The Pinacol Rearrangement

What the pinacol rearrangement does

Take a 1,2-diol (two hydroxyls on adjacent carbons, also called a vicinal glycol), add strong acid and heat, and it collapses into a carbonyl compound — a ketone or an aldehyde — with one carbon rearranged. The archetype gives the reaction its name: pinacol (2,3-dimethyl-2,3-butanediol) becomes pinacolone (3,3-dimethyl-2-butanone), a tert-butyl methyl ketone.

The trick is that a carbon–carbon or carbon–hydrogen bond physically slides — migrates — from one carbon to the next-door carbon. This is a 1,2-shift, the same move that scrambles carbocations in S_N1 reactions. What makes the pinacol version special is that the migration terminus carries an oxygen, so the shift ends not in another carbocation but in a resonance-stabilized oxocarbenium ion — a protonated carbonyl. That built-in trap is why the reaction is so clean and so useful.

    HO  OH                              O
     \  /            H⁺, heat           ||
  Me₂C–CMe₂    ───────────────────►   Me–C–CMe₃   +  H₂O
   pinacol                              pinacolone
 (2,3-dimethyl-                       (3,3-dimethyl-
  2,3-butanediol)                      2-butanone)

The mechanism, arrow by arrow

Four elementary steps, each a familiar move. Follow the electrons:

1. Protonate a hydroxyl. One lone pair on an –OH oxygen grabs a proton from the acid, converting a bad leaving group (⁻OH, hydroxide) into a good one (H₂O, water). The two hydroxyls are equivalent in pinacol, so it doesn't matter which one is protonated.
2. Lose water — form the carbocation. The C–O bond breaks heterolytically; both electrons leave with the water molecule. This unmasks a tertiary carbocation on that carbon. (In an unsymmetrical diol, the diol ionizes to give the more stable of the two possible cations — this is the regiochemistry-setting step.)
3. 1,2-migration. A bonding pair from the adjacent carbon — a C–C or C–H bond — swings over to fill the empty p orbital of the cation. The migrating group moves with its two electrons in a concerted, backside-like push. The positive charge lands on the carbon the group left behind — but that carbon still carries a hydroxyl, whose oxygen lone pair immediately shares into the empty orbital. The result is an oxocarbenium ion, drawn as R₂C=O⁺H ↔ R₂C⁺–OH.
4. Deprotonate. A base — a spectator water, HSO₄⁻, or the solvent — plucks the proton off the oxygen, dropping the charge and revealing a neutral C=O double bond. The carbonyl is born; the catalyst (H⁺) is regenerated.

  step 1:  HO–CMe₂–CMe₂–OH  +  H⁺   →   HO–CMe₂–CMe₂–OH₂⁺
  step 2:  HO–CMe₂–CMe₂–OH₂⁺         →   HO–CMe₂–CMe₂⁺  +  H₂O   (3° cation)
  step 3:  HO–CMe₂–CMe₂⁺             →   [Me migrates] →  ⁺HO=CMe–CMe₃
                                                          (oxocarbenium)
  step 4:  ⁺HO=CMe–CMe₃  +  base     →   O=CMe–CMe₃  +  H⁺   (pinacolone)

Steps 2 and 3 are often concerted — water departs as the neighboring group migrates, so a discrete high-energy secondary/tertiary carbocation may never fully form. This concert is exactly why an anti-periplanar geometry between the departing water and the migrating bond speeds the reaction: the migrating σ-bond has to overlap with the developing empty orbital, backside to the C–O that is breaking.

Reagents, catalyst, and real conditions

• Brønsted acids. Concentrated sulfuric acid is the textbook choice; H₃PO₄, aqueous HCl, p-toluenesulfonic acid (TsOH), and triflic acid (TfOH) all work. Neat pinacol with a few percent H₂SO₄ rearranges on heating to reflux.
• Temperature. The parent reaction wants heat — commonly 80–200 °C. Pinacol itself is often distilled from dilute acid; pinacolone boils at 106 °C and is removed as it forms, driving the equilibrium.
• Lewis acids. BF₃·OEt₂, montmorillonite K10 clay, and other oxophilic Lewis acids run milder, lower-temperature versions, useful for sensitive substrates.
• Water management. Because the first two steps expel water, keeping the medium relatively anhydrous (or removing water by distillation) pushes the equilibrium toward product. Too much water reverses the dehydration and lowers yield.
• Making the diol. Symmetric pinacols come cheaply from a pinacol coupling — reductive dimerization of two ketones (e.g. acetone) by Mg, Na, or SmI₂ — so the diol → ketone rearrangement is often paired with the ketone → diol coupling that precedes it.

Regiochemistry, migratory aptitude, and stereochemistry

Two questions decide the product of any unsymmetrical pinacol rearrangement: which hydroxyl leaves (which cation forms) and which group migrates.

• Cation stability sets ionization. The diol loses whichever water gives the more stabilized carbocation. A hydroxyl flanked by two aryl or two alkyl groups leaves preferentially, because the resulting cation is best stabilized. Tertiary > secondary > primary, and benzylic/allylic beat all.
• Migratory aptitude ranks the shift. Once the cation is set, the group anti to the empty orbital that can best stabilize positive charge in the transition state migrates. The classic order is aryl (electron-rich) > H ≈ alkyl, with para-methoxyphenyl among the fastest migrators and para-nitrophenyl among the slowest. Because migration builds partial positive character on the migrating carbon, groups that donate electrons into it move fastest.
• Anti-periplanar geometry. The migrating σ-bond must be aligned antiperiplanar to the leaving water. In rigid rings this stereoelectronic requirement can override migratory aptitude — only the group locked anti can move — which is how ring systems dictate a single rearrangement pathway.
• Stereospecificity. Because the shift is concerted and suprafacial, the migrating group moves with retention of its own configuration, and the geometry at both carbons is transmitted into the product. This is the basis for the asymmetric semipinacol rearrangements used in total synthesis.

Pinacol vs related 1,2-shift reactions

	Pinacol rearrangement	Semipinacol rearrangement	Simple carbocation shift
Substrate	1,2-diol (vicinal glycol)	β-functional alcohol (halohydrin, epoxide, β-OMs)	Any carbocation precursor (RX, ROH, alkene)
How the cation forms	Protonate + lose water (strong acid)	Ionize a dedicated leaving group (mild)	Ionize LG / protonate alkene
What migrates	C–C or C–H, by migratory aptitude	C–C or C–H, chosen by leaving-group placement	H or alkyl, toward more stable cation
Migration terminus	Carbon bearing –OH → oxocarbenium	Carbon bearing –OH → oxocarbenium	Another carbon → new carbocation
Product	Ketone or aldehyde	Ketone/aldehyde, ring-expanded, stereodefined	Rearranged alkyl cation → substitution/elimination
Regiocontrol	Set by cation stability (poor if symmetric)	Excellent — you place the leaving group	Thermodynamic (most stable cation)
Conditions	Conc. H₂SO₄ / H₃PO₄, heat	Mild base or Lewis acid, often RT	Varies (SN1, E1, Friedel-Crafts)
Typical use	Pinacolone, quaternary-carbon ketones	Enantioselective ring expansions in synthesis	Usually an unwanted side reaction

Worked example: pinacol → pinacolone

The canonical run makes pinacolone, a fragrant ketone used as a solvent and a synthetic intermediate.

    (CH₃)₂C(OH)–C(OH)(CH₃)₂   ──6% H₂SO₄, distill (Δ)──►   (CH₃)₃C–C(=O)–CH₃  +  H₂O
      pinacol, mp 38 °C                                     pinacolone, bp 106 °C

• Reagents. Pinacol (or its hexahydrate) and dilute sulfuric acid (≈ 6% by volume), no other solvent needed.
• Conditions. Heat to distillation; pinacolone (bp 106 °C) codistills with water and is collected as the reaction proceeds, which continuously removes product and drives the equilibrium.
• What happens mechanistically. Symmetric diol → protonate either OH → lose water → tertiary cation → a methyl group migrates from the neighboring carbon → oxocarbenium → deprotonate → pinacolone. The migrating methyl is the only C–C bond available to shift; the product carries the tell-tale quaternary tert-butyl carbon.
• Yield. 60–75% pinacolone on a teaching scale; higher with efficient water removal.

Note that the product has a new quaternary carbon — the tert-butyl group — assembled by a bond migration, not by an S_N2 (which can never build a quaternary center). Being able to forge a quaternary carbon α to a carbonyl is one of the reaction's most prized capabilities.

Ring expansion and contraction: the synthetic payoff

The pinacol (and semipinacol) rearrangement shines when the migrating C–C bond is part of a ring. If the cation sits on a carbon exocyclic to a ring and a ring bond migrates, the ring expands by one carbon; if the cation is in the ring and an exocyclic bond migrates in, the ring contracts.

• Ring expansion. 1-(1-hydroxycyclohexyl)-substituted diols rearrange to ring-expanded cycloheptanones — a one-carbon homologation that is hard to do any other way. This "pinacol-type ring expansion" is a staple for building medium rings and spirocycles.
• Tiffeneau–Demjanov variant. A β-amino alcohol on a ring is treated with nitrous acid (HNO₂); diazotization makes an in-situ diazonium that loses N₂ to give the cation, the ring bond migrates, and the ring grows by one carbon. It is a semipinacol in disguise and a classic route from cyclohexanone to cycloheptanone.
• Total-synthesis workhorse. Asymmetric semipinacol ring expansions (via chiral catalysts on β-halo or β-epoxy alcohols) install quaternary stereocenters and expand rings enantioselectively — used in syntheses of terpenoids and complex natural products.

Limitations & side reactions

• Symmetric diols give clean products; unsymmetric ones can be messy. If two comparably stable cations can form, or two groups have similar migratory aptitude, you get product mixtures. This is why the controllable semipinacol version — where you pre-place the leaving group — largely superseded the parent reaction for real synthesis.
• Competing dehydration to a diene or allylic alcohol. Under strong acid and heat, the intermediate cation can also just lose a proton (E1) to give an alkene instead of rearranging. Tertiary substrates that can form conjugated dienes are especially prone to this.
• Reversibility / acid sensitivity. The dehydration is an equilibrium; excess water or too-mild conditions can strand the diol or send it back. Acid-sensitive functional groups (acetals, tert-butyl esters, epoxides elsewhere in the molecule) may not survive concentrated H₂SO₄.
• Carbocation scrambling. If the initially formed cation is long-lived, it can undergo further 1,2-shifts before the oxygen quenches it, giving skeletal isomers. Concerted (semipinacol) conditions suppress this.

Discovery: Fittig, 1859

The reaction is named for its founding substrate, pinacol, whose name comes from the Greek pinax (a tablet or plate) — the diol crystallizes in flat plate-like crystals. In 1859 the German chemist Wilhelm Rudolph Fittig (better known for the Wurtz–Fittig coupling) observed that treating pinacol with acid produced a new ketone, pinacolone, that had lost a molecule of water yet had the same carbon count. The carbon-skeleton rearrangement was puzzling before structural theory matured; it became a foundational example of the 1,2-shift and of what would later be understood as carbocation chemistry.

Georg Wagner and Hans Meerwein's early-1900s work on camphene rearrangements generalized the 1,2-shift into the broader Wagner–Meerwein rearrangement family, of which the pinacol rearrangement is the oxygen-terminated member. The concept of migratory aptitude was quantified through the 1920s–30s by comparing rates of aryl migration in substituted pinacols — a body of physical-organic work that helped establish how substituents stabilize cationic transition states.

Industrial and practical notes

• Pinacolone production. Pinacolone (3,3-dimethyl-2-butanone) is made industrially, partly via pinacol rearrangement chemistry, and is a building block for herbicides and pharmaceuticals (its tert-butyl-ketone motif appears in triazole fungicides and other agrochemicals).
• Safety. The reaction uses hot concentrated mineral acid — corrosive, with an exothermic dehydration. Add acid to substrate slowly, control the temperature, and quench spent acid into ice water (never the reverse). Pinacolone is flammable (flash point ≈ 5 °C) and should be distilled and stored away from ignition sources.
• Green variants. Solid-acid catalysts — montmorillonite clays, zeolites, and sulfated metal oxides — run the rearrangement heterogeneously, are recyclable, and avoid the neutralization waste of stoichiometric H₂SO₄. These are favored where the substrate tolerates the milder conditions.