Read Radical Stability Before Predicting the Major Halogenation Product
Free-radical halogenation questions come down to two things — which C-H bond breaks to give the most stable radical (deciding the major product), and how many chemically distinct hydrogens a molecule has (deciding how many monohalogenation products are possible).
Free-Radical Halogenation, Combustion and Nitration of Alkanes is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Counting monohalogenation products from symmetry
The number of monochlorination products (ignoring stereoisomers) equals the number of chemically DISTINCT hydrogen environments in the molecule — n-butane, with only 2 distinct environments (the two equivalent terminal CH3 groups, and the two equivalent internal CH2 groups), gives exactly 2 monochloro products, while a more substituted, less symmetrical molecule gives more.
Major product follows radical stability: 3° > 2° > 1°
n-Butane brominated with light gives predominantly the 2-bromo (secondary) product, not the 1-bromo (primary) product, because bromination is selective for the more stable secondary radical intermediate — bromination is considerably MORE selective for the more-substituted position than chlorination is, which is comparatively indiscriminate.
Halogen reactivity in the chain-propagation step
Fluorine reacts fastest in the chain-propagating step (most exothermic, least selective, often too violent to control usefully); iodination is so endothermic it barely proceeds at all under normal conditions — reactivity order F > Cl > Br > I is the mirror image of selectivity order (I would be most selective if it reacted at all, F is least selective).
Combustion and nitration are separate radical-adjacent alkane reactions
Complete combustion of methane gives CO2 + H2O; INCOMPLETE combustion (limited O2) gives elemental carbon (soot) + H2O instead — distinguishing complete from incomplete combustion is about oxygen sufficiency, not about which hydrocarbon is burnt. Alkane nitration (vapour-phase, high temperature, unlike aromatic nitration's different mechanism) can break C-C bonds during the process, so propane's nitration gives a MIXTURE including smaller nitroalkanes, not just position-specific substitution on the intact carbon skeleton.
OlympIQ exam lens
- ✓Number of monohalogenation products = number of chemically distinct H environments in the molecule (by symmetry).
- ✓Radical halogenation favours the more substituted (more stable) radical: 3° > 2° > 1° — bromination is far more selective for this than chlorination.
- ✓Halogen reactivity in the propagation step: F > Cl > Br > I — the opposite order from selectivity.
- ✓Incomplete combustion (limited O2) gives elemental carbon + water, not CO2 + water.
Don't assume chlorination and bromination give the same major product ratio — bromination is much more selective for the more substituted (more stable) radical, while chlorination reacts comparatively indiscriminately across different C-H environments.
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