34. Nitrogen compounds

Primary and secondary amines can be prepared by nucleophilic substitution of halogenoalkanes with ammonia or a primary amine, and by reducing suitable nitrogen-containing compounds such as nitriles or amides where specified.

The nitrogen substituent count determines the product: ammonia gives a primary amine after one alkylation, while a primary amine can give a secondary amine. Excess amine helps limit further substitution.

Bromoethane + excess NH₃ → ethylamine; ethylamine + bromoethane can then form diethylamine if further substitution is allowed.

Using ammonia does not guarantee only one product unless conditions control successive alkylation; primary, secondary and tertiary amines can form.

Ammonia or an amine reacts with an acyl chloride at room temperature to form an amide. The nitrogen nucleophile attacks the carbonyl carbon, chloride leaves, and HCl is produced.

Ammonia gives a primary amide; a primary amine gives an N-substituted secondary amide. A second equivalent of amine or another base can remove the HCl.

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl overall. With methylamine, the product is N-methylacetamide rather than acetamide.

Do not call this simple acid–base neutralisation or write an alcohol product; the nitrogen remains attached to the acyl carbon.

An amine is a Brønsted–Lowry base: its nitrogen lone pair accepts a proton from water, producing an alkylammonium ion and hydroxide.

The equilibrium is partial, so aqueous amines are weak bases. The position depends on how available the lone pair is and on electron-donating or withdrawing groups.

Ethylamine + H₂O ⇌ ethylammonium ion + OH⁻. The solution is alkaline even though most ethylamine molecules remain unprotonated.

A weak base is not a base that cannot react; it is one whose protonation equilibrium is incomplete.

Benzene is first nitrated to nitrobenzene. Hot tin and concentrated hydrochloric acid reduce the nitro group to an ammonium salt; aqueous sodium hydroxide then liberates phenylamine.

Treat it as a three-stage route: electrophilic substitution, reduction in acid, then basification. Separating the stages prevents the acid–base work-up from being missed.

C₆H₆ → C₆H₅NO₂ → C₆H₅NH₃⁺Cl⁻ → C₆H₅NH₂. NaOH converts the ammonium salt to the free amine.

The reduction product in the acidic mixture is not immediately free phenylamine; it is protonated until alkali is added.

The –NH₂ group activates the aromatic ring: phenylamine reacts with bromine water at room temperature to give rapid substitution. With nitrous acid made from NaNO₂ and dilute acid below 10 °C, it forms a diazonium salt; warming with water gives phenol.

Keep the conditions separate: bromination is an aromatic substitution, while diazotisation requires cold acid and the next hydrolysis step requires warming.

Phenylamine + Br₂(aq) gives 2,4,6-tribromophenylamine; the diazonium salt route then releases N₂ as phenol forms.

Do not warm during diazotisation or use bromine conditions to infer the diazonium route.

In water the usual basicity order is ethylamine > ammonia > phenylamine. An ethyl group donates electron density towards nitrogen, while in phenylamine the lone pair is delocalised into the benzene ring.

A more available lone pair accepts H⁺ more readily. Solvation and the aqueous environment matter, so use the syllabus order rather than a gas-phase shortcut.

Ethylamine produces a higher equilibrium concentration of OH⁻ than ammonia at comparable conditions; phenylamine’s ring delocalisation makes proton acceptance less favourable.

Phenylamine is still basic. Delocalisation reduces its basicity; it does not remove the nitrogen lone pair.

Azo compounds contain an –N=N– group connecting carbon-containing groups, commonly two aromatic rings. Their extended conjugation allows strong absorption in the visible region and often produces intense colours.

In syllabus chemistry, azo compounds are linked to diazonium salts and coupling reactions; the N=N unit is the identifying structural feature.

An azo dye can be represented as Ar–N=N–Ar′, where substituents on either ring tune colour and solubility.

The azo link is not an amide or an amine: it has two nitrogens joined by a nitrogen–nitrogen double bond.

Ammonia reacts with an acyl chloride to make a primary amide; a primary amine makes an N-substituted amide. Both are room-temperature condensation reactions with HCl produced.

The nitrogen lone pair attacks the carbonyl carbon and chloride leaves. Use a second equivalent of ammonia or amine to remove the HCl in the overall equation.

CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl; CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl.

Do not count the second base molecule as part of the amide structure; it neutralises the HCl by-product.

Hydrolysis breaks the amide C–N bond. Aqueous acid gives a carboxylic acid and an ammonium ion; aqueous alkali gives a carboxylate salt and ammonia or an amine.

Heating is normally required because the amide is resonance-stabilised. The reaction conditions determine whether the nitrogen product is protonated.

CH₃CONH₂ + H₂O/H⁺ → CH₃CO₂H + NH₄⁺; with aqueous NaOH the products are CH₃CO₂⁻Na⁺ and NH₃.

Do not write the same nitrogen product for acid and alkaline hydrolysis; the acid–base work-up changes its form.

In an amide, the nitrogen lone pair overlaps with the carbonyl π system. Delocalisation gives the C–N bond partial double-bond character and makes the lone pair much less available to accept H⁺ than an amine lone pair.

The carbonyl also withdraws electron density, reinforcing the lower basicity. Protonation is more favourable at oxygen than at nitrogen in many contexts.

An aqueous amine readily forms an ammonium ion, whereas an amide is only weakly protonated under comparable conditions.

Amides still contain a nitrogen lone pair; “weak base” does not mean “no basic behaviour”.

An amino acid contains both a carboxylic acid and an amine. In water it can transfer a proton internally, forming a zwitterion with –NH₃⁺ and –COO⁻ groups.

At low pH the molecule is more positively charged; at high pH it is more negatively charged. The isoelectric point is the pH at which the average net charge is zero.

Glycine is mainly a zwitterion near its isoelectric point, but becomes cationic in acid and anionic in alkaline solution.

A zwitterion is not electrically neutral at every site: it has internal charges whose sum may be zero.

Condensation between the carboxylic acid group of one amino acid and the amino group of another forms an amide (peptide) bond, –CO–NH–, and removes water.

The order of amino acids matters: the free amino end is the N-terminus and the free carboxyl end is the C-terminus. A dipeptide has two residues; a tripeptide has three.

Glycine + alanine can form Gly–Ala or Ala–Gly, which are different sequences even though they contain the same two amino acids.

Peptide formation is not ionic attraction between zwitterions; it is covalent condensation at the functional groups.

In an electric field, an amino acid or peptide moves toward the electrode of opposite charge. Its direction and speed depend on net charge, pH relative to its isoelectric point, and size/shape.

At pH below pI the species is net positive; above pI it is net negative; at pI it has no net migration. Compare every component at the stated pH.

At a pH below glycine’s pI, glycine moves toward the negative electrode; at a pH above its pI it moves toward the positive electrode.

“All amino acids move to the same side” ignores their different pI values and the chosen buffer pH.