---
title: "Radical Selectivity, Stability and Allylic Positions"
description: "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 domin"
canonical: https://lightmysky.com/learn/science/radical-selectivity-stability-and-allylic-positions-mt_tgIlRLvc9x
source: https://lightmysky.com/learn/science/radical-selectivity-stability-and-allylic-positions-mt_tgIlRLvc9x.md
retrieved: 2026-09-12
---

> **Agent view.** This is the Markdown twin of the page, for tools and assistants.
> When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md
> API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md
> Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt
> Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json
> Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`.

# 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.

Subject: Science · Area: Chemistry · Ages 19 to 21
Page: https://lightmysky.com/learn/science/radical-selectivity-stability-and-allylic-positions-mt_tgIlRLvc9x

## Ready when they can

- 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

## Lesson: Which hydrogen gets taken first

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.

**Example.** 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.

**Recap.** Radicals take the weakest C-H bond first, bromine is choosier than chlorine, and hydrogen count times reactivity gives the product ratio.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Hydroboration, Oxidation and Cleavage of Alkenes and Alkynes](https://lightmysky.com/learn/science/hydroboration-oxidation-and-cleavage-of-alkenes-and-alkynes-mt_Ny7vC57rqc)
- [Free-Radical Substitution in Alkanes](https://lightmysky.com/learn/science/free-radical-substitution-in-alkanes-mt_QkreYC9e2d)

## Opens up

- [Chain Growth and Step Growth: Two Routes to a Polymer](https://lightmysky.com/learn/science/chain-growth-and-step-growth-two-routes-to-a-polymer-mt_LADBgarlKs)
- [Nucleophilic Addition and the Tetrahedral Intermediate](https://lightmysky.com/learn/science/nucleophilic-addition-and-the-tetrahedral-intermediate-mt_N9Eydek3s7)
- [Photoredox and Electrochemical Routes to Radical Intermediates](https://lightmysky.com/learn/science/photoredox-and-electrochemical-routes-to-radical-intermediates-mt_YhqzHGcx31)