A representative example — structures drawn live.
The leaving-group problem
Everything about alcohol chemistry flows from one fact: hydroxide (OH–) is a bad leaving group. Leaving groups are best when they are weak bases — stable on their own after they depart. Hydroxide is a strong base, so it clings to carbon and refuses to leave. If you simply mix an alcohol with a nucleophile, nothing happens.
Chemists get around this in two ways. Either protonate the OH so it leaves as neutral water (a much weaker base), or replace OH with a genuinely good leaving group before doing the reaction you actually want.
Substitution: making the OH leave
Under strongly acidic conditions, the oxygen is protonated to give an oxonium ion (R–OH2+). Now water can leave. Tertiary and secondary alcohols react with HX (HCl, HBr) through an SN1 pathway — water leaves to form a carbocation, then the halide attacks. This works well for 3° alcohols because their carbocations are stable; watch for rearrangements.
Primary alcohols cannot form a stable carbocation, so acidic substitution is sluggish and unreliable. For those, use reagents that convert OH into a good leaving group without a carbocation:
- SOCl2 (thionyl chloride) converts R–OH to R–Cl. The byproducts (SO2 and HCl) are gases, driving the reaction forward.
- PBr3 (phosphorus tribromide) converts R–OH to R–Br. Both reagents proceed through an SN2-like displacement, so 1° and 2° alcohols work cleanly with inversion at the carbon and no rearrangement.
- TsCl (tosyl chloride) converts R–OH into a tosylate (R–OTs). The tosylate leaves the oxygen–carbon bond intact, so configuration is retained in this step. A tosylate is an excellent leaving group, so the alcohol carbon is now primed for any SN2, SN1, E1, or E2 reaction you like.
Dehydration: elimination to an alkene
Heating an alcohol with a strong acid catalyst (typically concentrated H2SO4 or H3PO4) removes water to form an alkene. This acid-catalyzed dehydration follows an E1 mechanism: the OH is protonated, water leaves to make a carbocation, and a base removes a β-hydrogen to form the double bond.
Because a carbocation forms, the reactivity order is 3° > 2° > 1°, matching carbocation stability. The regiochemistry follows Zaitsev's rule: the more substituted, more stable alkene predominates. As with any carbocation process, hydride and methyl shifts can rearrange the skeleton before elimination, so the product may not sit where you first expect.
Oxidation: from alcohols to carbonyls
Oxidation of an alcohol removes hydrogens and installs C=O character. The outcome depends on the alcohol's class:
- Primary alcohols can stop at the aldehyde or go all the way to the carboxylic acid. A mild, anhydrous oxidant such as PCC (pyridinium chlorochromate) stops at the aldehyde. Stronger, aqueous oxidants such as the Jones reagent (CrO3/H2SO4) or hot KMnO4 push all the way to the carboxylic acid.
- Secondary alcohols oxidize to ketones with any of these oxidants; there is no further oxidation because there is no C–H left on the carbinol carbon to remove.
- Tertiary alcohols resist oxidation entirely — the carbinol carbon bears no hydrogen, so there is nothing for the oxidant to abstract without breaking a C–C bond.
Choosing the right route
Read the substrate first. Need a halide from a 1° or 2° alcohol without rearrangement? Reach for SOCl2 or PBr3. Need to set up a later SN2 or E2? Make the tosylate. Want an alkene? Dehydrate and expect the Zaitsev product. Want a carbonyl? Match the oxidant strength to the target — PCC for an aldehyde, Jones or KMnO4 for a carboxylic acid, and remember that 3° alcohols simply will not oxidize.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, curved arrows, wedge/dash bonds and a clickable periodic table built in. No account needed.