Q1(a). Cycloheptatrienyl (tropylium) bromide: an ionic organic bromide

Introduction. An ordinary alkyl halide is a covalent, weakly polar molecule: it dissolves in non-polar solvents and not in water. Cycloheptatrienyl bromide (C₇H₇Br) behaves like a salt, because its cation, C₇H₇⁺, is aromatic and very stable.

Scheme: The C–Br bond breaks because the cation formed is aromatic: the compound is tropylium bromide, C₇H₇+Br⁻.

1. Structure of the cation. In C₇H₇⁺ all seven carbons are sp² and the ring is planar. Seven p orbitals overlap in a closed loop and hold six π electrons (4n+2, n=1). The cation is aromatic (Hückel rule), and the positive charge is spread equally over all seven carbons; all seven C–H protons are equivalent.
2. Consequence. Ionisation of the C–Br bond turns a non-aromatic triene into an aromatic ion, so the compound exists as an ion pair C₇H₇+Br⁻ even in the solid. It is a crystalline salt that melts above 200°C, unlike liquid alkyl bromides of similar mass.
3. Water. Water is highly polar (high dielectric constant) and solvates both ions by ion–dipole forces: the O lone pairs face the cation and the O–H dipoles hydrogen-bond to Br⁻. The lattice energy is repaid, so the salt is readily soluble in water.
4. Non-polar solvents. Hexane, benzene or CCl₄ cannot solvate ions; dissolving the ionic lattice would cost the lattice energy with nothing in return, so the compound is insoluble in non-polar solvents.
5. Supporting test. Aqueous AgNO₃ gives an immediate precipitate of AgBr at room temperature, a test that a covalent alkyl bromide passes only slowly. Br⁻ is already present as an ion.
6. Contrast. 5-Bromocyclopenta-1,3-diene does not ionise: its cation would hold four π electrons and be antiaromatic. Ring size and electron count, not just the C–Br bond, decide the behaviour.

Conclusion. Cycloheptatrienyl bromide is really tropylium bromide, an ionic salt of the aromatic 6π cation C₇H₇⁺. Like any salt, it dissolves in water, which solvates the ions, and not in non-polar solvents.

Application. The tropylium ion is a hydride acceptor used to oxidise C–H bonds, and the same ion (m/z 91) is the base peak in the mass spectra of toluene and other benzyl compounds.