A-LevelChemistryHalogen derivatives9701/22 · May/June 2022 · Q513 Marks

Fig. 5.1 shows three reactions of 2-bromopropane, CH3CH(Br)CH3. CH3CH(Br)CH3 reaction 1 / \ reaction 2 CH3CH(OH)CH3 CH3CH(NH2)CH3 H2C=CHCH3 Fig. 5.1

(a)Complete Table 5.1 for each reaction, by: • stating the reagent and conditions used • identifying the type of reaction that occurs. Table 5.1 reaction reagent and conditions type of reaction 1 2 3[6]
(b)A sample of 2-iodopropane, CH3CH(I)CH3, reacts under the same conditions as reaction 1 to produce CH3CH(OH)CH3. Explain why 2-iodopropane reacts at a faster rate than 2-bromopropane.[2]
(c)(i)Fig. 5.2 shows how butan-1-ol can be made from 1-bromopropane in three steps. Br-CH₂-CH₂-CH₃ --step 1--> N≡C-CH₂-CH₂-CH₃ --step 2--> HOOC-CH₂-CH₂-CH₃ --step 3--> HO-CH₂-CH₂-CH₂-CH₃ Fig. 5.2 In step 1, 1-bromopropane reacts with CN- to form butanenitrile. Complete Fig. 5.3 to show the mechanism for step 1. Include charges, dipoles, lone pairs of electrons and curly arrows as appropriate. H H H H H H H-C-C-C-Br (curly arrow) H-C-C-C-C≡N H H H H H H C N Br- Fig. 5.3[2]
(c)(ii)In step 2, butanenitrile is heated with HCl(aq). A hydrolysis reaction occurs. Construct an equation for the reaction in step 2.[1]
(c)(iii)Step 3 is a reduction reaction. Construct an equation for the reduction reaction in step 3. Use [H] to represent one atom of hydrogen from the reducing agent.[1]
(c)(iv)State the identity of a suitable reducing agent in step 3.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[6] reaction reagent and conditions type of reaction 1 NaOH(aq) [M1] substitution 2 NH3 [M2] in ethanol AND heat under pressure [M3] substitution 3 NaOH in ethanol AND heat [M4] elimination M5 and M6 types of reaction [3 correct types = 2 marks and 2 correct types = 1 mark] All reagents can be identified as (correct) formula or in words
[2] M1 C-I (covalent bond) is weaker ora M2 lower activation energy / lower Ea (with 2-iodopropane) OR M2 explain in terms of the SN₁ mechanism (that is dominant/preferred/occurring with 2-iodopropane is fast(er)) (identical) carbocation is made (more) quickly (with 2-iodopropane) OR low/less energy is required to make the (same)carbocation / intermediate (from 2-iodopropane) OR M2 iodine / I (of C-I) has weaker attraction of nucleus to bonding / shared pair (of electrons) due to any one of: more / high shielding (of electrons from inner shells) more/5 electron shells in iodine C-I greater distance from nuclei (to bonding pair of electrons)
[2] M1 dipole Cδ+-Brδ- AND curly arrow from C–Br bond to Br- M2 lone pair on C of -CN AND curly arrow from lone pair to C (on C-1 of bromopropane)
[1] CN(CH2)2CH3 + 2H2O + HCl → HOOC(CH2)2CH3 + NH4Cl OR (structure of butanenitrile) + 2H2O + HCl → (structure of butanoic acid) + NH4Cl
[1] HOOC(CH2)2CH3 + 4[H] → HO(CH2)3CH3 + H2O OR (structure of butanoic acid) + 4[H] → (structure of butan-1-ol) + H2O
[1] LiAlH4 OR lithium tetrahydridoaluminate (in ether)

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