Source: Purdue University Organic Chemistry lecture notes
Tags: bromine radical, free radical halogenation, radical chain mechanism, homolytic cleavage, initiation propagation termination, alkane halogenation, Br2 hv
Difficulty: Intermediate | Prerequisites: basic bond-line structures, Lewis structures, electronegativity, bond dissociation energies
Free radical halogenation is one of the first reaction mechanisms you meet in organic chemistry, and it sets the template for every chain reaction that follows. It explains how a simple alkane (which is otherwise quite unreactive) can be converted into an alkyl halide using molecular bromine and either heat or UV light. The mechanism runs on radicals, species with an unpaired electron, and understanding their behaviour here gives you the toolkit for later topics like polymerisation and atmospheric chemistry. You should already be comfortable drawing Lewis structures and recognise the difference between homolytic and heterolytic bond cleavage.
When Br2 is exposed to heat or UV light (hv), it splits into two bromine radicals. Those radicals then abstract hydrogen atoms from an alkane, creating a carbon radical that reacts with another Br2 molecule, replacing one H with Br. The cycle repeats until two radicals collide and cancel each other out.
Free radical (radical)
A species with an unpaired electron. Radicals are highly reactive because that lone electron wants a partner. Think of it as a molecule with an "empty seat" that will grab the nearest available electron.
Homolytic cleavage (homolysis)
Bond breaking in which each atom keeps one electron from the shared pair, producing two radicals. In simple terms, this means the bond splits evenly, one electron to each side.
Half-arrow (fishhook arrow)
A curved arrow with a single barb, showing the movement of one electron rather than a pair. Think of it as the radical version of a regular curved arrow.
Initiation
The first step of a radical chain reaction, where energy input (heat or light) breaks a bond homolytically to generate radicals from non-radical starting materials. In simple terms, this is the spark that starts the chain.
Chain propagation
The self-sustaining middle steps where one radical reacts and produces a new radical, keeping the chain going. Think of it as a relay race: every runner hands the baton (the unpaired electron) to the next.
Chain termination
Any step in which two radicals combine, destroying both unpaired electrons and ending the chain. In simple terms, two radicals bump into each other and cancel out.
Radical intermediate
The short-lived carbon radical formed mid-reaction when a hydrogen is abstracted from the alkane. It exists only long enough to react with the next Br2 molecule.
Radical stability order
The ranking of how stable a carbon radical is, based on substitution: 3° > 2° > 1° > methyl. More alkyl groups around the radical carbon stabilise it through hyperconjugation. Think of it as the same trend you already know from carbocation stability.
If the reaction conditions include heat (Δ) or light (hv), expect a radical mechanism.
Radical reactions involve homolytic bond cleavage and half-arrows (single-barbed curved arrows), not the full arrows used in ionic mechanisms.
Energy input (UV light or heat) breaks the Br–Br bond homolytically.
Each bromine atom keeps one electron from the shared pair.
Result: two bromine radicals (Br·), each with an unpaired electron.
Notation: half-arrows (fishhook arrows) show one electron moving to each Br atom.
Propagation has two sub-steps, each consuming one radical and producing another:
Sub-step A: Hydrogen abstraction. A bromine radical (Br·) removes a hydrogen atom from the alkane (e.g. ethane). This forms HBr and leaves behind a carbon radical on the alkane.
Sub-step B: Halogen transfer. The carbon radical attacks a Br2 molecule, taking one bromine atom. This forms the alkyl bromide product and regenerates a new Br· radical.
The new Br· feeds back into Sub-step A, so the cycle repeats. This is why it is called a chain reaction.
Termination occurs whenever two radicals collide and their unpaired electrons form a new covalent bond. Possible termination events:
Br· + Br· → Br2 (reforms molecular bromine)
Carbon radical + Br· → alkyl bromide (forms product, but ends the chain)
Carbon radical + carbon radical → a coupled alkane (a side product)
Termination is statistically rare because radical concentrations are low at any given moment, so most radicals undergo propagation instead.
Two carbon radicals can combine during termination to form a longer-chain alkane (e.g. two ethyl radicals joining to make butane).
These coupling products are typically minor because the chance of two carbon radicals meeting is small compared to a carbon radical meeting Br2.
The stability order for carbon radicals mirrors the order for carbocations:
3° (tertiary) > 2° (secondary) > 1° (primary) > methyl
More substituted radicals are more stable because neighbouring C–H bonds can donate electron density through hyperconjugation. This means bromine preferentially abstracts a hydrogen from the most substituted carbon, making the most stable radical intermediate and yielding the most substituted alkyl bromide as the major product.
Bromine is notably more selective than chlorine. Chlorine radicals are so reactive they abstract almost any hydrogen with little preference, whereas bromine is selective enough to favour the more stable radical.
Overall reaction (ethane example):
CH3CH3 + Br2 → (hv or Δ) → CH3CH2Br + HBr
Initiation:
Br–Br → (hv) → 2 Br·
Propagation step A:
Br· + CH3CH3 → HBr + CH3CH2·
Propagation step B:
CH3CH2· + Br2 → CH3CH2Br + Br·
Termination examples:
Br· + Br· → Br2
CH3CH2· + Br· → CH3CH2Br
CH3CH2· + CH3CH2· → CH3CH2CH2CH3
Radical stability order:
3° > 2° > 1° > methyl
Free radical halogenation is used industrially to produce alkyl halides, which serve as intermediates in manufacturing plastics, refrigerants, and solvents.
The same radical chain mechanism underlies how UV light degrades polymers (plastics yellowing and cracking in sunlight), and it is central to understanding ozone depletion by chlorofluorocarbons (CFCs) in the upper atmosphere.
"Light breaks the C–H bond." It does not. The UV light (or heat) breaks the Br–Br bond in initiation. The C–H bond is broken by the bromine radical during propagation, not by the light source itself.
"Termination is the main pathway." Termination is actually rare. Radical concentrations are very low at any given moment, so two radicals meeting is statistically unlikely. Propagation dominates, which is why the reaction produces useful amounts of product.
"Any hydrogen is equally likely to be abstracted." With bromine, selectivity matters. Br· preferentially abstracts hydrogens that produce the more stable (more substituted) radical. This is less true for chlorine, which is far less selective.
"Half-arrows and full curved arrows mean the same thing." They do not. A full curved arrow moves two electrons (heterolytic, ionic mechanisms). A half-arrow moves one electron (homolytic, radical mechanisms). Using the wrong arrow type in a mechanism is a common mark-losing mistake.
⚠️ You will almost certainly be asked to draw the full mechanism (initiation, both propagation steps, termination) with correct half-arrows. Practise until you can do it from memory.
⚠️ Questions often ask you to predict the major product of radical bromination on a molecule with multiple types of C–H bonds. Apply the radical stability order: the most substituted position wins.
⚠️ Be ready to explain why bromine is more selective than chlorine. The answer is that the Br· radical is less reactive (the C–H abstraction step is endothermic for Br· but exothermic for Cl·), so it is more discriminating about which hydrogen it takes.
⚠️ Side-product identification: you may be asked what other products form. Remember the coupling products from termination (e.g. two carbon radicals joining) and the HBr co-product from propagation.
True or false: UV light breaks the C–H bond in the initiation step.
Fill in the blank: In propagation step A, Br· abstracts a ______ atom from the alkane.
True or false: Termination produces new radicals.
Fill in the blank: The radical stability order is 3° > 2° > 1° > ______.
True or false: Bromine is less selective than chlorine in radical halogenation.
Answers: 1. False (it breaks the Br–Br bond). 2. Hydrogen. 3. False (it destroys radicals). 4. Methyl. 5. False (bromine is more selective).
Q: Write the three stages of the radical bromination of ethane, naming each stage.
A: Initiation: Br2 → 2 Br· (homolytic cleavage by hv or heat). Propagation step A: Br· + CH3CH3 → HBr + CH3CH2·. Propagation step B: CH3CH2· + Br2 → CH3CH2Br + Br·. Termination: any two radicals combine (e.g. Br· + Br· → Br2).
Q: What type of arrow is used in radical mechanisms, and what does it represent?
A: A half-arrow (fishhook arrow) with a single barb, representing the movement of one electron.
Q: Predict the major product of radical bromination of propane (CH3CH2CH3). Explain your reasoning.
A: The major product is 2-bromopropane (CH3CHBrCH3). Bromine abstracts the hydrogen from the secondary (2°) carbon because the resulting 2° radical is more stable than the 1° radical that would form at a terminal carbon.
Q: Why is radical bromination more selective than radical chlorination?
A: The hydrogen-abstraction step is endothermic for Br· but exothermic for Cl·. By Hammond's postulate, the endothermic transition state resembles the radical product, so differences in radical stability have a larger effect on the activation energy for bromine. Chlorine reacts so exothermically that the transition state barely reflects the product radical's stability.
Q: Identify two possible side products of radical bromination of ethane and explain how each forms.
A: (1) Butane (CH3CH2CH2CH3), from two ethyl radicals combining during termination. (2) Br2, reformed when two Br· radicals combine during termination. Both result from radical-radical coupling.
This connects to carbocation stability because the stability order for radicals (3° > 2° > 1° > methyl) follows the same pattern and for the same underlying reason: hyperconjugation from adjacent C–H bonds.
Radical mechanisms reappear in polymer chemistry (radical polymerisation of alkenes uses the same initiation-propagation-termination framework) and in atmospheric chemistry (Cl· radicals from CFCs destroy ozone via an analogous chain mechanism).
The selectivity comparison between Br· and Cl· ties directly to Hammond's postulate and reaction coordinate diagrams, which you will use repeatedly when predicting transition-state geometry and product distribution.
free radical halogenation, radical chain reaction, radical substitution, homolytic cleavage, homolysis, fishhook arrow, half-arrow mechanism, bromine radical Br·, alkane bromination, initiation propagation termination, radical intermediate, radical stability order, tertiary secondary primary methyl radical, Hammond's postulate selectivity, NBS bromination, anti-Markovnikov addition, radical polymerisation, alkyl halide synthesis, hv light-initiated reaction, bond dissociation energy BDE