Unit 24: Organic Nitrogen Compounds — Short Questions

1. Ammonolysis of chlorobenzene: C₆H₅Cl + 2NH₃ --Cu₂O, 250 °C, 50 atm--> C₆H₅NH₂ + NH₄Cl.
2. Reduction of nitrobenzene: C₆H₅NO₂ + 6[H] --Sn/conc. HCl--> C₆H₅NH₂ + 2H₂O.
(Phenylamine can also be made by the Hofmann reaction on benzamide: C₆H₅CONH₂ + Br₂ + 4NaOH → C₆H₅NH₂ + 2NaBr + Na₂CO₃ + 2H₂O.)

The chromophore in azo dyes is the azo group, –N=N– (as in Ph–N=N–Ph). This group is responsible for the intense colour of the dye; depending on the other groups attached to the benzene rings, it produces colours ranging from yellow and orange to red, blue and green.

In an amide, the lone pair on the nitrogen atom is delocalised (by resonance) into the carbonyl group, i.e. R-C(=O)-NH₂ ↔ R-C(-O⁻)=NH₂⁺. This delocalisation reduces the C–N bond length (132 pm, shorter than a normal C–N single bond of 147 pm) and greatly reduces the availability of the nitrogen lone pair for donation to a proton, so amides are much weaker bases than ammonia and amines and give neutral solutions in water.

A zwitter ion is the dipolar form of an amino acid in which a proton has transferred internally from the –COOH group to the –NH₂ group, giving a –COO⁻ group and an –NH₃⁺ group on the same molecule. It carries both a positive and a negative charge but has no overall net charge, and is also called the internal salt of the amino acid.

Primary amines (RNH₂) have one alkyl/aryl group attached to the nitrogen atom, e.g. CH₃NH₂ (methanamine). Secondary amines (R₂NH / RNHR′) have two alkyl/aryl groups attached to nitrogen, e.g. CH₃NHCH₃ (N-methylmethanamine). Tertiary amines (R₃N) have three alkyl/aryl groups attached to the same nitrogen atom, e.g. (CH₃)₃N (N,N-dimethylmethanamine). The branching that decides the class occurs at the nitrogen atom, not at the carbon attached to it (unlike alcohols or haloalkanes).

1. Propan-1-amine (1°): CH₃-CH₂-CH₂-NH₂
2. Propan-2-amine (1°): CH₃-CH(NH₂)-CH₃
3. N-methylethanamine (2°): CH₃-CH₂-NH-CH₃
4. N,N-dimethylmethanamine / trimethylamine (3°): (CH₃)₃N

A peptide bond (peptide link) is the amide bond (–CO–NH–) formed between the –COOH group of one amino acid and the –NH₂ group of another amino acid, with the elimination of a water molecule (a condensation reaction). A tripeptide is formed from three amino acids joined together, and since each new linkage forms one peptide bond, a tripeptide contains two peptide bonds.

With two different amino acids and three positions in the chain, 2³ = 8 tripeptides are possible: Gly-Gly-Gly, Gly-Gly-Ala, Gly-Ala-Gly, Ala-Gly-Gly, Gly-Ala-Ala, Ala-Gly-Ala, Ala-Ala-Gly, Ala-Ala-Ala.

A = Nitrobenzene, C₆H₅NO₂ (nitration of benzene).
B = Phenylamine (aniline), C₆H₅NH₂ (reduction of the nitro group by Sn/HCl).
C = 2,4,6-Tribromophenylamine (excess Br₂ substitutes at the two ortho and the para positions directed by –NH₂).
D = 2,4,6-Tribromobenzenediazonium chloride (diazotisation of the amine with NaNO₂/HCl below 10 °C).
E = 1,3,5-Tribromobenzene (H₃PO₂ reductively replaces the diazonium group –N₂⁺ with –H, and the three Br's, originally 2,4,6 relative to the amino carbon, become equivalent to the 1,3,5 positions once that carbon bears only H).

1. Nucleophilic substitution of a halogenoalkane with excess ethanolic ammonia under pressure (to favour the primary amine over secondary/tertiary products): CH₃CH₂Br + NH₃(excess) --ethanol, high pressure--> CH₃CH₂NH₂ + HBr.
2. Reduction of an alkanenitrile with LiAlH₄ or H₂/Ni: CH₃CH₂CN + 4[H] --LiAlH₄--> CH₃CH₂CH₂NH₂.
(Primary amines can also be made by reducing a nitroalkane or an unsubstituted amide with LiAlH₄, or by the Hofmann reaction on an amide.)