C–O bond cleavage in alcohols, not phenols The focus shifts from breaking O–H bonds to reactions that cleave the carbon–oxygen bond. In alcohols, the carbon bonded to oxygen is sp3 hybridized, while in phenols it is sp2 and part of an aromatic system with resonance. Resonance shortens and strengthens the phenolic C–O bond, discouraging cleavage, so phenols generally do not undergo this reaction (apart from reaction with zinc, not treated here). Consequently, the following transformations concern alcohols, beginning with reactions using hydrogen halides and phosphorus trihalides.
Alcohols + hydrogen halides give alkyl halides and water Alcohols react with hydrogen halides to form alkyl halides and water: ROH + HX -> RX + H2O (X is a halogen). The C–O bond in the alcohol and the H–X bond both break; OH combines with H to produce water, and the halogen replaces OH on carbon. The overall transformation is the same for primary, secondary, and tertiary alcohols, but their mechanisms differ.
Protonation activates the OH group and generates the halide A lone pair on oxygen bonds to the hydrogen of HX, while the electron pair from H–X moves to the halogen, producing a halide ion. The oxygen now bears a positive charge because it donates the electron pair to form the new O–H bond. This protonated species contains a water unit poised to depart and is relatively unstable, setting up the next step.
Slow C–O bond cleavage forms a stabilized tertiary carbocation The C–O bond breaks in the slow, rate‑determining step: oxygen takes the bonding electrons, becomes neutral water, and carbon is left positively charged. In a tertiary system, neighboring alkyl groups donate electron density, stabilizing the carbocation and favoring this pathway. The halide ion then rapidly bonds to the carbocation to yield the alkyl halide. Secondary alcohols follow the same route but form less stable carbocations.
Primary alcohols avoid carbocations via a bimolecular pathway Forming a primary carbocation would be highly unstable, so the reaction does not proceed by stepwise carbocation formation. Instead, loss of water and attack by the halide occur simultaneously in a single, bimolecular rate‑determining step. This concerted process produces the primary alkyl halide but is much slower than the pathway favored by tertiary substrates.
Lucas reagent distinguishes alcohol classes by turbidity Lucas reagent (concentrated HCl and ZnCl2) exploits these mechanistic differences. Alcohols are soluble in the reagent, whereas the alkyl chlorides formed are insoluble and make the solution turbid. Tertiary alcohols react immediately and do not even require ZnCl2; secondary alcohols react more slowly; primary alcohols are slowest. At room temperature, tertiary solutions turn turbid at once, secondary turbidity appears after a delay, and primary alcohols show no turbidity unless warmed.
Phosphorus trihalides also convert alcohols to alkyl halides Alcohols react with phosphorus trihalides to furnish alkyl halides and phosphorous acid, providing another route to C–O bond cleavage. For example, ROH + PBr3 -> RBr + H3PO3. Both hydrogen halides and phosphorus trihalides thus transform alcohols into alkyl halides.