Radical Selectivity, Stability and Allylic Positions
Radicals attack whichever hydrogen is easiest to remove, and the ranking follows the same stability logic as cations. Where a double bond sits next door, the resulting radical is delocalised and dominates the product mixture.
What a learner can do afterwards
- Ranks radical stability and links it to bond dissociation enthalpies
- Explains why bromination is more selective than chlorination using the relative energies of the abstraction step
- Predicts allylic and benzylic substitution and draws the delocalised radical that explains it
- Calculates a product ratio from stated relative reactivities and the number of hydrogens of each type
1 · Read
Placing the next atom where you want it starts with knowing which hydrogen leaves first, as you did with oxygen on propene. A radical takes the easiest hydrogen: tertiary beats secondary beats primary beats methyl. Allylic and benzylic hydrogens jump higher because the radical left behind is delocalised over neighbouring carbons. The same order rules carbocations, seen in bond strengths: the weaker the C-H bond, the stabler the radical and the faster the abstraction.
This ranking explains why bromine is picky and chlorine is not. Abstraction by bromine is uphill, so its transition state arrives late and looks like the radical product, so stability gaps bite hard. Abstraction by chlorine is downhill with an early transition state, so it barely distinguishes one C-H type from another.
Shine light on propene with bromine and you get 3-bromopropene, with bromine next to the double bond. The allylic radical spreads over two carbons, draw both forms to find each landing site. Vinylic hydrogens, sitting on the double bond, almost never break because their radicals cannot delocalise at all.
To predict a mixture, score each hydrogen type: multiply its hydrogen count by its relative reactivity, then compare. A substrate with 9 primary hydrogens of reactivity 1 scores 9, while its single tertiary hydrogen of reactivity 1600 scores 1600, so the tertiary product wins by a mile. Statistics only matter when the reactivities are close.
Radicals take the weakest C-H bond first, bromine is choosier than chlorine, and hydrogen count times reactivity gives the product ratio.
The rewritten reading was written for this dataset.
More to read
Free Radical Generation
A free radical carries an unpaired electron and forms by homolysis, one electron per fragment. Initiation creates them, propagation keeps the chain alive as each radical makes the next, and termination pairs them off. This three act structure repeats in halogenation, polymerization, and combustion alike.
Count reactive hydrogen sites
Alkanes have C-H bonds at different positions. A tertiary hydrogen is usually abstracted faster than a primary hydrogen, with selectivity depending on the radical. Primary hydrogens often outnumber tertiary ones, so product proportions reflect both their counts and reactivity.
20.1: Hydrocarbons
Hydrocarbon skeletons show where the weak C-H bonds live: branched, allylic, and benzylic sites. The bond that breaks most easily leaves behind the most stabilized radical, so bond strengths and radical stabilities are two faces of one ranking. Tables of dissociation enthalpies turn this ranking into numbers.
16.7: Polymers
Chain growth polymerization is radical chemistry at scale: an initiator radical adds to alkene after alkene, growing a chain that terminates by coupling. Allylic radicals also explain why some polymers crosslink and harden with age. The same selectivity rules from small molecule halogenation govern these giant chains.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.