyl bromide, while the last compound is a 3° alkyl bromide. As alkyl groups are electron donating (electron releasing), the carbonium ion is stabilized by inductive effect (to spread the positive charge). Hence, the reactivity of the 3° alkyl bromide is higher than the 1° alkyl bromide.