Carbon–carbon double bonds are present in most organic and biological molecules, so a good understanding of their behavior is needed. In this chapter, we’ve looked at some consequences of alkene stereoisomerism and at the details of the broadest class of alkene reactions—the electrophilic addition reaction.

An alkene is a hydrocarbon that contains a carbon–carbon double bond. Because they contain fewer hydrogens than alkanes with the same number of carbons, alkenes are said to be unsaturated.

Because rotation around the double bond can’t occur, substituted alkenes can exist as cis–trans stereoisomers. The configuration of a double bond can be specified by applying the Cahn–Ingold–Prelog sequence rules, which rank the substituents on each double-bond carbon. If the higher-ranking groups on each carbon are on the same side of the double bond, the configuration is Z (zusammen, “together”); if the higher-ranking groups on each carbon are on opposite sides of the double bond, the configuration is E (entgegen, “apart”).

Alkene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted alkene, Markovnikov’s rule predicts that the H will add to the carbon having fewer alkyl substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic alkene π bond with electrophilic H⁺.

Carbocation stability follows the order: R₃C⁺ > R₂CH⁺ > RCH₂⁺ > CH₃⁺

Markovnikov’s rule can be restated by saying that, in the addition of HX to an alkene, a more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an alkene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation.

Evidence in support of a carbocation mechanism for electrophilic additions comes from the observation that structural rearrangements often take place during reaction.

Rearrangements occur by shift of either a hydride ion, :H⁻ (a hydride shift), or an alkyl anion, :R⁻, from a carbon atom to the neighboring positively charged carbon. This results in isomerization of a less stable carbocation to a more stable one.