Organic Chemistry · Alkenes: Reactions and Synthesis
Radical Additions to Alkenes: Chain-Growth Polymers
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A radical Species with an unpaired electron Full entry → is any species with an unpaired electron — an atom or molecule with one electron alone in an orbital. Radicals are electron-hungry, so they add to the π bond of an alkene, and the radical product adds to another alkene, and so on: a chain reaction. This topic uses that chain chemistry twice: HBr adds to an alkene with peroxides to give the anti-Markovnikov product (the "peroxide effect Peroxides switch HBr addition to the radical (anti-Markovnikov) pathway Full entry →"), and the same chain-growth logic, run with alkene monomers and a radical initiator, produces chain-growth polymers — polyethylene, polypropylene, PVC, polystyrene, Teflon, and more. The mechanistic skeleton — initiation Step that creates the first radical (e.g., ROOR homolysis) Full entry →, propagation Steps that consume and regenerate a radical while growing the product Full entry →, termination Steps where radicals combine or disproportionate, ending chains Full entry → — is identical in both.
Why this matters
Polymers are the highest-volume products of the chemical industry — plastics, fibers, and rubbers touch nearly every object you own. Understanding chain-growth polymerization explains why bags stretch, why some plastics are crystalline and others rubbery, and why recycling streams separate polymer types. The peroxide effect is a favorite exam trap: HBr is the only hydrogen halide that adds anti-Markovnikov with peroxides — for thermodynamic reasons. Radical chemistry also underlies oxidative damage in biology (Topic 11) and controlled radical polymerization methods for advanced materials.
The college version
Core Concepts
Radical structure and stability
A radical carbon is roughly trigonal (sp²-like), the unpaired electron in a p-type orbital. Radical stability follows the order:
benzyl ≈ allyl > 3°> 2°> 1°> methyl
Stability comes from hyperconjugation and, for allyl/benzyl radicals, resonance delocalization. This order decides the regiochemistry of radical additions.
Anti-Markovnikov HBr addition (the peroxide effect)
In the presence of peroxides (ROOR), HBr adds to an unsymmetrical alkene against Markovnikov's rule: H goes to the more substituted carbon, Br to the less substituted one. Mechanism in words:
- Initiation: the weak O–O bond homolyses (heat or light) into two alkoxy radicals, RO•.
- Propagation, step 1: RO• abstracts H from HBr, giving ROH and Br•.
- Propagation (repeated): Br• adds to the less substituted carbon so the radical forms at the more substituted (stable) carbon; that radical abstracts H from another HBr, giving the alkyl bromide and regenerating Br•.
- Termination: two radicals combine.
Only HBr works: the H–Br bond is weak enough (~366 kJ/mol) for Br• to be regenerated and for the carbon radical to abstract H from HBr. HCl fails (H–Cl too strong for that step); HI fails (I• adds too slowly and reversibly). Without peroxides, ionic HBr addition still follows Markovnikov's rule — the peroxide (with light or heat) switches the pathway.
Chain-growth polymerization: from monomers to plastics
Chain-growth (addition) polymerization repeats an alkene's radical addition thousands of times. The monomer Small alkene unit that repeats in the polymer Full entry → is an alkene; substituted alkenes polymerize well: styrene, propylene, vinyl chloride, methyl methacrylate, acrylonitrile, butadiene, tetrafluoroethylene. Mechanism:
- Initiation: the initiator (benzoyl peroxide, AIBN) homolyses, and the radical adds to the first monomer.
- Propagation: the radical adds to the next monomer — head-to-tail, substituents on alternate carbons — one monomer at a time, each step regenerating a chain-end radical.
- Termination: two chains combine (combination) or one transfers a hydrogen (disproportionation).
The product is a long saturated chain. Because termination is random, chains vary in length, so polymers are characterized by an average molar mass and degree of polymerization.
Structure–property links
- Branching: radical polymerization of ethylene at high pressure gives LDPE — branched chains that cannot pack tightly → flexible, low-density plastic (bags). Catalytic (Ziegler–Natta) polymerization gives linear HDPE → crystalline, stiff, higher density (bottles, pipes).
- tacticity Regularity of stereocenters along a substituted polymer chain Full entry →: for substituted alkenes such as propylene, each repeat-unit stereocenter can be regular (isotactic — methyls all on the same side; syndiotactic — alternating) or random (atactic). Regular chains crystallize; atactic polypropylene is rubbery.
- Cross-linking: dienes like butadiene leave a C=C in the chain; vulcanization (sulfur cross-links) turns sticky polydiene into elastic rubber.
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| "The peroxide effect works for HCl and HI too." | Only HBr: the radical chain is thermodynamically favorable for HBr alone (H–Cl is too strong; I• adds too slowly/reversibly). |
| "Anti-Markovnikov product forms through a more stable carbocation." | No carbocation is involved — the intermediate is the more substituted radical; Br ends up on the less substituted carbon. |
| "Radical and carbocation stability orders are identical." | Both favor more substitution, but resonance stabilization (allyl, benzyl) matters more for radicals. |
| "Chain-growth polymers form by losing water between monomers." | That is step-growth/condensation; chain-growth adds alkene monomers with no atoms lost. |
| "All polyethylene is the same material." | LDPE is branched and flexible; HDPE is linear, crystalline, and stiff. |
| "Polymerization stops only when the monomer runs out." | Chains also terminate when two radicals meet (combination or disproportionation). |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a line of people passing a hot potato (the radical): the first grabs it, then passes a copy to the next, who passes a copy to the next — each pass adds one more person to the line. That's a chain reaction: one radical starts it, and every step creates the next. If the "people" are alkene monomers, the line they build is a polymer.
Worked example
Example 1: Predicting the peroxide-effect product
Question: 1-hexene reacts with HBr in the presence of benzoyl peroxide. What is the major product?
Step 1 — the alkene: CH₂=CH–CH₂CH₂CH₂CH₃ — unsymmetrical, with a terminal CH₂ and an internal CH.
Step 2 — radical logic: Br• adds so the radical forms at the more substituted carbon — Br bonds to the terminal CH₂, placing the radical on C2 (secondary > primary).
Step 3 — H abstraction from HBr puts H on C2: 1-bromohexane, CH₃CH₂CH₂CH₂CH₂CH₂Br — the anti-Markovnikov product. (Without peroxide, the ionic path gives 2-bromohexane.)
Example 2: Degree of polymerization with dimensional analysis
Question: A polystyrene sample has average molar mass 312,000 g/mol; styrene (C₈H₈) has M = 104.15 g/mol. What is the average degree of polymerization?
Step 1 — formula:
n = MpolymerMmonomer
Step 2 — substitute with units:
n = 312 000 g mol-1104.15 g mol-1 = 2.996 × 103 ≈ 3.0 × 103
The g mol⁻¹ units cancel: each average chain holds about 3,000 styrene units.
Key takeaways
- Radical: species with an unpaired electron; stability benzyl ≈ allyl > 3° > 2° > 1° > methyl.
- Peroxide effect: HBr + ROOR adds anti-Markovnikov (Br to the less substituted carbon). HCl and HI do NOT show this effect.
- Radical addition of Br• proceeds through the more substituted (more stable) carbon radical.
- Chain-growth polymerization: initiation → propagation (chain reaction) → termination.
- Monomers are alkenes; addition is head-to-tail; the polymer is a saturated chain.
- Degree of polymerization: n = Mpolymer/Mmonomer (average).
- LDPE branched (flexible) vs HDPE linear (stiff); tacticity controls crystallinity; vulcanization cross-links rubber.
- Termination by combination or disproportionation.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Why does the radical intermediate in HBr + peroxide addition form at the more substituted carbon, yet the product puts Br on the less substituted carbon?
Show answer
Br• adds to the less substituted alkene carbon so the unpaired electron ends up on the more substituted carbon, which is the more stable radical (2° > 1°); the subsequent H• transfer from HBr places H there, leaving Br on the less substituted carbon.
Name the three phases of a radical chain reaction and what happens in each.
Show answer
Initiation (peroxide homolysis creates RO•; RO• + HBr → Br•), propagation (Br• + alkene → carbon radical; carbon radical + HBr → alkyl bromide + Br•, repeated), termination (radicals combine or disproportionate).
Why does only HBr show the peroxide effect?
Show answer
The chain steps are energetically balanced only for HBr: Br• forms readily, and the carbon radical can abstract H from HBr (relatively weak H–Br bond). For HCl the H-abstraction step is too endothermic; for HI the iodine radical adds too slowly and reversibly.
A polypropylene sample has average M = 210,000 g/mol. Propylene is C₃H₆ (M = 42.08 g/mol). What is the average degree of polymerization?
Show answer
n = 210,000 / 42.08 ≈ 4.99 × 10³ ≈ 5.0 × 10³ monomer units per average chain.
Explain how branching differs between LDPE and HDPE, and one physical consequence of that difference.
Show answer
LDPE chains have many branches (high-pressure radical process), so they pack poorly — flexible, lower density; HDPE chains are essentially linear (catalytic process) and pack into crystalline regions — stiffer, higher density.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- radical
- Species with an unpaired electron
- anti-Markovnikov addition
- H goes to the more substituted carbon, X to the less substituted one
- peroxide effect
- Peroxides switch HBr addition to the radical (anti-Markovnikov) pathway
- initiation
- Step that creates the first radical (e.g., ROOR homolysis)
- propagation
- Steps that consume and regenerate a radical while growing the product
- termination
- Steps where radicals combine or disproportionate, ending chains
- chain-growth polymer
- Polymer built by repeated addition of alkene monomers to a growing chain
- monomer
- Small alkene unit that repeats in the polymer
- degree of polymerization (n)
- Average number of monomer units per chain
- tacticity
- Regularity of stereocenters along a substituted polymer chain
Sources & references
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