DP Chemistry (first assessment 2025)

Question 19M.2.SL.TZ1.1

Date	May 2019	Marks available	[Maximum mark: 7]	Reference code	19M.2.SL.TZ1.1
Level	SL	Paper	2	Time zone	TZ1
Command term	Draw, Identify, State	Question number	1	Adapted from	N/A

[Maximum mark: 7]

19M.2.SL.TZ1.1

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

Bromine reacts with alkanes.

(a)

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

Number of signals:

Ratio:

[2]

Markscheme

Number of signals:
2 [✔]

Ratio:
3 : 2
OR
6 : 4 [✔]

Note: Accept any correct integer or fractional ratio.

Accept ratios in reverse order.

Examiners report

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

(b)

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

[1]

Markscheme

[✔]

Examiners report

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

(c(i))

Identify the initiation step of the reaction and its conditions.

[2]

Markscheme

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.

Examiners report

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

(c(ii))

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]

Markscheme

[✔]

HBr [✔]

Note: Accept condensed formulae, such as CH₃C₆H₄CH₂Br.

Accept skeletal structures.

Examiners report

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.

Syllabus sections

Structure 3. Classification of matter » Structure 3.2—Functional groups: Classification of organic compounds » Structure 3.2.10—Proton nuclear magnetic resonance spectroscopy (1H NMR) gives information on the different chemical environments of hydrogen atoms in a molecule. Interpret 1H NMR spectra to deduce the structures of organic molecules from the number of signals, the chemical shifts, and the relative areas under signals (integration traces).

Structure 3. Classification of matter » Structure 3.2—Functional groups: Classification of organic compounds » Structure 3.2.6—Structural isomers are molecules that have the same molecular formula but different connectivities. Recognize isomers, including branched, straight-chain, position and functional group isomers.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.12—Benzene, C6H6, is an important example of a molecule that has resonance. Discuss the structure of benzene from physical and chemical evidence.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.3—Electron sharing reactions » Reactivity 3.3.2—Radicals are produced by homolytic fission, e.g. of halogens, in the presence of ultraviolet (UV) light or heat. Explain, including with equations, the homolytic fission of halogens, known as the initiation step in a chain reaction.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.4—Electron-pair sharing reactions » Reactivity 3.4.13—Electrophilic substitution reactions include the reactions of benzene with electrophiles. Describe and explain the mechanism of the reaction between benzene and a charged electrophile, E+.

Question 19M.2.SL.TZ1.1

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