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.
2 · Watch
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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.