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Date May 2019 Marks available 1 Reference code 19M.2.sl.TZ1.1
Level SL Paper 2 Time zone TZ1
Command term Draw Question number 1 Adapted from N/A

Question

Xylene is a derivative of benzene. One isomer is 1,4-dimethylbenzene.

Bromine reacts with alkanes.

State the number of 1H NMR signals for this isomer of xylene and the ratio in which they appear.

Number of signals:

Ratio:

[2]
a.

Draw the structure of one other isomer of xylene which retains the benzene ring.

[1]
b.

Identify the initiation step of the reaction and its conditions.

[2]
c(i).

1,4-dimethylbenzene reacts as a substituted alkane. Draw the structures of the two products of the overall reaction when one molecule of bromine reacts with one molecule of 1,4-dimethylbenzene.

[2]
c(ii).

Markscheme

Number of signals:
[✔]

Ratio:
3 : 2
OR
6 : 4 [✔]

 

Note: Accept any correct integer or fractional ratio.

Accept ratios in reverse order.

a.

  [✔]

b.

Br2 → 2Br•  [✔]

«sun»light/UV/hv
OR
high temperature  [✔]

 

Note: Do not penalize missing radical symbol on Br.

Accept “homolytic fission of bromine” for M1.

c(i).

  [✔]

HBr  [✔]

 

Note: Accept condensed formulae, such as CH3C6H4CH2Br.

Accept skeletal structures.

c(ii).

Examiners report

Most students gained M1 but very few gained M2, suggesting that the correct answer of 2 signals may have been a guess.

a.

Another isomer of xylene was generally correctly drawn, but some candidates drew the original compound.

b.

Drawing or describing the homolytic fission of bromine was generally done well.

c(i).

Very few students gained 2 marks finding hard to apply their knowledge of free radical substitution to a benzene containing compound. Many thought that the bromine will attach to the benzene ring or would substitute the alkyl group twice and not produce HBr.

c(ii).

Syllabus sections

Core » Topic 10: Organic chemistry » 10.1 Fundamentals of organic chemistry
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