A-LevelChemistryHalogen derivatives9701/21 · Oct/Nov 2017 · Q324 Marks

Some reactions based on 1-bromobutane, CH3(CH2)3Br, are shown. [Figure X.X] (Diagram shows a reaction scheme with CH3(CH2)3OH, CH3(CH2)3Br, CH3CH2CH=CH2, CH3(CH2)2CHO, CH3(CH2)3C≡N, CH3(CH2)2COOH, CH3(CH2)3COOH and reactions 1-6)

(a)For each of the reactions state the reagent(s), the particular conditions required, if any, and the type of reaction. For the type of reaction choose from the list. Each type may be used once, more than once or not at all. Each reaction may be described by more than one type. elimination hydrolysis substitution oxidation addition condensation reaction | reagent(s) and conditions | type(s) of reaction 1 | | 2 | | 3 | | 4 | | 5 | | 6 | | [6]
(b)Complete the diagram to show the SN2 mechanism of reaction 1. R represents the CH3(CH2)2 group. Include all necessary charges, dipoles, lone pairs and curly arrows. [Figure X.X] (Diagram shows R-C-Br and R-C-OH with H atoms around C, requiring curly arrows, lone pairs and dipoles)[2]
(c)2-bromo-2-methylpropane is a tertiary halogenoalkane that is a structural isomer of 1-bromobutane.
(c)(i)Define the term structural isomer and name the three different types of structural isomerism. definition ................................................................................................................................................................................ ................................................................................................................................................................................................ types of structural isomerism 1 .............................................................................................................................................................................................. 2 .............................................................................................................................................................................................. 3 ..............................................................................................................................................................................................[4]
(c)(ii)2-bromo-2-methylpropane is treated with the same reagents as in reaction 1. Methylpropan-2-ol is formed. Identify the mechanism for this reaction. Explain why this reaction proceeds via a different mechanism from that of reaction 1. mechanism ........................................................................................................................................................................... explanation ............................................................................................................................................................................[3]
(d)The product of reaction 2, but-1-ene, does not show stereoisomerism. However, but-1-ene reacts with HCl to form a mixture of structural isomers X and Y. X (exists as a pair of stereoisomers and is produced in higher yield than Y) but-1-ene + HCl Y (does not show stereoisomerism)
(d)(i)Explain the meaning of the term stereoisomers.[2]
(d)(ii)Give two reasons why but-1-ene does not show stereoisomerism.[2]
(d)(iii)Name X and Y. X .............................................................................................................................................................................................. Y ..............................................................................................................................................................................................[2]
(d)(iv)Name the type of stereoisomerism shown by X.[1]
(d)(v)Use the conventional representation to draw the two stereoisomers of X.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[1] aqueous / aq / dilute NaOH / KOH OR water
[1] substitution OR hydrolysis
[1] alcoholic / ethanolic NaOH / KOH
[1] elimination
[1] NaCN / KCN in ethanol / alcohol
[1] substitution
[1] aqueous /dilute H2SO4 / H+(aq)
[1] hydrolysis OR substitution OR addition-elimination
[1] acidified / H+ (with) K2Cr2O7 / Cr2O72- (and distil) NOT reflux
[1] oxidation OR elimination
[1] acidified / H+ K2Cr2O7 / Cr2CrO7 2- Fehling's / Tollens' / Benedict's (reagent)
[1] oxidation
[1] M1 lone pair on O of ¯OH AND curly arrow from lone pair to C(-Br)
[1] M2 correct dipole on Cδ⁺—Brδ⁻ AND curly arrow from bond to Br
[1] (different molecules) with same molecular formula / same numbers of atoms of (each type) of element
[1] different structural formulae / displayed formulae
[2] chain / skeletal ; functional group ; position(al) / regioisomerism (two types correct = 1 mark, all three correct = 2 marks)
[1] SN1 / nucleophilic substitution
[1] ((CH3)3CBr / tertiary halogenoalkane) forms a stable (carbo)cation / stable intermediate (as charge density on cation is reduced) OR (in) 1-bromobutane / primary halogenoalkane there is no (stable) (carbo)cation / intermediate formed
[1] (because) there are (3/more) alkyl / methyl groups AND (+) I / (greater) inductive effect OR (because) there is only one / fewer alkyl / methyl group(s) (compared to reaction with 2-bromo-2-methyl propane / tertiary halogenoalkane) AND limited (+) I / (less) inductive effect
[1] (different molecules) with the same (molecular and) structural formula /
[1] with different arrangements of atoms in space / spatial arrangement of atoms
[1] mirror images are super(im)posable / no chiral carbon / no chiral centre / it is achiral
[1] (one) C of double bond has identical groups / H (atoms) (attached) OR (one) end of double bond has identical groups / 2 H (atoms) (attached)
[1] X = 2-chlorobutane
[1] Y = 1-chlorobutane
[1] optical (isomerism)
[1] one acceptable 3D structure of 2-chlorobutane
[1] the 2nd optical isomer EITHER drawn as a mirror image of the first OR the same bond pattern is shown but two of the groups swap positions.