CAIE A-Level Chemistry 15.1.5 SN1 and SN2 Substitution Mechanisms

Practise drawing SN1 and SN2 halogenoalkane mechanisms with dipoles, intermediates, transition states and curly arrows.

Exam points

for SN1, show C–X heterolysis first, then nucleophile attack on the planar carbocation intermediate
for SN2, show one concerted step with nucleophile attack as the C–X bond breaks
start arrows at a lone pair or bond, include charges and explain carbocation stabilisation by alkyl groups

[Maximum number: 3]

Species such as $\mathrm{NH}_{4}{ }^{+}, \mathrm{CO}_{3}{ }^{2-}$ and $\mathrm{PO}_{4}{ }^{3-}$ are examples of molecular ions.

$\mathrm{OH}^{-}(\mathrm{aq})$ reacts with 2-bromo-2-methylpropane in an $\mathrm{S}_{\mathrm{N}} 1$ reaction. The molecular ion $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}^{+}$ forms as the intermediate in this reaction.

Draw the mechanism for the $\mathrm{S}_{\mathrm{N}} 1$ reaction of $\mathrm{OH}^{-}$ with 2-bromo-2-methylpropane. Include charges, dipoles, lone pairs of electrons and curly arrows as appropriate. Draw the structures of the organic reactant and organic product.

Figure for Question 15.1.5—SN1/SN2 substitution mechanisms question 1 — CAIE A-Level Chemistry AS

Correct SN1 mechanism is shown in the attached official markscheme figure.

Text form of product:
$(\mathrm{CH}_{3})_{3}\mathrm{COH}$

Marking points:
M1 correct dipole on $\delta+\mathrm{C}-\mathrm{Br}^{\delta-}$ in tert-butyl bromide and curly arrow from C-Br to Br.
M2 curly arrow from electron pair on O of $-:\mathrm{OH}$ / $:\mathrm{OH}^{-}$ to $\mathrm{C}^{+}$ in carbocation.
M3 tert-butyl alcohol product.

Figure for Answer to Question 15.1.5—SN1/SN2 substitution mechanisms question 1 — CAIE A-Level Chemistry AS

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15.1.5—SN1/SN2 substitution mechanisms question 1