Alcohol → Alkyl Bromide with PBr₃ (Primary/Secondary: SN2 Inversion; Tertiary: Not Suitable)

Phosphorus tribromide (PBr₃) swaps alcohol OH groups for bromine in a clean, closed-shell SN2 sequence. The oxygen first becomes a good leaving group (alkoxy-dibromophosphite), and bromide executes a backside attack that inverts configuration at the reacting carbon. Because no carbocation is involved, rearrangements are avoided — a major advantage over direct HX conditions. Primary and most secondary alcohols behave beautifully; tertiary and highly hindered substrates hardly move the needle.

Need to contrast options? Benchmark against the SOCl₂ playbook, the Appel reaction, or acid-driven HX substitutions when rearrangements or harsh acidity are in play.

Quick Summary

• Reagents & conditions: PBr₃ (≈0.33 eq per OH) in dry ether or CH₂Cl₂, 0–25 °C; pyridine or another base often suppresses HBr buildup.
• Scope: primary & secondary alcohols → alkyl bromides with inversion; tertiary substrates remain unchanged.
• Mechanistic spine: alcohol oxygen attacks PBr₃ → alkoxy-dibromophosphite → bromide SN2 displacement → R–Br.
• Selectivity: allylic systems may deliver SN2′ mixtures; benzylic substrates react rapidly but remain SN2.
• Advantage: avoids carbocations/rearrangements that plague strong-acid HX routes.

Mechanism — PBr₃ (3 Frames)

Step 1 — Formation of the phosphorus ester (activation)

Step 2 — SN2 displacement by bromide (configuration inversion)

Step 3 — Workup / stoichiometry

Mechanistic Checklist

• No carbocation: pathway stays closed-shell; hydride/methyl shifts and rearrangements are absent.
• SN2 at carbon: a single backside attack gives Walden inversion at the reacting centre.
• Substrate class: primary and most secondary alcohols excel; tertiary and neopentyl variants are effectively inert.
• Allylic/benzylic: allylic substrates may deliver SN2′ mixtures; benzylic sites react quickly but remain SN2.
• Acid management: HBr forms in situ; a base (pyridine, imidazole, Et₃N) prevents elimination or side reactions.

Edge Cases & Exam Traps

• “Retention with PBr₃?” No — only one substitution at carbon occurs, so overall inversion is observed.
• Allylic SN2′: expect mixtures if an allylic π-system is present; draw both primary SN2 and SN2′ products.
• Competing E2: minimal under cold, neutralized conditions; rises with hindered secondary substrates or excess HBr.
• Tertiary alcohols: essentially no reaction — acid-promoted HX routes or other halogenating agents are required.
• Neighboring heteroatoms: sulfur/phosphorus handles can chelate PBr₃; keep glassware dry and add base judiciously.

Worked Examples

Secondary, chiral centre (inversion)

(S)-2-butanol undergoes backside attack to give (R)-2-bromobutane — a single Walden inversion with no rearrangement.

Primary (clean displacement)

Primary alcohols substitute cleanly: 1-hexanol delivers 1-bromohexane in high yield with negligible side reactions.

Allylic (SN2 vs SN2′ mixture)

Allylic systems deliver mixtures: direct SN2 gives 2-bromobut-3-ene, while SN2′ attack shifts bromide to the terminal carbon (3-bromobut-1-ene).

Tertiary (not suitable)

Tertiary centres are too hindered for concerted backside attack, so PBr₃ offers no useful substitution — use HX or another halogenating strategy instead.

Practical Tips

• Keep it dry: PBr₃ is moisture-sensitive; dry glassware/solvent and add alcohol to reagent (never the reverse).
• Use a base: pyridine, imidazole, or Hunig’s base mops up HBr and suppresses elimination.
• Temperature control: 0 °C → rt covers most cases; colder runs help rein in SN2′ for allylic systems.
• Stoichiometry: ≈0.33 eq PBr₃ per OH (or slight excess) ensures full conversion for sluggish substrates.
• Workup: quench with cold aqueous NaHCO₃/Na₂CO₃, vent carefully, and segregate phosphorus-containing waste.

Exam-Style Summary

PBr₃ converts primary/secondary alcohols to alkyl bromides by first forming an alkoxy-dibromophosphite and then promoting an SN2 displacement that inverts configuration. Rearrangements are off the table, allylic substrates can show SN2′ mixtures, and tertiary centres are a non-starter — reach for HX or other halogenating tactics instead.

Interactive Toolbox

• Mechanism Solver — render the three PBr₃ frames with electron pushing and toggle SN2′ arrows for allylic inputs.
• Reaction Solver — compare PBr₃ versus HX outcomes (rearrangement risk, stereochemical control, product identity).
• IUPAC Namer — confirm R–Br product names from your drawn substrates without exposing structural encodings.