Edexcel IAL Chemistry 10.9 Nucleophilic substitution mechanisms

Place δ+ on the carbon and δ− on the halogen, start one arrow at the nucleophile lone pair and the other at the C–X bond toward X.

Exam points

Draw SN2 attack from the HO⁻ or NH₃ lone pair to the δ+ carbon of a primary haloalkane.
Show C–X bond electrons moving to X with correct dipoles, charges and leaving ion.
Recognise primary halogenoalkane reactions as nucleophilic substitution and select SN2.

[Maximum number: 4]

This question is about sodium hydroxide.

(a)

Aqueous sodium hydroxide reacts with 1-bromopropane to produce propan-1-ol.

[ 4 ]

(i)

State the type and mechanism of this reaction.

[ 1 ]

An answer that makes reference to the following points:

nucleophilic and substitution

Allow nucleophile

Ignore $\mathrm{S}_{\mathrm{N}} 2$

Do not award $\mathrm{S}_{\mathrm{N}} 1$ , halogenation, nuclear

(1)

(ii)

Complete the mechanism for this reaction.

Include curly arrows, and relevant lone pairs and dipoles.

Figure for Question (ii) — Edexcel A-Level Chemistry AS

[ 3 ]

An answer that makes reference to the following points:

- dipole on C-Br bond

- lone pair on O of $\mathrm{OH}^{-}$

- curly arrow from lone pair to C of C - B r. If no lone pair shown, allow curly arrow from O

- arrow from C-Br to Br or just beyond

- organic product

- $\mathrm{Br}^{-}$

Figure for Answer to Question (ii) — Edexcel A-Level Chemistry AS

Example of mechanism

Allow product as structural formula rather than displayed

Allow NaBr

Ignore $\mathrm{Na}^{+}$

Do not award HBr 6 points correct scores (3)

4 / 5 points correct scores ( 2 )

2 / 3 points correct scores (1) Ignore intermediate/ transition state if shown

(3)

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10.9—The mechanisms of the nucleophilic substitution reactions between primary halogenoalkanes and: i aqueous potassium hydroxide question 1

Question (a)

Question (i)

Question (ii)