Alkenes are the first genuinely reactive functional group you study in depth, and their chemistry introduces a new reaction type: addition, where the C=C pi bond breaks and two new groups add across the former double bond (the reverse, conceptually, of elimination). The underlying logic is everything you learned about nucleophiles, electrophiles, and carbocations, now applied to a pi bond.

Why do alkenes react this way? The pi bond is electron-rich and exposed — sitting above and below the molecular plane, weakly held, and eager to donate its electrons. That makes the alkene itself the nucleophile, and it reacts with electrophiles. This is the reverse of the substitution chapter, where carbon was the electrophile; here carbon (the alkene) is the electron donor. The general mechanism, called electrophilic addition, goes: (1) the pi electrons attack an electrophile, forming a carbocation on the more stable carbon, then (2) a nucleophile attacks that cation.

Because a carbocation forms, one rule governs the regiochemistry of nearly all these reactions — Markovnikov's rule. When an unsymmetrical reagent like HBr adds across an unsymmetrical alkene, the hydrogen adds to the carbon that already has more hydrogens, and the halogen (or other group) adds to the more substituted carbon. The reason is pure carbocation stability: the proton adds first in whatever way generates the more stable carbocation (3° over 2° over 1°), and the nucleophile then goes wherever that cation formed. The catchphrase "the rich get richer" captures it, but understanding it as "the pathway through the more stable carbocation wins" is what lets you handle every variant. And because carbocations are involved, Markovnikov additions can show rearrangements — another callback to Track 14.

A crucial exception is the radical addition of HBr in the presence of peroxides, which gives anti-Markovnikov product (bromine on the less substituted carbon). This proceeds by a radical mechanism (fishhook arrows, Track 3) rather than a carbocation, and because the selectivity is reversed, exams love to hide "peroxides" or "ROOR" in the reagent line as the tell.

The broader family of alkene additions you'll learn to recognize includes: hydrohalogenation (add H–X), hydration (add H–OH to make an alcohol, acid-catalyzed and Markovnikov), halogenation (add X₂ to give a vicinal dihalide, notably with anti stereochemistry through a bridged "halonium" intermediate), and hydrogenation (add H₂ across the double bond over a metal catalyst to give the alkane, with syn addition). Each is really the same story — pi bond as nucleophile, plus an electrophile — differing only in the reagent and the resulting stereochemistry. Learning to see the common thread rather than memorizing each in isolation is exactly the mechanistic maturity this unit is designed to build.

"Add It On"