Concept Explanation: SN2 Reaction Rates with Alkyl Halides

In SN2 reactions, the rate depends on steric hindrance and the leaving group ability. For alkyl halides reacting with KI in acetone (a polar aprotic solvent that favors SN2):

• Steric hindrance is the primary factor: less steric crowding around the carbon leads to faster SN2 rates.
• Leaving group ability: I⁻ is a good leaving group, but since all substrates are alkyl halides, sterics dominate here.
• The order of reactivity for alkyl halides in SN2 is: methyl > primary > secondary > tertiary (tertiary hardly reacts via SN2).

Step-by-Step Analysis of Each Compound:

Step 1: Identify the carbon center and its substitution

P: CH₃-CH₂-Br (primary alkyl bromide)

Q: (CH₃)₂CH-Br (secondary alkyl bromide)

R: CH₂=CH-CH₂-Br (allylic bromide, primary but with resonance stabilization)

S: Ph-CH₂-Br (benzylic bromide, primary with resonance stabilization)

Step 2: Compare steric hindrance

Primary halides (P, R, S) have less steric hindrance than secondary (Q). So Q is slowest.

Step 3: Consider resonance effects

Allylic (R) and benzylic (S) halides have resonance stabilization of the transition state, making them react faster than simple primary halides (P). Between R and S, benzylic (S) has better resonance stabilization than allylic (R) due to the aromatic ring.

Step 4: Order the rates

Benzylic (S) > Allylic (R) > Simple primary (P) > Secondary (Q)

So the order is: S > R > P > Q

Final Answer: The correct option is S > R > P > Q

Related Topics:

• SN1 vs SN2 mechanisms: SN2 is bimolecular and favored by less steric hindrance, while SN1 is unimolecular and favored by carbocation stability.
• Effect of solvent: Polar aprotic solvents (like acetone) favor SN2 by solvating cations but not anions, making nucleophiles more reactive.
• Leaving group ability: I⁻ > Br⁻ > Cl⁻ in SN2 reactions.
• Resonance effects: Allylic and benzylic positions stabilize transition states and intermediates.

Important Formulae and Theory:

The rate law for SN2 reaction is: $\mathrm{Rate}=k\left[\mathrm{alkyl\; halide}\right]\left[\mathrm{nucleophile}\right]$

General reactivity order for SN2: CH₃X > RCH₂X (primary) > R₂CHX (secondary) > R₃CX (tertiary, very slow)

Special cases: Benzylic and allylic halides undergo SN2 faster than simple alkyl halides due to resonance stabilization of the transition state.

Carbocation stability order (for SN1): tertiary > secondary > primary > methyl, and benzylic ≈ allylic > tertiary.