A-LevelChemistryHydrocarbons9701/23 · May/June 2017 · Q417 Marks

A, B and C all have the formula C₄H₈. They all decolourise bromine and are structural isomers of each other.

(a)State the name of the process by which A, B and C could be obtained from C10H22.[1]
(b)Draw the structures of these three structural isomers.[1]
(c)(i)Only A shows geometrical isomerism. Explain the meaning of the term geometrical isomerism.[2]
(c)(ii)Draw the displayed formula of A and use it to show the mechanism of the reaction of A with HBr. Include all necessary charges, dipoles, lone pairs and curly arrows.[4]
(d)(i)B does not show geometrical isomerism. B reacts with HBr to form a mixture of two structural isomers, X and Y. X (has a chiral centre and is produced in higher yield than Y) B + HBr → X Y (does not have a chiral centre) State the meaning of the term chiral centre.[1]
(d)(ii)Name B.[1]
(d)(iii)X exists as a pair of optical isomers. Draw these isomers using the conventional three-dimensional representation.[2]
(d)(iv)Explain why X is produced in higher yield than Y.[2]
(e)(i)C does not show geometrical isomerism. C reacts with HBr to form a mixture of two structural isomers, neither of which has a chiral centre. Name C.[1]
(e)(ii)Draw the displayed formula of each of the structural isomers produced by the reaction of C with HBr.[2]

Mark scheme

Marking points from the official mark scheme (May/June 2017). The mark or mark type is shown in brackets.

[1] cracking
[1] In any order: CH2=CHCH2CH3 AND CH3CH=CHCH3 AND (CH3)2C=CH2 (or skeletal representations)
[1] (different) molecules with the same (molecular and) structural formula
[1] (due to) different arrangement in space caused by C=C / double bond
[1] arrow from the C=C double bond drawn to the H
[1] dipole on H-Br in correct orientation AND arrow from the H-Br bond to the Brδ⁻
[1] correct carbocation from the structure with C=C drawn
[1] Br⁻ with lone pair, negative charge AND arrow from lone pair to the positively charged carbon atom of intermediate
[1] a (tetrahedral) atom with four different groups / atoms / substituents attached OR a carbon (atom) with four different groups / atoms / substituents attached
[1] but-1-ene
[1] One 3D structure of 2-bromobutane which must have 2 bonds shown the same and two different, i.e. three bond types altogether, e.g. two solid lines, one wedge and one dash. If two bonds are drawn in the plane of the paper, i.e. single solid lines, they must not be at 180 degrees to each other.
[1] Second structure either mirror of first OR all bonds drawn the same with position of two groups swapped.
[1] intermediate / (secondary carbo) cation from X is more stable ora
[1] charge density of C⁺ (of the intermediate of X) is reduced
[1] (due to) electron-releasing character / (positive) inductive effect of alkyl groups / / due to electron releasing alkyl group
[1] (2-)methylpropene / (2-)methylprop-1-ene
[1] Displayed formula for 2-bromo-2-methylpropane
[1] Displayed formula for 1-bromo-2-methylpropane