A-LevelChemistryHalogen derivatives9701/23 · May/June 2019 · Q512 Marks

Halogenoalkanes react with a number of different reagents in nucleophilic substitution reactions. (a) A sample of potassium cyanide dissolved in ethanol is added to a sample of 1-bromobutane, CH3(CH2)3Br, and heated under reflux. A nucleophilic substitution reaction occurs and compound A is formed. (i) Name compound A. [1] (ii) What is meant by the term nucleophile? [1] (iii) Identify the nucleophile in this reaction. [1] (iv) Explain why this reaction is described as a substitution reaction. [1] (b) State the reagent(s) and conditions needed for CH3(CH2)3Br to react to form CH3(CH2)3NH2. reagent(s) conditions [2] (c) Equal amounts of three different halogenoalkanes are added to three separate test-tubes. An equal amount of aqueous silver nitrate and ethanol is added to each test-tube. The time taken for a precipitate to form is recorded for each halogenoalkane. halogenoalkane time taken for precipitate to form/s (CH3)3CCl 460 (CH3)3CBr 190 (CH3)3CI 40 (i) Describe and explain the trend in reactivity of the different halogenoalkanes shown in this experiment. [2] (ii) All three halogenoalkanes tend to react via the SN1 mechanism. Explain why the SN1 mechanism is favoured. [3] (iii) Identify a halogenoalkane which tends to react with an aqueous solution of silver nitrate and ethanol via the SN2 mechanism. [1] [Total: 12]

(a)(i)Name compound A.[1]
(a)(ii)What is meant by the term nucleophile?[1]
(a)(iii)Identify the nucleophile in this reaction.[1]
(a)(iv)Explain why this reaction is described as a substitution reaction.[1]
(b)State the reagent(s) and conditions needed for CH3(CH2)3Br to react to form CH3(CH2)3NH2. reagent(s) conditions[2]
(c)(i)Describe and explain the trend in reactivity of the different halogenoalkanes shown in this experiment.[2]
(c)(ii)All three halogenoalkanes tend to react via the SN1 mechanism. Explain why the SN1 mechanism is favoured.[3]
(c)(iii)Identify a halogenoalkane which tends to react with an aqueous solution of silver nitrate and ethanol via the SN2 mechanism.[1]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] pentanenitrile
[1] a lone pair / electron pair donor
[1] (:)CN-/-(:)CN / cyanide ion
[1] Br (atom) is replaced (with / by CN / nitrile)
[2] M1 reagent Ammonia
[2] M2 conditions heat with under pressure / heat in a sealed tube
[2] M1 Increasing reactivity from Cl → Br → I
[2] M2 Due to decreasing strength of C-X bond (from C-Cl to C-Br to C-I) OR Less energy needed to break C-X (from C-Cl to C-Br to C-I)
[3] M1 tertiary / 3° halogenoalkane
[3] M2 (carbo)cation / intermediate is stable
[3] M3 due to (3) electron releasing/donating methyl groups / + I groups (attached to central C) OR (positive) inductive effect of the (three) methyl groups /
[1] Any formula / name for any primary halogenoalkane i.e. 1-chlorobutane / 1-bromobutane / 1-iodobutane

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