Alkanes react with halogens (Cl₂, Br₂) in presence of UV light or heat via free radical mechanism. The reaction involves three steps: Initiation (Cl₂ → 2Cl• by UV), Propagation (Cl• abstracts H from alkane → alkyl radical; alkyl radical + Cl₂ → alkyl chloride + Cl•), and Termination (two radicals combine). Selectivity: Br₂ is more selective (tertiary > secondary > primary) while Cl₂ is less selective. UV light or high temperature is necessary — reaction does NOT occur in dark at room temperature.

HNO₂ = nitrous acid, formed in situ: NaNO₂ + HCl → HNO₂ + NaCl. Primary aliphatic amines (R-NH₂): Highly unstable diazonium salt [RN₂]⁺ formed instantly decomposes at room temperature itself → gives alcohol (R-OH) with water, or alkene, or other products with loss of N₂. Cannot be isolated. Primary aromatic amines (Ar-NH₂): Stable diazonium salt [ArN₂]⁺Cl⁻ forms at 0–5°C — can be isolated and used in coupling reactions (azo dyes). Aryl diazonium salts are stable below 5°C because resonance stabilises the positive charge.

Amines are basic due to the lone pair on nitrogen. Basicity order in gas phase: tertiary > secondary > primary > NH₃. In aqueous solution: secondary > primary > tertiary > NH₃ (due to solvation effects — tertiary amine's bulky groups hinder solvation of the conjugate acid). Aromatic amines are much less basic than aliphatic amines because the lone pair on N is delocalised into the benzene ring, making it less available for protonation. Aniline (pKb ≈ 9·4) is far less basic than methylamine (pKb ≈ 3·4).

When dehydrohalogenation can give multiple alkenes: Saytzeff's rule: more substituted (stable) alkene is the major product — applies with small base (like KOH in alcohol). Hofmann's rule: less substituted alkene is major product — applies with bulky base (like t-BuOK). In this reaction (C₂H₅Cl + alc. KOH): only one possible alkene (ethene), so no ambiguity. The alkyl chloride has only primary hydrogens — only one product possible.

Aromatic diazonium salts react with Cu₂X₂ (X = Cl, Br, CN) to replace -N₂⁺ with -X group — called Sandmeyer reaction. ArN₂⁺Cl⁻ + Cu₂Cl₂ → Ar-Cl + N₂. ArN₂⁺Cl⁻ + Cu₂Br₂ → Ar-Br + N₂. ArN₂⁺Cl⁻ + CuCN → Ar-CN + N₂. Balz-Schiemann: ArN₂⁺BF₄⁻ → Ar-F + N₂ + BF₃. Gattermann: ArN₂⁺ + HX + Cu → Ar-X (uses Cu powder, not Cu₂X₂). These are important methods to introduce halogens onto benzene ring — direct halogenation gives different orientation.

Aromatic diazonium salts couple with phenols or tertiary aromatic amines to form azo compounds (R-N=N-R') — these are intensely coloured and used as dyes (azo dyes). Coupling occurs at the para position (or ortho if para is blocked). The reaction is electrophilic aromatic substitution where the diazonium ion is the electrophile. Example: benzenediazonium chloride + aniline → para-aminoazobenzene (orange-red dye). Congo red, Methyl orange are important azo dyes tested in NEET.