The class of the carbinol carbon — 1°, 2°, or 3° — decides almost everything that follows.
1. The –OH is a poor leaving group, so an alcohol must be activated before it reacts
Hydroxide is a strong base, so it clings to carbon and refuses to leave; the only fixes are to protonate the OH (it leaves as water) or replace it with a halide or tosylate.
2. Oxidation turns 1° alcohols into aldehydes or acids and 2° alcohols into ketones
Mild anhydrous PCC stops a 1° alcohol at the aldehyde, while strong aqueous oxidants (H2CrO4/Jones, hot KMnO4) drive it to the carboxylic acid.
PCC stops a 1° alcohol at the aldehyde — 1-propanol → propanal.
Secondary alcohols oxidize to ketones and stop there; tertiary alcohols do not oxidize — that carbon has no H to lose.
3. SOCl₂ and PBr₃ convert alcohols to alkyl halides without rearrangement
PBr3 gives R–Br and SOCl2 gives R–Cl in one concerted, SN2-like step — no free carbocation, so the skeleton never rearranges.
PBr3 converts 1-propanol to 1-bromopropane — no rearrangement.
4. With HX, protonation lets water leave — but 3° alcohols go SN1 and can rearrange
Concentrated HX protonates the oxygen so water leaves and halide takes its place; 3° alcohols react fast by SN1 through a stable carbocation.
tert-butanol + HCl → 2-chloro-2-methylpropane via a 3° carbocation.
With 2° substrates that cation can undergo a hydride or methyl shift, scrambling the product — which is exactly why PBr3 and SOCl2 exist.
5. Tosylation converts a bad leaving group into an excellent one for SN2 and E2
TsCl/pyridine caps R–OH as a tosylate by attacking the O–H bond, never C–O, so configuration at carbon is retained.
TsCl caps the oxygen — the C–O bond is untouched, so configuration is retained.
The sulfonate spreads its charge over three oxygens, making –OTs a superb leaving group — a launchpad for a clean SN2 or E2.
6. Acid and heat dehydrate an alcohol to the Zaitsev alkene by an E1 path
Strong acid and heat lose water to an alkene by E1: protonate the OH, lose water to a carbocation, then a base plucks a β-hydrogen.
2-propanol dehydrates to propene under conc. H2SO4 and heat.
Reactivity tracks cation stability (3° > 2° > 1°) and regiochemistry follows Zaitsev's rule — the more substituted alkene dominates.
How each reagent works — the arrow pushing
Electron flow only. Follow the arrows; the structures do the talking.
7. Summary
Carbonyl → oxidize (PCC to aldehyde, H2CrO4/KMnO4 to acid, any oxidant for 2° → ketone; 3° won't) · Clean halide → PBr3/SOCl2 · 3° alcohol → HX by SN1 · Set up SN2/E2 → tosylate · Alkene → acid + heat, Zaitsev by E1.
Quiz yourself
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Because –OH is a poor leaving group. Hydroxide is a strong base, so it will not depart from carbon. You must first protonate it (so it leaves as water) or convert it to a halide or tosylate.
PCC (pyridinium chlorochromate) — a mild, anhydrous oxidant. Strong aqueous oxidants like H2CrO4 (Jones) or hot KMnO4 push all the way to the carboxylic acid.
PBr3 reacts by a concerted, SN2-like displacement with no free carbocation, so the skeleton does not rearrange. HBr goes through a carbocation that can undergo hydride/methyl shifts, scrambling the product.
Propene, by E1. The acid protonates the OH, water leaves to give a 2° carbocation, and a base removes a β-hydrogen to form the alkene (Zaitsev product).
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.