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 electron-withdrawing (−I) ethyl group pulls electron density away from the nitrogen, which increases the positive charge on the atom and attracts protons
• CEthylamine is less soluble in water than ammonia
• DEthylamine has a much higher molar mass than ammonia, and the heavier molecule holds on to a proton more strongly once it has been accepted

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 the nitrogen atom, so the lone pair is held too tightly to be shared with a proton
• 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; the large excess of ammonia favours a second substitution at the same carbon atom
• BEthanenitrile; the excess ammonia provides the extra carbon atom needed to lengthen the chain
• CEthanol; the ammonia acts as a base and hydrolyses the carbon–halogen bond in the sealed tube
• 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, (CH₃)₂CHNH₂
• 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 with heating; Step 2: aqueous HCl added in excess
• BStep 1: NaBH₄ in methanol; Step 2: warm water added to the mixture
• CStep 1: tin and concentrated HCl; Step 2: aqueous NaOH✓
• DStep 1: acidified KMnO₄ under reflux; Step 2: solid NaOH pellets

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 of its groups
• BThe pH numerically equal to the Ka of the carboxyl group
• CThe pH at which the amino acid is fully deprotonated at both ends
• 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 (–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 built from monomers containing C=C double bonds, whereas the C–C and ester links in the two chains are equally easy to hydrolyse with warm aqueous alkali
• BPoly(ethene) is a condensation polymer; the polyester is an addition polymer
• CBoth are condensation polymers, each formed when the monomers join together with the loss of a small molecule of water at each linkage along the growing polymer chain
• 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 still contain C=C double bonds (left over from the monomers) that micro-organisms attack easily
• DThey are made from petroleum, so micro-organisms recognise the monomers

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.

Why are amines basic?

• AThe nitrogen lone pair can donate a proton
• BThe nitrogen atom carries a full negative charge
• CThe nitrogen lone pair can accept a proton✓
• DThe amine group releases hydroxide ions in water

Answer: A lone pair on nitrogen accepts a proton, which is the Brønsted-Lowry definition of a base. That also makes amines nucleophiles, which is why they attack acyl chlorides and halogenoalkanes.

Why is phenylamine a weaker base than ethylamine?

• AThe nitrogen lone pair is delocalised into the benzene ring✓
• BThe benzene ring donates electron density onto the nitrogen
• CPhenylamine cannot hydrogen bond with any water molecules
• DThe nitrogen in phenylamine carries no lone pair of electrons

Answer: In phenylamine the lone pair on nitrogen overlaps the delocalised π system of the ring, so it is less available to accept a proton. Alkyl groups do the opposite, releasing electron density onto the nitrogen, which is why ethylamine is the stronger base of the two.

What is a zwitterion?

• AAn amino acid with both a positive and a negative charge✓
• BAn amino acid carrying no charges anywhere on the molecule
• CAn amino acid carrying two separate positive charges on it
• DAn amino acid that has lost its amine group entirely

Answer: At intermediate pH the carboxyl group donates its proton to the amine group, so the molecule carries both charges but is neutral overall. That is why amino acids have unexpectedly high melting points for their size.

What is the isoelectric point of an amino acid?

• AThe pH at which it exists mainly as a positive ion
• BThe pH at which it exists mainly as a negative ion
• CThe pH at which it exists mainly as a zwitterion✓
• DThe temperature at which the amino acid melts

Answer: At the isoelectric point the net charge is zero, so the amino acid does not migrate in an electric field — which is the basis of separating proteins by electrophoresis. It differs for each amino acid.

How does addition polymerisation differ from condensation polymerisation?

• AAddition releases a small molecule; condensation loses none
• BAddition needs two monomer types; condensation needs one
• CAddition produces only polyesters; condensation only polyalkenes
• DAddition loses no small molecule; condensation releases one✓

Answer: Addition polymers form from alkene monomers with nothing eliminated, so the repeat unit has the same formula as the monomer. Condensation polymers such as polyesters and polyamides lose water or hydrogen chloride at each link.

Why are polyesters biodegradable while poly(ethene) is not?

• AEster linkages can be hydrolysed but C–C bonds cannot✓
• BEster linkages cannot be hydrolysed but C–C bonds can
• CPolyesters are made only from natural monomers
• DPoly(ethene) dissolves readily in water over time

Answer: The polar ester bond is attacked by water, so the chain can be broken back into its monomers. Poly(ethene) is an unreactive hydrocarbon chain with no such vulnerable linkage, which is why it persists.

What linkage joins amino acids in a protein?

• AAn ester bond between hydroxyl and carboxyl groups
• BA glycosidic bond between two hydroxyl groups
• CA peptide bond between amine and carboxyl groups✓
• DA disulfide bond between two sulfur atoms

Answer: Condensation between the amine of one amino acid and the carboxyl of the next releases water and forms the peptide bond. Disulfide bridges do occur in proteins, but they cross-link the chain rather than build it.

How can a primary amine be prepared from a halogenoalkane?

• AHeating with excess water in a sealed tube
• BHeating with dilute sulfuric acid under reflux
• CHeating with acidified potassium dichromate
• DHeating with excess ammonia in a sealed tube✓

Answer: Ammonia acts as a nucleophile and displaces the halide. Excess ammonia is used because the amine produced is itself nucleophilic and will otherwise react on to secondary and tertiary amines.

Why does the secondary structure of a protein form?

• AHydrogen bonding between the R groups of the various amino acids
• BHydrogen bonding between backbone amine and carbonyl groups✓
• CIonic attraction between the oppositely charged R groups present
• DCovalent bonding between sulfur atoms in the backbone

Answer: Regular hydrogen bonds along the backbone produce alpha helices and beta pleated sheets. Interactions between R groups — ionic, disulfide, hydrogen — fold the chain further into the tertiary structure.