A-LevelChemistryChemical bonding9701/23 · May/June 2024 · Q416 Marks

NH3(g) reacts with HCl(g) to produce NH4Cl(s), as shown. NH3(g) + HCl(g) → NH4Cl(s)

(a)Draw a diagram to show the ionic, covalent and coordinate bonding present in a formula unit of NH4Cl.[2]
(b)(i)An exothermic reaction occurs when NH4+(aq) is added to OH¯(aq). Identify the type of reaction.[1]
(b)(ii)Construct an ionic equation for the reaction of NH4+ and OH¯.[1]
(c)(i)Substitution reactions of NH3 and OH¯ with halogenoalkanes both involve a lone pair of electrons. Name the role of NH3 and OH¯ in these reactions.[1]
(c)(ii)Suggest which species, NH3 or OH¯, is more reactive during these reactions. Explain your answer.[1]
(d)When 2-bromo-2-methylpropane reacts with OH¯, two mechanisms, SN1 and SN2, both occur. The SN2 mechanism has a slower rate. Fig. 4.1 shows the reaction pathway diagram for the SN1 mechanism. Sketch a graph on Fig. 4.1 to show the reaction pathway for the SN2 mechanism. [Figure 4.1][2]
(e)(i)Complete Fig. 4.2 to show the mechanism for the SN1 reaction that occurs when CH3CHBrC2H5 reacts with NH3 to produce CH3CH(NH2)C2H5. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate. [Figure 4.2][3]
(e)(ii)Identify the inorganic product that forms in the reaction in Fig. 4.2.[1]
(e)(iii)Give the systematic name for the organic product CH3CH(NH2)C2H5.[1]
(f)(i)Complete Table 4.1 by drawing the structural formula of the intermediate that is formed when 2-bromo-2-methylpropane reacts in an SN1 reaction. Table 4.1 structural formula of intermediate in SN1 reaction 2-bromobutane H C⁺—C2H5 CH3 2-bromo-2-methylpropane[1]
(f)(ii)Identify the halogenoalkane in Table 4.1 that has the greater tendency to react using the SN1 mechanism. Explain your answer.[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 (May/June 2024). The mark or mark type is shown in brackets.

[2] Correct representation or label showing M1 (ionic bond between) NH4+ and Cl- AND 4 N—H covalent bonds OR 4 shared pairs of electrons in NH4 M2 1 N-H coordinate bond OR arrow on bond/line in correct direction or only 1 bonding pair using the same symbol for electrons 1
[1] acid-base reaction
[1] NH4+ + OH- → NH3 + H2O
[1] nucleophile
[1] OH-(more reactive because) negatively charged species more attracted to halogenoalkane and / or Cδ+ and / or intermediate and/or C+ OR NH3 (more reactive because) N electrons less tightly held OR N is more able to donate (lp) electrons (to make the covalent bond) OR N is less electronegative ORA
[2] M1 same start energy and finishing energy AND single hump M2 transition state / activation energy shown in answer is higher than both transition state energies shown in reaction profile (as the reaction is slower)
[3] M1 dipole and correct arrow on C-Br of 2-bromobutane M2 lone pair on NH3 attacking C(+) of the intermediate drawn M3 curly arrow showing bond between N—H breaking and pointing towards the N
[1] NH4Br / ammonium bromide
[1] butan-2-amine OR butyl-2-amine
[1] Structural formula of intermediate in SN1 reaction for 2-bromo-2-methylpropane shows a tertiary carbocation: a central C+ bonded to three methyl groups (CH3) and no H atoms attached to the C+.
[2] M1 2-bromo-2-methylpropane AND more stable carbocation / C+ M2 alkyl groups have an inductive effect (of alkyl groups) AND 2-bromo-2-methylpropane has more / most alkyl groups attached (to C+)/carbocation contains more alkyl groups