CAIE A-Level Chemistry 15.1 Halogenoalkanes

Practise preparing and classifying halogenoalkanes and choosing substitution or elimination conditions and mechanisms.

Exam points

classify the C–X carbon as primary, secondary or tertiary from its carbon attachments
choose aqueous OH−, ethanolic CN− or ethanolic NH3 for the required nucleophilic substitution product
use hot ethanolic hydroxide for elimination to an alkene and distinguish it from aqueous substitution

Covered syllabus points

15.1.1—Preparation of halogenoalkanes 15.1.2—Classify halogenoalkanes into primary,15.1.3—Nucleophilic substitution 15.1.4—Elimination reaction with NaOH in ethanol 15.1.5—SN1/SN2 substitution mechanisms 15.1.6—SN2/SN1 trends in halogenoalkanes 15.1.7—Rate of hydrolysis of halogenoalkanes

[Maximum number: 1]

Allyl chloride is an important chemical used in the manufacture of plastics, pharmaceuticals and pesticides.

allyl chloride

Allyl chloride can be produced by many different methods. The most common method is chlorination of propene which proceeds via a free-radical substitution mechanism.

Figure for Question 15.1 Halogenoalkanes question 1 — CAIE A-Level Chemistry AS

Allyl chloride can also be formed by the following substitution reaction.

Suggest the identity of reagent X.

$\mathrm{SOCl}_{2}$ OR $\mathrm{PCl}_{5}$ OR $\mathrm{PCl}_{3}$ OR $\underline{\mathrm{c}}$ (oncentrated) HCl

[Maximum number: 4]

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

(a)

$\mathrm{OH}^{-}(\mathrm{aq})$ 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.

[ 4 ]

(i)

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 (i) — CAIE A-Level Chemistry AS

[ 3 ]

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 (i) — CAIE A-Level Chemistry AS

(ii)

2-bromo-2-methylpropane is a tertiary bromoalkane.

Define tertiary bromoalkane.

[ 1 ]

[Maximum number: 1]

Fig. 4.1 shows how propane, $\mathrm{C}_{3} \mathrm{H}_{8}$ , can be converted to propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ .

Fig. 4.1

State the reagent and solvent required for reaction 2 .

All question bank results loaded

Covered syllabus points

15.1 Halogenoalkanes question 1

15.1 Halogenoalkanes question 2

Question (a)

Question (i)

Question (ii)

15.1 Halogenoalkanes question 3