Amines, Amino Acids & Polymers

Why is ethylamine (CH₃CH₂NH₂) a stronger base than ammonia (NH₃)?

• AThe electron-donating (+I) ethyl group increases the electron density on the nitrogen lone pair, so it accepts a proton more readily✓
• BThe ethyl group withdraws electron density, increasing the charge on nitrogen
• CEthylamine is less soluble in water than ammonia
• DEthylamine has a higher molar mass so holds protons more strongly

Answer: Base strength of amines depends on the availability of the nitrogen lone pair. The alkyl (ethyl) group is electron-releasing (positive inductive effect), pushing electron density onto N, making the lone pair more available to accept (bond to) a proton. Hence ethylamine is a stronger base than ammonia.

Phenylamine (C₆H₅NH₂) is a much weaker base than ethylamine. What is the reason?

• APhenylamine is a secondary amine
• BThe benzene ring donates electron density onto nitrogen
• CThe nitrogen lone pair is delocalised into the benzene ring, reducing its availability to accept a proton✓
• DPhenylamine cannot form hydrogen bonds

Answer: In phenylamine the nitrogen lone pair overlaps with (is delocalised into) the π system of the benzene ring. This pulls electron density away from the nitrogen, making the lone pair far less available to accept a proton, so phenylamine is a weaker base than ammonia or aliphatic amines.

Bromoethane is heated with an excess of concentrated ammonia in a sealed tube. What is the main organic product and why is excess ammonia used?

• ADiethylamine; excess ammonia favours double substitution
• BEthanenitrile; excess ammonia provides the extra carbon
• CEthanol; ammonia acts as a base
• DEthylamine; excess ammonia minimises further substitution to secondary/tertiary amines✓

Answer: Ammonia acts as a nucleophile, substituting the halogen to give a primary amine (ethylamine). The amine product is itself nucleophilic and can react further, so a large excess of ammonia is used to make single substitution (ethylamine) the major product.

Propanenitrile (CH₃CH₂CN) is reduced using LiAlH₄ (or H₂ with a Ni catalyst). What is the amine product?

• AEthanamine, CH₃CH₂NH₂
• BPropan-1-amine, CH₃CH₂CH₂NH₂✓
• CPropan-2-amine, CH₃CH(NH₂)CH₃
• DPropan-1-ol, CH₃CH₂CH₂OH

Answer: Reduction of a nitrile adds four hydrogens across the C≡N and converts it to a primary amine with the SAME number of carbons: CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂ (propan-1-amine). The nitrile route is useful because it increases the carbon chain length by one compared with the parent halogenoalkane.

Nitrobenzene is converted to phenylamine in two stages. What reagents are used for the reduction (step 1) and to liberate the free amine (step 2)?

• AStep 1: concentrated H₂SO₄; Step 2: aqueous HCl
• BStep 1: NaBH₄; Step 2: water
• CStep 1: tin and concentrated HCl; Step 2: aqueous NaOH✓
• DStep 1: acidified KMnO₄; Step 2: NaOH

Answer: Nitrobenzene is reduced to phenylamine using tin (Sn) and concentrated hydrochloric acid, which produces the protonated salt (C₆H₅NH₃⁺). Adding excess aqueous NaOH then liberates the free amine, phenylamine (C₆H₅NH₂).

The amino acid glycine (H₂NCH₂COOH) exists predominantly as a zwitterion in the solid state and in neutral solution. What is the structure of the glycine zwitterion?

• AH₂NCH₂COOH
• B⁺H₃NCH₂COO⁻✓
• C⁺H₃NCH₂COOH
• DH₂NCH₂COO⁻

Answer: A zwitterion is internally ionised but overall neutral: the basic –NH₂ group is protonated to –NH₃⁺ and the acidic –COOH group is deprotonated to –COO⁻, giving ⁺H₃NCH₂COO⁻. This explains the high melting points and water solubility of amino acids.

At a pH well below its isoelectric point, the dominant form of an amino acid carries a net positive charge. Which statement best explains the isoelectric point?

• AThe pH at which the amino acid is fully protonated on both groups
• BThe pH equal to the Ka of the carboxyl group
• CThe pH at which the amino acid is fully deprotonated on both groups
• DThe pH at which the amino acid exists predominantly as the zwitterion with no overall charge✓

Answer: The isoelectric point is the pH at which the amino acid has no overall net charge, existing mainly as the zwitterion. Below this pH the –COO⁻ is protonated (net +); above it the –NH₃⁺ is deprotonated (net −).

Two amino acids join to form a dipeptide. Which functional group links them, and what small molecule is eliminated?

• AAn ester linkage (–COO–); methanol is eliminated
• BA peptide/amide linkage (–CONH–); water is eliminated✓
• CAn ether linkage (–O–); hydrogen is eliminated
• DA disulfide bridge (–S–S–); hydrogen sulfide is eliminated

Answer: Amino acids undergo a condensation reaction in which the –COOH of one reacts with the –NH₂ of another to form a peptide (amide) bond, –CONH–, with the elimination of a molecule of water.

Which pair of monomers would form a polyamide by condensation polymerisation?

• AA diol and a dicarboxylic acid
• BA single molecule containing one C=C double bond
• CA diamine and a dicarboxylic acid✓
• DA diol and a diamine

Answer: Polyamides (e.g. nylon-6,6) form by condensation between a diamine (two –NH₂ groups) and a dicarboxylic acid (two –COOH groups), creating repeated amide (–CONH–) links and eliminating water. A diol + dicarboxylic acid would instead give a polyester.

Poly(ethene) and a polyester such as Terylene differ in their formation and breakdown. Which statement is correct?

• ABoth are addition polymers and both are readily hydrolysed
• BPoly(ethene) is a condensation polymer; the polyester is an addition polymer
• CBoth are condensation polymers releasing water on formation
• DPoly(ethene) is an addition polymer with an unreactive C–C backbone, whereas the polyester is a condensation polymer that can be hydrolysed at its ester linkages✓

Answer: Poly(ethene) is made by addition polymerisation of ethene, giving a saturated, non-polar C–C chain with no bonds that water can attack, so it is non-biodegradable. Polyesters are condensation polymers whose ester (–COO–) linkages can be hydrolysed (e.g. by acid or alkali), making them more readily broken down.

Why are condensation polymers such as polyesters and polyamides generally more biodegradable than addition polymers such as poly(propene)?

• AThey have lower molar masses
• BThey contain polar bonds (ester or amide links) that can be hydrolysed, breaking the chain✓
• CTheir carbon backbones contain C=C double bonds that are easily attacked
• DThey are made from petroleum so micro-organisms recognise them

Answer: Condensation polymers contain polar, hydrolysable linkages (–COO– or –CONH–) within the backbone. These can be broken by hydrolysis (and by enzymes/micro-organisms), so the polymer can be broken down. Addition polymers have inert, non-polar C–C backbones with no such hydrolysable groups, so they persist.