Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 11.9 SN1 and SN2 from rate equations-Study Notes - New Syllabus

11.9 Hydrolysis Mechanisms of Halogenoalkanes

The rate equations for the hydrolysis of halogenoalkanes provide evidence for whether the reaction follows an \( \mathrm{S_N1} \) or \( \mathrm{S_N2} \) mechanism, depending on the structure of the halogenoalkane.

Key Idea

• Reaction mechanism is deduced from rate equation
• Order of reaction indicates number of species in rate-determining step

(i) SN1 Mechanism (Tertiary Halogenoalkanes)

In an \( \mathrm{S_N1} \) reaction, the rate depends only on the concentration of the halogenoalkane.

\( \mathrm{rate = k[RX]} \)

• First-order reaction
• Rate-determining step involves only halogenoalkane
• Forms a carbocation intermediate

Mechanism:

Step 1 (slow): \( \mathrm{RX \rightarrow R^+ + X^-} \)

Step 2 (fast): \( \mathrm{R^+ + OH^- \rightarrow ROH} \)

Evidence:

• • Rate independent of \( \mathrm{[OH^-]} \)
  • Indicates nucleophile not in slow step

(ii) SN2 Mechanism (Primary Halogenoalkanes)

In an \( \mathrm{S_N2} \) reaction, the rate depends on both the halogenoalkane and the nucleophile.

\( \mathrm{rate = k[RX][OH^-]} \)

• Second-order reaction
• Rate-determining step involves both species
• No intermediate formed (single step)

Mechanism:

Single step: \( \mathrm{RX + OH^- \rightarrow ROH + X^-} \)

Evidence:

• • Rate depends on both concentrations
  • Indicates both species in slow step

Comparison Summary

• SN1: first order, carbocation intermediate, tertiary favoured
• SN2: second order, single step, primary favoured