Home › Subjects › A-Level H2 Chemistry › Amines, amides and amino acids
A-Level Amines, amides and amino acids
What the A-Level syllabus expects for Amines, amides and amino acids, and how to practise it.
What the syllabus expects
- Describe how amines are formed: ethylamine by reducing an amide with LiAlH4 or a nitrile with LiAlH4 or H2(g) over Ni; and phenylamine by reducing nitrobenzene with Sn and concentrated HCl under heat, then NaOH(aq).
Scope: see also Section 11.5 - Describe how an amine reacts to form a salt.
- Describe and account for how basic primary, secondary and tertiary amines are in the gas phase, treating them as Lewis bases.
- Explain, from their structures, why ammonia, ethylamine and phenylamine differ in basicity in aqueous medium.
- Describe what phenylamine does with aqueous bromine.
- Describe how an amide forms when RNH2 condenses with R'COCl.
- Explain the neutrality of an amide through delocalisation of the lone pair on nitrogen.
- Describe amide chemistry through the following reactions:
- hydrolysis with aqueous acid or aqueous alkali and heat
- reduction to an amine with LiAlH4
- Describe how an amino acid behaves as both an acid and a base.
How it's examined
Questions on this topic most often ask you to compare, name, outline. About 3% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Compare the relative basicities of tryptophan and lysine, giving your prediction along with an explanation.
Show the worked answer
Lysine has a side chain -(CH2)4-NH2 ending in a primary aliphatic amine. The amino nitrogen has a lone pair that is not delocalised, and the alkyl chain is electron-donating, making the lone pair readily available to accept a proton. Tryptophan's side chain contains an indole ring; the nitrogen lone pair of the indole is part of the aromatic pi system (delocalised into the ring to preserve aromaticity), so it is far less available for protonation. Therefore lysine is more basic than tryptophan.
Example 2 (3 marks)
Propose a synthetic route of three stages that converts acetanilide (C6H5NHCOCH3) into sulfapyridine (H2N-C6H4-SO2NH-(2-pyridyl)), beginning from acetanilide together with chlorosulfuric acid (HOSO2Cl) held at 80 C. For every stage, name the intermediate produced along with the reagents and reaction conditions used.
Show the worked answer
Stage 1: Chlorosulfonation. React acetanilide (C6H5NHCOCH3) with chlorosulfuric acid, HOSO2Cl, at 80 C. The -SO2Cl group is introduced para to the acetamido group (which is o/p-directing). Intermediate: 4-acetamidobenzenesulfonyl chloride, CH3CONH-C6H4-SO2Cl. Stage 2: Sulfonamide formation. React this sulfonyl chloride with 2-aminopyridine (2-pyridylamine), usually with a base such as pyridine, at room temperature. The -SO2Cl reacts with the amine to form the sulfonamide. Intermediate: 4-acetamido-N-(2-pyridyl)benzenesulfonamide, CH3CONH-C6H4-SO2NH-(2-pyridyl). Stage 3: Hydrolysis of the amide (removal of the acetyl protecting group). Reflux with dilute aqueous acid (e.g. dilute HCl) - or dilute NaOH - then neutralise. This hydrolyses the CH3CONH- amide to the free -NH2 group. Product: sulfapyridine, H2N-C6H4-SO2NH-(2-pyridyl).
Example 3 (2 marks)
In nicotine, a pyridine ring (with ring nitrogen labelled N-1) is joined to an N-methylpyrrolidine ring (with ring nitrogen labelled N-2). The pyridine nitrogen N-1 has a pKb of 10.88. Compare how basic the nitrogen atom in pyrrole is relative to N-1 in nicotine, and give reasons for your comparison.
Show the worked answer
The nitrogen in pyrrole is much less basic than N-1 (the pyridine-type nitrogen) in nicotine. In pyrrole, the nitrogen's lone pair is delocalised into the aromatic ring: it forms part of the 6 pi-electron aromatic system. Because this lone pair is tied up in maintaining aromaticity, it is not available to accept a proton, so pyrrole is a very weak base (essentially non-basic). In nicotine, N-1 is a pyridine-type nitrogen: its lone pair sits in an sp2 orbital in the plane of the ring and is NOT part of the aromatic pi system, so it is available to accept a proton (pKb 10.88). Therefore pyrrole's nitrogen is less basic than N-1 in nicotine.
More worked questions on this topic
- Pyrrole and imidazole, like pyridine, are heterocyclic compounds. Pyrrole is more prone to elec (2 marks)
- Psilocin is a 4-hydroxy tryptamine bearing a -CH2CH2N(CH3)2 side chain, while the neurotransmit (2 marks)
- Both 2-chloropyridine and 4-chloropyridine take part in nucleophilic aromatic substitution with (2 marks)
- This question concerns heterocyclic compounds. The NAD+/NADH couple acts as a biological redox (2 marks)
- Sulfonamides (sulfa drugs) are bacteriostatic: rather than killing bacteria, they simply preven (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry