One Goal, Two Faces of It
Earlier in this rung you watched an alkene meet an acid and undergo hydrohalogenation — H and a halogen straddling the old double bond. Now we change the thing being added: instead of H-Br, we add the elements of water, H and -OH, across the C=C. The product is an alcohol, and turning a cheap alkene into an alcohol is one of the workhorse transformations of the whole subject. But 'add water' hides a real question. The double bond has two carbons; the H goes on one and the -OH on the other. Which carbon gets which? And does the addition come from one face of the flat alkene or from both?
The honest punchline of this guide: the alkene does not decide — the reagent does. Three classic recipes all hydrate an alkene, yet they fall into two camps. Acid-catalyzed hydration and oxymercuration place the -OH on the more substituted carbon (the Markovnikov outcome). Hydroboration-oxidation places it on the LESS substituted carbon (the anti-Markovnikov outcome) and, as a bonus, controls which face it adds from. Same starting alkene, opposite alcohols, simply by reaching for a different bottle. That control is the deep idea worth carrying away.
Acid-Catalyzed Hydration: Markovnikov via a Carbocation
The most direct route is acid-catalyzed hydration: alkene plus water with a strong acid catalyst (dilute sulfuric or phosphoric acid). Its mechanism is the mirror image of the hydrohalogenation you already know — same first step, just a different nucleophile mopping up at the end. The pi electrons of the double bond are electron-rich and act as the nucleophile; the proton of the acid is the electrophile. Walk it through.
- Protonation. The pi electrons reach out and grab a proton from the acid. The arrow's tail is on the C=C pi bond, its head on the H. One carbon now holds the new H; the other carbon, robbed of its share of the pi electrons, becomes a positively charged carbocation.
- Water attacks. A water molecule, with its lone pairs, drifts in and bonds to the empty, hungry carbocation carbon. The arrow runs from water's oxygen lone pair to the positive carbon. The carbon is now bonded to oxygen — but that oxygen still carries the two hydrogens it came with, so it bears a positive charge (an oxonium, -OH2 plus).
- Deprotonation. A second water molecule plucks one proton off that oxonium, leaving a neutral -OH. The catalyst's proton, borrowed in step one, is handed back to solution here — which is exactly what 'catalytic' means: the acid is regenerated, not consumed.
Regiochemistry falls out of step one. The proton adds so as to make the MORE stable carbocation — tertiary beats secondary beats primary, because more attached carbons donate electron density through hyperconjugation and the inductive effect to soothe the positive charge. Water then bonds to that more-substituted carbon, so the -OH ends up there. That is exactly Markovnikov's rule, and here is the honest framing the rule is often taught badly: Markovnikov is NOT really 'H adds to the carbon with more H's already.' That rhyme just happens to give the right answer for simple cases. The true cause is carbocation stability — the proton goes wherever leaves behind the steadiest cation.
Oxymercuration: Markovnikov Without the Rearrangement
Suppose you want the Markovnikov alcohol but the carbocation's bad habits — rearrangement and harsh acid — are ruining your yield. Oxymercuration (more fully oxymercuration-demercuration) is the cleaner Markovnikov route. The alkene is treated with mercuric acetate (Hg(OAc)2) in water, then the carbon-mercury bond is removed with sodium borohydride. The net result is the same regiochemistry as acid hydration — -OH on the more substituted carbon — but with essentially no rearrangement. The trick is what replaces the open carbocation.
When the pi electrons attack the mercury electrophile, they do not make a fully open cation. Instead the mercury bridges across BOTH carbons, forming a three-membered ring called a mercurinium ion — picture mercury sitting like a lid over the top of the old double bond, sharing a bond with each carbon. That bridge is the key. There is no naked, flat cation sitting still long enough to rearrange, so the skeleton stays put. Yet the bridge is lopsided: the more substituted carbon bears more of the positive character, so when water attacks the ring from the back, it goes to that carbon. Markovnikov regiochemistry, delivered through a bridge instead of a free cation.
This is a beautiful illustration of how a reagent buys you a particular outcome. The acetate-mercury reagent is engineered to bridge rather than to open, and that single structural choice — a closed three-membered ring versus a flat empty p orbital — is the whole difference between 'rearranges' and 'does not.' The halonium-ion bridges you met in halogenation earlier in this rung work on exactly the same principle. Same family of trick: cap the carbocation with a bridge and you keep Markovnikov selectivity while taming the rearrangement.
Hydroboration-Oxidation: Anti-Markovnikov and Syn
Now flip the answer over. Hydroboration-oxidation takes the very same alkene and puts the -OH on the LESS substituted carbon — the anti-Markovnikov product — and it does so with clean, predictable stereochemistry. It runs in two stages. First, borane (BH3, often as its BH3-THF complex) adds across the double bond. Then a second reagent set, hydrogen peroxide in base, swaps the boron for an -OH with retention of position. The magic all happens in stage one, where there is no carbocation at all.
Boron is electron-poor — it has only six electrons around it, an empty orbital begging for a pair — so boron is the electrophile and adds to the carbon, while the hydrogen it carries goes to the other carbon. Crucially, the boron and the hydrogen add at the same instant, in one concerted four-membered transition state: the C=C, the boron, and one of boron's hydrogens all touch at once. Nothing ever comes apart, so there is no flat cation, no chance to rearrange. Two consequences follow directly, and they are the whole personality of this reaction.
Regiochemistry first. Why does boron prefer the less substituted carbon? Two forces, pulling the same way. Sterically, boron is bulky (it brings two more H's, and in practice often two whole alkyl groups), so it fits more comfortably on the roomier, less crowded carbon. Electronically, in that concerted transition state partial positive charge builds on the carbon NOT bonding to boron; that partial charge is better tolerated on the more substituted carbon, which is exactly the LESS substituted carbon for boron. Both effects park boron on the less substituted carbon — so after oxidation the -OH sits there too. Anti-Markovnikov, and notice it arises not from breaking Markovnikov's logic but from a different reagent shifting which carbon goes slightly positive.
Stereochemistry second. Because boron and hydrogen add together in one motion from the same face of the flat alkene, the -OH and the new H end up on the SAME side — a syn addition. (The oxidation step replaces boron with -OH at the very position boron held, with retention, so the geometry set in stage one survives.) Where acid hydration's flat cation gave a racemic, uncontrolled stereocenter, hydroboration is stereospecific: the concerted, one-face mechanism locks in syn geometry. This is the same theme as E2 from the elimination rung — concerted, geometry-locked reactions hand you stereochemical certainty, while stepwise cationic ones scramble it.
Reading the Toolkit: Reagent as Switch
ALKENE: R2C = CH2 (R = alkyl on the left carbon) H3O+ , H2O --------> R2C(OH)-CH3 Markovnikov, racemic, may rearrange Hg(OAc)2/H2O; NaBH4 -> R2C(OH)-CH3 Markovnikov, no rearrangement 1) BH3 2) H2O2/OH- -> R2C(H)-CH2OH anti-Markovnikov, SYN, no rearrangement carbocation stability order: 3 > 2 > 1 > methyl
Step back and the real lesson is not three recipes to memorize but one habit of mind. When you want to predict — or design — an addition, ask two separate questions. First, regiochemistry: which carbon does the -OH want? Trace the mechanism to the cation or the partial charge; whichever carbon is more positive in the rate-determining transition state is where the nucleophile (and so the -OH) goes. Second, stereochemistry: from which face? A flat free cation is open to both faces and racemizes; a bridged or concerted pathway adds to one face and gives a defined syn (or anti) product. Both answers are written into the mechanism before you ever name the rule.