CAIE A-Level Chemistry AS 15.1.5 Sn1 Sn2 Substitution Mechanisms Questions

Practise drawing and comparing SN1 and SN2 halogenoalkane mechanisms with charges, dipoles, curly arrows and rate-determining steps.

Exam points

show SN1 C–X heterolysis first, then nucleophile attack on the planar carbocation
show SN2 as one concerted backside attack while the C–X bond breaks
compare substrate structure, carbocation stability, transition states and stereochemical outcomes

[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 CAIE A-Level Chemistry AS 15.1.5 Sn1 Sn2 Substitution Mechanisms Questions 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 CAIE A-Level Chemistry AS 15.1.5 Sn1 Sn2 Substitution Mechanisms Questions question 1 — CAIE A-Level Chemistry AS

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CAIE A-Level Chemistry AS 15.1.5 Sn1 Sn2 Substitution Mechanisms Questions question 1