Lecture 17 – Nitrogen Compounds (H2 Chemistry 9729)

A comprehensive set of question-and-answer flashcards covering nomenclature, preparation, reactions, physical properties, and biological roles of amines, amides, amino acids, and proteins, with emphasis on reaction conditions, mechanistic reasoning, and acidity/basicity trends relevant to A-level H2 Chemistry.

Name the four main classes of organic nitrogen compounds covered in this lecture.

Amines, Amides, Amino Acids, and Proteins (polypeptides).

What differentiates primary, secondary and tertiary amines?

The number of alkyl/aryl groups directly attached to the nitrogen atom (1, 2 or 3 respectively).

Give the general formulae for (i) a primary amine and (ii) a tertiary amide.

(i) RNH₂ (ii) RCONR’R’’

Which type of ammonium species is ionic and therefore solid at room temperature?

Quaternary ammonium salts (R₄N⁺ X⁻).

Why do 1° and 2° amines have higher boiling points than alkanes of similar Mr?

They can form intermolecular hydrogen bonds; alkanes can only form weak instantaneous dipole–induced dipole interactions.

Why do 3° amines have lower boiling points than 1° and 2° amines of similar Mr?

3° amines lack an N–H bond and therefore cannot hydrogen-bond with each other.

State two laboratory reagents (with conditions) that can reduce a nitrile to a primary amine.

(i) H₂(g) with Ni catalyst, heat (ii) LiAlH₄ in dry ether at room temperature.

Why is LiAlH₄ a stronger reducing agent than NaBH₄?

Aluminium is less electronegative than boron, so the Al–H bond is more polar and releases hydride (H⁻) more readily.

Outline the reagents and conditions to convert nitrobenzene into phenylamine.

Sn and conc. HCl, heat under reflux, then add NaOH(aq) to liberate phenylamine (or catalytic hydrogenation with H₂/Pd).

Write the overall ionic half-equation for the reduction of nitrobenzene with Sn/HCl.

C₆H₅NO₂ + 6 H⁺ + 6 e⁻ → C₆H₅NH₂ + 2 H₂O

What observation is made when aqueous bromine is added to phenylamine at room temperature?

Orange Br₂(aq) is decolourised and a white precipitate of 2,4,6-tribromophenylamine forms with white fumes of HBr.

How can phenylamine be distinguished from an aliphatic amine?

Phenylamine rapidly decolourises Br₂(aq) giving a white ppt; aliphatic amines do not react with Br₂(aq).

Explain why phenylamine is a much weaker base than ethylamine.

The lone pair on N is delocalised into the benzene π-system, decreasing its availability for protonation.

Arrange the following in order of increasing basicity in aqueous medium: ammonia, ethylamine, phenylamine.

Phenylamine < Ammonia < Ethylamine.

Why is a tertiary amine a stronger Lewis base in the gas phase yet the weakest Bronsted base in water?

Gas phase: three electron-donating alkyl groups increase electron density on N. In water: bulky substituents produce steric hindrance and poor solvation of R₃NH⁺, lowering basicity.

What type of reaction occurs between an acyl chloride and a primary amine, and what are the products?

Condensation; a secondary amide and HCl (steamy white fumes).

Why do carboxylic acids not form amides with amines under normal conditions?

The acid–base neutralisation (salt formation) is favoured; the condensation step is not sufficiently activated.

Explain why simple amides are neutral to litmus.

The lone pair on N is delocalised into the C=O group, removing its basicity; hence amides neither accept protons readily nor donate them.

Which carbonyl derivative is most electrophilic: acyl chloride, ester, or amide?

Acyl chloride (due to strong –Cl electron-withdrawing effect).

State reagents and conditions for acidic hydrolysis of an amide.

Dilute HCl(aq) or H₂SO₄(aq), heat under reflux.

What gaseous product is evolved when a primary amide is heated with NaOH(aq)?

Ammonia gas, which turns moist red litmus blue.

What is the outcome when an amide is reduced by LiAlH₄?

The carbonyl is reduced to a methylene group, giving an amine (RCONH₂ → RCH₂NH₂).

Give two reasons for Kevlar’s exceptional tensile strength.

(i) Extensive hydrogen bonding between polymer chains (ii) Rigid, linear chains due to aromatic rings and amide linkages.

Define a zwitterion.

A dipolar ion containing both positive and negative charges but overall electrically neutral.

Why do amino acids have high melting points compared with comparable Mr molecules?

They exist in the solid state as ionic zwitterions that form a giant ionic lattice with strong ionic attractions.

What is the isoelectric point (pI) of an amino acid?

The pH at which the amino acid exists predominantly as a zwitterion with zero net charge.

Describe how a zwitterion acts as a buffer when small acid is added.

The carboxylate (–COO⁻) group accepts H⁺, shifting equilibrium to maintain pH.

What happens to an amino acid with pI 5.5 placed in buffer pH 8.0 during electrophoresis?

It becomes negatively charged and migrates towards the anode (+).

State the functional groups involved in a peptide (amide) bond.

The α-carboxyl group of one amino acid and the α-amino group of another.

During protein acid hydrolysis, what form do the amino groups take?

They are protonated to –NH₃⁺.

Write the abbreviation sequence for the tripeptide formed from glycine, alanine and cysteine in that order.

Gly-Ala-Cys.

How many water molecules are removed when four amino acids form a tetrapeptide?

Three water molecules (one per peptide bond).

Which reagent and conditions are used for complete alkaline hydrolysis of proteins?

Excess NaOH(aq), heat under reflux for several hours.

Explain how partial hydrolysis aids peptide sequencing.

It yields overlapping fragments whose amino-acid sequences can be assembled by matching overlaps to deduce the overall order.

What two half-equivalence points appear on the titration curve of a simple amino acid without ionisable R group?

pH = pKa₁ (COOH/COO⁻) and pH = pKa₂ (NH₃⁺/NH₂).

Calculate the approximate pI of glycine given pKa₁ 2.34 and pKa₂ 9.60.

pI ≈ (2.34 + 9.60)/2 = 5.97.

For lysine (pI 9.7) placed in pH 6.5 buffer, predict its charge.

Positively charged (protonated) and will migrate towards the cathode (–).

What observation indicates formation of an amine salt when HCl is added to an amine?

The fishy odour disappears and a white crystalline solid (amine hydrochloride) may form on evaporation.

Give the order of boiling points for compounds of similar Mr: propane, ethylamine, ethanol, methanoic acid.

Propane (lowest) < Ethylamine < Ethanol < Methanoic acid (highest).

Why can tertiary amines still dissolve in water despite lacking N–H bonds?

They accept hydrogen bonds from water (lone pair on N), forming hydrogen-bond interactions with solvent molecules.

State two solvents in which amines are soluble apart from water.

Ether and benzene (due to compatible id-id interactions with hydrocarbon chains).

What type of isomerism arises due to restricted rotation about the C–N bond in an amide?

Cis-trans (geometric) isomerism around the planar amide linkage.

Which of the following is more hydrolysis-resistant and why: ester or amide?

Amide, because greater delocalisation of N lone pair into C=O lowers electrophilicity of carbonyl carbon and strengthens C–N bond.

Name the reaction type and conditions for converting an acid chloride to a primary amide with ammonia.

Condensation; react acid chloride with excess NH₃ at room temperature (anhydrous).

Why does NaBH₄ fail to reduce nitriles while LiAlH₄ succeeds?

NaBH₄ is a weaker hydride donor; nitriles require a stronger reducing agent to break the C≡N bond.

What distinguishes N-methylphenylamine from 2-methylphenylamine in nomenclature?

‘N-’ indicates the methyl group is attached to nitrogen, whereas the number indicates substitution on the benzene ring.

Provide the formula for the conjugate acid of dimethylamine.

(CH₃)₂NH₂⁺

Give a qualitative test to differentiate a primary amide from a primary amine using NaOH.

Heat with NaOH(aq): primary amide evolves NH₃(g) (turns moist red litmus blue), primary amine does not release gas.

Which polymer is made from 1,4-benzene-dicarboxylic acid and 1,4-diaminobenzene?

Kevlar (a polyamide).

Explain the trend of amine basicity in the gas phase.

3° > 2° > 1° because more electron-donating alkyl groups increase electron density on N, enhancing Lewis basicity.

Explain why trimethylamine is less basic than dimethylamine in water.

Bulky three alkyl groups hinder proton approach and the R₃NH⁺ ion is poorly solvated, lowering base strength.

What is produced when phenylamine reacts with acyl chloride?

An aromatic amide (e.g., C₆H₅NHCOCH₃) and HCl.

Describe the hybridisation of nitrogen in (i) ammonia, (ii) phenylamine, (iii) an amide.

(i) sp³, trigonal pyramidal (ii) sp², trigonal planar (lone pair in p orbital overlapping with benzene) (iii) sp², trigonal planar (lone pair in p orbital delocalised into C=O).

Why is pyridine more basic than phenylamine?

Pyridine’s lone pair resides in an sp² orbital orthogonal to the aromatic π system, remaining available for protonation; phenylamine’s lone pair is delocalised into the ring.

State the outcome of reacting bromine water with phenol versus phenylamine.

Both give white 2,4,6-tribromo products and decolourise Br₂(aq); phenol additionally gives violet colour with neutral FeCl₃, allowing distinction.

What structural feature makes β-alanine different from α-alanine?

In β-alanine (3-aminopropanoic acid), the –NH₂ is on the β-carbon (next-but-one) rather than the α-carbon adjacent to –COOH.

How many different dipeptides can be formed from lysine and serine?

Two: Lys-Ser and Ser-Lys (side-chain amino of lysine does not form the peptide bond).

Which reagent set fully reduces an amide but does not affect a benzene ring?

LiAlH₄ in dry ether (room temperature).

Predict the product of acidic hydrolysis of CH₃CONHCH₃.

CH₃COOH and CH₃NH₃⁺Cl⁻ (if HCl used).

In amino-acid electrophoresis at pH 3, which amino acid (glu pI 3.1, his pI 7.6) migrates towards the cathode?

Histidine (positively charged); glutamic acid is near its pI and barely moves.

Give the Henderson–Hasselbalch relationship for an amino acid’s first acidic group.

pH = pKa₁ + log([A⁻]/[HA]) for the COOH/COO⁻ pair.

What is the maximum number of peptide linkages in a tripeptide?

Two peptide (amide) bonds.

Define a polyamide and give one synthetic example.

A polymer whose repeating units are connected by amide linkages; examples include Nylon-6,6 and Kevlar.

Explain why energy released on forming id-id interactions is insufficient to dissolve long-chain amides in water.

Long hydrophobic chains increase id-id attractions among amide molecules; energy from interactions with water cannot overcome strong H-bonds in water plus id-id in amide, so solubility decreases.

Which functional group in proteins is cleaved during digestion by protease enzymes?

The peptide (amide) linkage –CONH–.

How does LiAlH₄ reduce an ester compared to an amide?

Ester → two alcohols; amide → amine (carbonyl reduced to CH₂).

During titration of alanine, at which added OH⁻ mole value does the pH equal the pI?

At 1.0 mol OH⁻ per mol alanine (first equivalence point).

State the pI expression for amino acids without ionisable side chains.

pI = ½(pKa₁ + pKa₂).

Which amino acid has no stereogenic centre and why?

Glycine; its α-carbon is bonded to two identical hydrogen atoms, so it is achiral.

Why does electron withdrawal by 2,4,6-trichloro groups decrease phenylamine’s basicity?

Inductive/mesomeric –I/–M effects pull electron density away from N, increasing lone-pair delocalisation into the ring and reducing availability for protonation.

Give an equation showing ethylamine acting as a Bronsted base in water.

CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻

What observation accompanies reaction of an acyl chloride with ammonia?

Steamy white fumes of HCl gas are produced.

Which amino acid side chain contains a thiol group and what is its approximate pKa?

Cysteine; pKa (SH) ≈ 8.3.