Once you can predict where two groups add across a double bond (regiochemistry, Track 18), the next layer is how they arrange in three dimensions — the stereochemistry. This matters because addition creates new sp³ carbons, and those carbons can become stereocenters. The central vocabulary is syn versus anti addition:

Syn addition delivers both new groups to the same face of the flat alkene.
Anti addition delivers the two new groups to opposite faces.

The mechanism dictates which you get, and this is the payoff for understanding mechanisms rather than memorizing products. Three mechanistic families to anchor:

Reactions through a bridged intermediate give anti addition. Halogenation (adding Br₂ or Cl₂) proceeds through a three-membered halonium ion — the halogen bridges both carbons, blocking one face. The nucleophile is therefore forced to attack from the opposite side, giving a strictly anti vicinal dihalide. The same bridged logic governs halohydrin formation (X₂ in water).
Reactions on a flat carbocation lose stereochemical control. Simple Markovnikov additions (HX, acid-catalyzed hydration) go through a planar carbocation that can be attacked from either face, so they are not stereospecific — you generally get a mix.
Concerted reactions deliver syn addition. Catalytic hydrogenation (H₂/metal) adds both hydrogens to the same face off the catalyst surface — syn. Hydroboration and the dihydroxylation reactions below are also syn for the same "everything happens at once on one face" reason.

The practical skill is a two-part prediction: use Markovnikov/anti-Markovnikov for which carbon gets what, then use syn/anti for which face — and, when the product has two stereocenters, translate that into whether you get a specific pair of enantiomers or diastereomers.

This track consolidates the important reactions whose regiochemistry runs opposite to the default Markovnikov outcome, because exams reliably test whether you can spot the "tell" in the reagent line and flip your prediction.

Hydroboration–oxidation (BH₃ or BH₃·THF, then H₂O₂/NaOH) converts an alkene into an alcohol with two special features simultaneously: the –OH ends up on the less substituted carbon (anti-Markovnikov) and the addition is syn. The reason is mechanistic and worth understanding rather than memorizing: boron is electron-poor and adds to the less hindered, less substituted carbon, while the hydrogen goes to the more substituted one — the reverse of proton-first additions. The whole four-center addition happens in one concerted step (hence syn), and critically there is no carbocation, so there are no rearrangements. Hydroboration is therefore the go-to method whenever you need the anti-Markovnikov alcohol.

Radical HBr addition (HBr with peroxides/ROOR/hν) gives the anti-Markovnikov alkyl bromide, as introduced in Track 18. The mechanism is a radical chain (initiation, propagation, termination): a bromine radical adds first, and it adds to give the more stable radical — which places Br on the less substituted carbon. The essential exam habit: whenever you see "peroxides" attached to HBr, reverse the regiochemistry. (This peroxide effect is specific to HBr; it does not work for HCl or HI.)

Contrast these deliberately with acid-catalyzed hydration and oxymercuration–demercuration, which both give the Markovnikov alcohol — the latter being useful precisely because it accomplishes Markovnikov hydration without rearrangements (it goes through a bridged mercurinium ion rather than a free carbocation). Lining these up as a set — Markovnikov vs. anti-Markovnikov, rearrangement-prone vs. rearrangement-free — turns a confusing pile of reagents into a small decision table.

Oxidative reactions of alkenes range from gently adding oxygen across the double bond to cleaving it entirely, and the trend to track is how much the pi bond is oxidized.

Epoxidation with a peroxyacid (mCPBA) converts the alkene into an epoxide — a strained three-membered ring containing oxygen. The oxygen is delivered to one face in a single concerted step, so the addition is syn and stereospecific. Epoxides are valuable because that ring strain makes them reactive toward ring-opening by nucleophiles (a topic revisited under ethers/epoxides).
Dihydroxylation with cold, dilute KMnO₄ or with OsO₄ adds two –OH groups across the double bond to give a vicinal diol (glycol), with syn stereochemistry (both –OH on the same face). Remember the pairing: syn-dihydroxylation gives a specific diastereomer, and dilute/cold conditions are the tell that you are hydroxylating rather than cleaving.
Oxidative cleavage breaks the double bond entirely. Ozonolysis (O₃, then a workup) cleaves the C=C and converts each end into a carbonyl. The workup determines the product: a reductive workup (Zn or dimethyl sulfide, (CH₃)₂S) gives aldehydes and ketones, while an oxidative workup (H₂O₂) pushes any aldehydes on to carboxylic acids. Hot, concentrated KMnO₄ likewise cleaves the alkene, giving carboxylic acids and ketones (and CO₂ from a terminal =CH₂).

Ozonolysis is also a powerful structure-determination tool, and exams exploit this "run it backwards" skill: given the carbonyl fragments produced, you can deduce the position of the original double bond by mentally rejoining the two C=O carbons into a C=C. Practice reasoning in both directions.

"The Other Way"