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Date May 2009 Marks available 2 Reference code 09M.2.hl.TZ1.9
Level HL Paper 2 Time zone TZ1
Command term Draw Question number 9 Adapted from N/A

Question

But-1-ene and 1-aminobutane (1-butylamine) can both be prepared from 1-bromobutane.

2-bromobutane and 2-bromo-2-methylpropane are two isomers of 1-bromobutane.

Identify the type of reaction and explain the mechanism for the preparation of but-1-ene from 1-bromobutane using curly arrows to represent the movement of electron pairs.

[3]
a.ii.

State the equation (using structural formulas) for the preparation of 1-aminobutane from 1-bromobutane. State the necessary reagents and conditions of the reaction.

[3]
a.iii.

Explain the mechanism for the preparation of 1-aminobutane from 1-bromobutane using curly arrows to represent the movement of electron pairs.

[4]
a.iv.

Draw the structures of the two mirror images of the isomer that can exhibit optical isomerism.

[2]
b.i.

Describe how the two optical isomers can be distinguished practically using plane-polarized light.

[2]
b.ii.

Explain why the mechanism of the reaction will be different if 1-bromobutane is replaced by 2-bromo-2-methylpropane to form 2-amino-2-methylpropane in the reaction in part (a) (iv).

[3]
b.iii.

Markscheme

elimination reaction ;

Then accept either E1 or E2 mechanism.

E1

M09/4/CHEMI/HP2/ENG/TZ1/09.a.ii_1/M

curly arrow showing bromine leaving the halogenoalkane;

OH acting as base on the intermediate carbocation;

E2

M09/4/CHEMI/HP2/ENG/TZ1/09.a.ii_2/M

curly arrow showingOH acting as base on H bonded to C;

concerted curly arrows showing Br leaving C–Br;

 

a.ii.

CH3CH2CH2CH2Br+NH3CH3CH2CH2CH2NH2+HBr;

ammonia/NH3;

warm / excess ammonia (to prevent secondary amines etc.);

a.iii.

M09/4/CHEMI/HP2/ENG/TZ1/09.a.iv/M

curly arrow from ammonia (to form transition state);

correct transition state;

curly arrow from bond to Br atom in either the first or second step;

formation of HBr and organic product;

Accept a second molecule of NH3 removing H+ from the transition state to give NH4+ and Bras products.

a.iv.

M09/4/CHEMI/HP2/ENG/TZ1/09.b.i/M

Award [1] for correct structure and [1] for correct 3-D representation of both enantiomers.

b.i.

polarimeter (to measure angle of rotation);

the plane of plane-polarized light rotates in opposite directions (by the different enantiomers);

b.ii.

2-bromo-2-methylpropane is tertiary / 1-bromobutane is primary;

2-bromo-2-methylpropane goes by SN1 / 1-bromobutane by SN2;

intermediate carbocation more stable for tertiary;

no space around tertiary carbon for five groups (in SN2 transition state);

b.iii.

Examiners report

This question was the least popular of the Section B questions. Some candidates were very well prepared and scored well, while many struggled to write correct mechanisms with curly arrows in the right place. For a small number of candidates, all parts of the question other than the identification of a functional group proved very difficult. Additionally, in part (a), few candidates knew the details of the reagents and conditions for a range of reaction types.

a.ii.

This question was the least popular of the Section B questions. Some candidates were very well prepared and scored well, while many struggled to write correct mechanisms with curly arrows in the right place. For a small number of candidates, all parts of the question other than the identification of a functional group proved very difficult. Additionally, in part (a), few candidates knew the details of the reagents and conditions for a range of reaction types.

a.iii.

This question was the least popular of the Section B questions. Some candidates were very well prepared and scored well, while many struggled to write correct mechanisms with curly arrows in the right place. For a small number of candidates, all parts of the question other than the identification of a functional group proved very difficult. Additionally, in part (a), few candidates knew the details of the reagents and conditions for a range of reaction types.

a.iv.

In part (b), very few candidates could draw three-dimensional structures of optical isomers, although many gained a mark for correctly identifying the structure which could have enantiomers.

b.i.

Few mentioned a polarimeter to distinguish between the two optical isomers, although several described in clear detail a practical method to do so.

b.ii.
[N/A]
b.iii.

Syllabus sections

Additional higher level (AHL) » Topic 20: Organic chemistry » 20.3 Stereoisomerism
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