Wohl-Ziegler Bromination

What Wohl-Ziegler bromination does

You have an alkene. You want a bromine on the carbon next to the double bond — the allylic position — and you want to leave the C=C intact. That sounds impossible: molecular bromine loves double bonds, and if you just add Br₂ to an alkene you get a 1,2-dibromide (anti addition across the π bond) every time. Wohl-Ziegler bromination is the trick that sidesteps this.

The answer is not a different bromine — it's a different concentration of bromine. N-bromosuccinimide (NBS) is a mild, crystalline solid that releases molecular Br₂ only in tiny, steady amounts. At those trace concentrations the ionic addition to the double bond becomes reversible and unproductive, while a radical chain — one that abstracts the weak allylic C–H bond — quietly grinds forward. The net result is clean substitution at the allylic (or benzylic) carbon:

                 NBS (1.0–1.1 eq)
                 AIBN (cat.), CCl₄
    cyclohexene  ─────────────────────→  3-bromocyclohexene
                 reflux, hν              (allylic bromide)

    the C=C survives untouched; the Br lands one carbon over

The same reaction works on benzylic C–H bonds — the carbon attached to an aromatic ring — because those radicals are stabilized by the ring exactly the way allyl radicals are stabilized by the double bond. Toluene → benzyl bromide is the textbook example.

The radical chain, step by step

The mechanism that actually operates is the Goldfinger mechanism (Paul Goldfinger, with J. Adam and P. A. Gosselain, 1953). The crucial and counter-intuitive point: the chain carriers are a bromine atom and an allyl radical — not the succinimidyl radical. NBS is only a slow-release reservoir of Br₂.

1. Initiation. The initiator (AIBN, peroxide, or a photon) makes the first radical, which abstracts a bromine to spit out a bromine atom Br•. AIBN homolyzes on gentle heating to two 2-cyanopropyl radicals plus N₂.
2. Propagation, step A — H abstraction. The bromine atom pulls the allylic (or benzylic) hydrogen off the substrate. Because it grabs the weakest C–H bond, it makes a resonance-stabilized allyl radical and releases H–Br. This is the selectivity-determining step.
3. Regenerate Br₂ (ionic side-reaction). That H–Br immediately reacts with NBS in a fast, ionic step: the N–Br of succinimide is cleaved to give a molecule of Br₂ and succinimide. This is how NBS "tops up" the trace bromine — and why [Br₂] stays low and buffered.
4. Propagation, step B — Br delivery. The allyl radical abstracts a bromine atom from that freshly made Br₂, forming the allylic bromide product and regenerating a new Br• to carry the chain onward.
5. Termination. Any two radicals that meet (Br• + Br•, or two carbon radicals) combine and end that chain. In a healthy reaction, initiation and termination are rare compared with the thousands of propagation cycles each chain runs.

  Initiation:   In-In  ──Δ or hν──→  2 In•        In• + Br-source → In-Br + Br•

  Propagation A:  R-H  +  Br•   →   R•  +  H-Br        (R• = allyl / benzyl radical)
  NBS refill:     H-Br +  NBS   →   Br₂ + succinimide   (ionic, keeps [Br₂] tiny)
  Propagation B:  R•   +  Br₂   →   R-Br +  Br•          (product + new chain carrier)

  net:  R-H  +  NBS  →  R-Br  +  succinimide

Notice that Br• appears on both sides of the propagation pair, so it never accumulates — that is the signature of a chain reaction. The whole cycle only works because [Br₂] is held at roughly 10⁻³ M or below; higher and the ionic addition to the double bond outcompetes the radical chain.

Reagents, initiator, and conditions

• NBS. 1.0–1.1 equivalents. It is a cheap, bench-stable white solid (mp 175–178 °C) that should be recrystallized from hot water if it has gone yellow (aged NBS liberates Br₂ and gives poor selectivity). NBS is denser than CCl₄ and sinks; the byproduct succinimide floats — a handy visual endpoint.
• Initiator. AIBN (azobisisobutyronitrile), 1–5 mol %, is the modern standard: it has a 10-hour half-life near 65 °C, so refluxing CCl₄ (bp 76 °C) or benzene gives a clean supply of radicals. Benzoyl peroxide works too. Photochemical initiation — a 300–500 W sunlamp or simply refluxing under a bright bulb — is the classic alternative.
• Solvent. Traditionally carbon tetrachloride, chosen because NBS and succinimide are both nearly insoluble in it, which keeps the reaction heterogeneous and the dissolved [Br₂] low. Because CCl₄ is a banned ozone-depleter and a suspected carcinogen, modern practice uses trifluorotoluene (PhCF₃), ethyl acetate, or even a water/MeCN emulsion.
• Temperature. Reflux, typically 60–80 °C, enough to decompose the initiator and drive the chain. The reaction is often exothermic once it starts; add NBS portionwise if the substrate is reactive.
• Endpoint / workup. When the dense NBS has been consumed and a fluffy layer of succinimide floats on top, the reaction is complete. Cool, filter off the succinimide, evaporate the solvent, and purify by distillation or chromatography.

Scope, selectivity, and regiochemistry

Wohl-Ziegler works wherever there is a weak, radical-stabilizing C–H bond:

• Allylic positions. The sp³ carbon adjacent to a C=C. Cyclohexene → 3-bromocyclohexene is the canonical case.
• Benzylic positions. The sp³ carbon attached to an aromatic ring. Toluene → benzyl bromide; ethylbenzene → (1-bromoethyl)benzene. Benzylic bromination is arguably the more common industrial use.
• Doubly activated positions. A carbon that is both allylic and benzylic, or allylic to two double bonds, reacts fastest of all.

The reason for the selectivity is a bond-strength argument. Abstracting an allylic hydrogen gives a resonance-stabilized allyl radical, and the allylic C–H bond dissociation energy is about 88 kcal/mol versus roughly 98 kcal/mol for an ordinary secondary alkyl C–H. Benzylic C–H is about 90 kcal/mol. The bromine atom is a highly selective abstractor — its H-abstraction is endothermic and has a late, product-like transition state (Hammond postulate), so it discriminates sharply in favor of the weakest bond.

The allylic-shift caveat. The intermediate allyl radical is delocalized, so bromine can attach at either end of the allylic system. Unsymmetrical alkenes therefore give mixtures — the "direct" product and the "transposed" product with the double bond shifted:

   1-octene  →  Br•  abstracts allylic H  →  [CH₂=CH-CH•-C₅H₁₁ ↔ •CH₂-CH=CH-C₅H₁₁]
                                              delocalized allyl radical
                                                    │
                                          Br₂ can cap either terminus
                                                    │
              ┌─────────────────────────────────────┴─────────────────────────────┐
        3-bromo-1-octene (direct)                              1-bromo-2-octene (transposed)

Symmetric allylic systems (cyclohexene, cyclopentene) sidestep this because both termini are equivalent. There is no stereochemistry to control in the simplest cases — the product allyl radical is planar — but chiral, non-racemic substrates racemize at the brominated center, and E/Z ratios of the shifted alkene can vary.

Wohl-Ziegler vs the alternatives

	Wohl-Ziegler (NBS)	Br₂ addition	Free-radical Br₂ (hν)
Reagent	NBS (trace Br₂ reservoir)	neat / concentrated Br₂	Br₂ + light, no double bond present
Mechanism	Radical chain substitution	Ionic electrophilic addition	Radical chain substitution
Product from an alkene	Allylic bromide (C=C kept)	1,2-dibromide (C=C destroyed)	Would add too — [Br₂] is high
Where Br lands	Allylic / benzylic C–H	Across the double bond	Weakest C–H, if no C=C
[Br₂] in flask	≈10⁻³ M, buffered low	High (molar)	High
Selectivity	High — one C–H type	N/A (adds)	Good (Br is selective)
Needs an alkene?	Yes (or an arene)	Yes	No
Typical use	Allylic/benzylic bromides	Alkene dibromides, Br₂ tests	Benzylic side-chain bromination

Note the last two columns are chemically the same radical bromination that NBS runs — the only difference is that NBS keeps [Br₂] low enough that an alkene in the flask survives instead of being consumed by ionic addition. Take the alkene away (as in toluene) and photobromination with dilute Br₂ does the same job; NBS is simply the convenient, low-hazard way to meter the bromine.

Worked example: 3-bromocyclohexene

Cyclohexene is the model substrate — symmetric allylic system, no regiochemistry to worry about.

    cyclohexene (1.0 eq)  +  NBS (1.05 eq)  ──AIBN (2 mol%), CCl₄, reflux, 1–2 h──→  3-bromocyclohexene

• Reagents. Cyclohexene 1.0 equiv, freshly recrystallized NBS 1.05 equiv, AIBN 2 mol % as initiator.
• Conditions. Dry CCl₄ (or PhCF₃ for a greener run), heated to a gentle reflux; if using light instead of AIBN, irradiate with a 300 W sunlamp. Reaction is usually complete in 1–2 h.
• Endpoint. The dense NBS at the bottom dissolves and reacts away; a light, fluffy layer of succinimide floats up. That flotation is the visual "done" signal.
• Workup. Cool, filter off succinimide, wash the filtrate, remove solvent, and distill (3-bromocyclohexene, bp ≈ 68–70 °C at 20 mmHg).
• Yield. 70–85% of the allylic bromide, with the ring double bond intact — no 1,2-dibromide, because [Br₂] never rose high enough to add.

3-Bromocyclohexene is a prized building block: the allylic bromide is set up for S_N2, S_N2′, elimination to 1,3-cyclohexadiene, or Grignard/organometallic couplings — all leveraging the C–Br installed next to the alkene.

Real-world applications

• Setting up 1,3-dienes. Allylic bromination followed by base-promoted dehydrohalogenation is a standard two-step route from a simple alkene to a conjugated diene — a Diels-Alder partner. Cyclohexene → 3-bromocyclohexene → 1,3-cyclohexadiene is the classic sequence.
• Benzylic bromides for cross-coupling and displacement. Side-chain bromination of methylarenes (toluene, xylenes, methylpyridines) gives benzyl bromides, the workhorse electrophiles for Williamson ethers, phosphonium salt formation (Wittig precursors), and quaternary ammonium synthesis. Industrial benzyl chloride/bromide and the bis-bromomethyl monomers for some polymers trace back to radical side-chain halogenation.
• Terpene and steroid functionalization. NBS allylic bromination lets total-synthesis chemists install a handle at a specific allylic carbon of a polyene without touching the alkenes — used in syntheses of vitamin A/carotenoid fragments and steroid side chains.
• Agrochemical and pharma intermediates. Bromomethyl-heteroarenes made by benzylic NBS bromination are common late-stage electrophiles for tethering fragments in medicinal chemistry (e.g. building the linkers in sartan-type and other drug scaffolds).
• Dibromination for double bond migration studies. Controlled mono- and bis-allylic bromination has long been used mechanistically to map allyl-radical delocalization and the allylic rearrangement.

Limitations and side reactions

• Allylic scrambling. The single biggest caveat — unsymmetrical substrates give regioisomeric mixtures because the allyl radical is delocalized (see the selectivity section). Plan for it or pick a symmetric substrate.
• Ionic addition creep. If NBS has decomposed (yellow crystals) or if you add it all at once to a very reactive alkene, [Br₂] spikes and you get the 1,2-dibromide instead. Use fresh, white NBS and add portionwise.
• Competing ring bromination. With very electron-rich arenes, electrophilic aromatic bromination of the ring can compete with benzylic side-chain bromination. Radical conditions (initiator, non-polar solvent, no Lewis acid) favor the side chain; polar solvents and Lewis acids push toward the ring.
• Over-bromination. A benzylic CH₃ can be brominated once, twice, or three times (→ CHBr₂, CBr₃). Control it with exactly one equivalent of NBS and monitor conversion.
• Radical inhibitors kill it. O₂ and phenolic antioxidants (BHT stabilizer in the solvent, trace hydroquinone) quench the chain and cause failed or sluggish runs. Degas and use inhibitor-free solvent.
• Not for isolated C–H bonds. With no allylic, benzylic, or otherwise activated position, NBS is unselective and low-yielding — it is a chemoselective reagent, not a general alkane brominator.

History: Wohl, Ziegler, and Goldfinger

The reaction carries two names because it was developed in two stages. Alfred Wohl first reported allylic bromination with a bromoamide reagent in 1919. It was Karl Ziegler (later the Nobel laureate of Ziegler-Natta polymerization fame) who, in 1942, established N-bromosuccinimide as the practical, general reagent and showed how broadly it worked for allylic and benzylic bromination — hence "Wohl-Ziegler."

For a decade the mechanism was misunderstood: many assumed the succinimidyl radical (the nitrogen radical from NBS) abstracted the hydrogen directly. In 1953 Paul Goldfinger, working with J. Adam and P. A. Gosselain, worked out the true picture — the low-bromine-concentration chain in which Br• and the allyl radical are the carriers and NBS merely regenerates trace Br₂ from HBr. That "Goldfinger mechanism" is the one taught today and is the reason the reaction is so selective.

Safety and practical notes

• NBS handling. An oxidizer and mild irritant; it liberates Br₂ if wet or old. Keep it dry, cool, and away from reducing agents. Recrystallize discolored NBS before use — both for selectivity and safety.
• Solvent hazard. The classic CCl₄ is toxic, hepatotoxic, and ozone-depleting (banned under the Montreal Protocol for most uses). Prefer trifluorotoluene, ethyl acetate, chlorobenzene, or an aqueous emulsion in modern work.
• Exotherm and gas. The chain, once initiated, can run away; the reaction evolves some HBr and, if things go wrong, Br₂ vapor. Work in a fume hood, add NBS portionwise, and have a base scrubber for the vent.
• Product reactivity. Allylic and benzylic bromides are lachrymators and alkylating agents — handle with gloves and treat as you would benzyl bromide.
• Initiator storage. AIBN and peroxides are shock- and heat-sensitive; store cold and use fresh. Never scale up a peroxide-initiated radical reaction without a thermal-hazard assessment.