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2025 H2 Chem P2 Q4: Phenylamine Synthesis Solution

Phenylamine Synthesis 2025 A-Level H2 Chemistry · Paper 2 · Q4 · 19 marks

Question

(a)(i)[1]

Phenylamine is made from benzene in a three-step process as shown in Fig. 4.1.

Figure fig_4_1

Describe the reagents and conditions required for step 1.

(a)(ii)[2]

Write an equation to describe the reaction to produce salt F in step 2.
Represent hydrogen from the reducing agent as [H].

(b)[1]

Phenylamine reacts with \(H_2(g)\) in the presence of a nickel catalyst to produce G.

Figure struct_4_b_G

Write an equation for this reaction.

(c)(i)[1]

A pyridine molecule is made from a ring of 5 carbon atoms and 1 nitrogen atom joined by six covalent bonds. The bonding is similar to that in benzene. Both rings have six \(\pi\) electrons.

Figure fig_4_2

Identify the hybridisation of the carbon atoms in the pyridine molecule shown in Fig. 4.2.

(c)(ii)[2]

The nitrogen atom in pyridine undergoes the same type of hybridisation as the carbon atoms.

Complete Fig. 4.3 to show all the valence electrons in the orbitals of carbon and nitrogen atoms in pyridine that are not involved in the formation of \(\sigma\) bonds.

Figure fig_4_3

(c)(iii)[1]

Describe how delocalisation of electrons in the pyridine ring occurs.

(c)(iv)[2]

Suggest why pyridine is less reactive than benzene in electrophilic substitution reactions.

(d)(i)[1]

The pyridine ring shown in Fig. 4.2 is found in nicotinamide and niacin. Both are types of vitamin B3, which is essential in a balanced diet.

State the molecular formula of niacin.

Figure struct_4_d_niacin

(d)(ii)[2]

Complete Table 4.1 to show the number of \(\sigma\) bonds and \(\pi\) bonds present in a nicotinamide molecule.

Figure table_4_1

(d)(iii)[1]

State the reagents and conditions required to convert nicotinamide into niacin in a single-step reaction.

Figure fig_4_d_nicotinamide_to_niacin

(e)(i)[1]

In plants and animals, niacin is used to produce two species that are represented as \(NAD^+\) and NADH.

The mechanism for the conversion of \(NAD^+\) to NADH is shown in Fig. 4.4. In the pyridine ring structure in \(NAD^+\), the ring is shown as alternating single and double covalent bonds.

Figure fig_4_4

Suggest the name of the mechanism for the reaction described in Fig. 4.4.

(e)(ii)[2]

The kinetics for the breakdown of ethanol using \(NAD^+\) and enzyme J was studied using a range of concentrations of \(NAD^+\), enzyme J and ethanol.

On Fig. 4.5, sketch the shapes of the graphs that show the order of reaction with respect to both \(NAD^+\) and ethanol is zero.

Figure fig_4_5

(e)(iii)[2]

Describe and explain the difference, if any, in the magnitude of the rate constant for the catalysed and uncatalysed reactions.

Video Solution

Overview

Question 4 is the long organic question. It opens with the three-step synthesis of phenylamine from benzene, asking for the reagents and conditions of the nitration step and the equation for the reduction step. It then explores the pyridine ring, a benzene-like ring containing nitrogen: the hybridisation of its atoms, how the electrons delocalise, and why pyridine is less reactive than benzene towards electrophiles. From there it moves to the vitamin B3 molecules niacin and nicotinamide, counting sigma and pi bonds and converting one into the other, and finishes with the kinetics of the NAD⁺ and NADH reaction that breaks down ethanol, including the orders of reaction and the effect of the enzyme on the rate constant.

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