Topic 7 of 34
Look for accessible electron-rich and electron-poor sites
Bond polarity, delocalisation and access explain why similar-looking compounds react differently.
A-Level 9476 (2026-2027)
Look for accessible electron-rich and electron-poor sites
Bond polarity, delocalisation and access explain why similar-looking compounds react differently.
| Family | Electronic structure | Characteristic response |
|---|---|---|
| Alkane | Strong, nearly non-polar C-C and C-H sigma bonds; no readily available pi cloud or strongly polar centre. | Generally unreactive towards polar reagents under mild conditions; radical initiation or combustion conditions can open other pathways. |
| Alkene | Accessible, localised pi electron density above and below the carbon framework. | Donates electron density to electrophiles in addition reactions. |
| Benzene | Six pi electrons delocalised over the entire aromatic ring. | More resistant to addition than an alkene; substitution can restore the stabilised aromatic system after temporary disruption. |
| Halogenoalkane | Polar C-X bond with partially positive carbon. | A nucleophile can replace X; bond strength and steric/carbocation factors affect the pathway. |
| Carbonyl compound | C=O is polar, with partially positive carbon and partially negative oxygen. | Nucleophile attacks the carbon while pi electrons move onto oxygen. |
For otherwise comparable halogenoalkanes, hydrolysis generally becomes easier from RCl to RBr to RI because C-Cl is stronger than C-Br, which is stronger than C-I. The trend is not explained by the C-X polarity alone. Carbon skeleton, solvent and mechanism must also be comparable before interpreting a rate comparison.
In chlorobenzene, a chlorine lone pair overlaps with the ring π system. The C-Cl bond has partial double-bond character and is harder to break than in a comparable halogenoalkane. The rigid ring framework hinders the usual backside-attack geometry at the carbon bearing chlorine, while direct ionisation would require a very unstable phenyl cation. Chlorobenzene therefore resists ordinary nucleophilic substitution under the hydrolysis conditions used for halogenoalkanes.
Benzene still has electron density that can attack an electrophile, but disrupting its delocalised system costs stabilisation. A stronger electrophile or catalyst is commonly needed than for alkene addition. Subsequent loss of H+ restores aromaticity, explaining substitution rather than permanent addition across the ring.