Addition reactions of alkenes and alkynes

A carbon-carbon double bond consists of two different kinds of bonds overlaid on each other: a sigma bond, formed by direct end-on orbital overlap along the axis between the two nuclei, and a pi bond, formed by sideways overlap of unhybridised p-orbitals above and below that axis. The pi bond is weaker than the sigma bond and, because it is formed by sideways rather than direct overlap, its electron density sits more exposed above and below the plane of the molecule rather than tucked directly between the two nuclei. This exposed, relatively loosely held electron density is exactly what makes alkenes (and, more so, alkynes with their two pi bonds) attractive targets for electrophiles — an electron-poor species approaching the molecule is drawn toward this available electron density long before it gets close to the more tightly held sigma-bond electrons.

The general mechanism for electrophilic addition proceeds in two main steps. First, the electrophile is attracted to the pi bond and forms a new sigma bond to one of the two alkene carbons, using the pi bond's electron pair; this converts the alkene into a positively charged carbocation intermediate at the other carbon, since that carbon lost its share of the pi electrons without gaining a new bond. Second, a nucleophile — often the counterion released alongside the electrophile — attacks this electron-poor carbocation, forming a new bond and completing the addition. The net result is that the carbon-carbon double bond becomes a single bond, and each of the two carbons that were part of the double bond has gained one new substituent, exactly matching the addition of one electrophile and one nucleophile across the original pi bond.

This same two-step logic — electrophile attacks the pi bond, nucleophile attacks the resulting cationic intermediate — extends to a wide family of alkene reactions, including addition of hydrogen halides, addition of halogens (Br2, Cl2), and, with water as the ultimate nucleophile and an acid catalyst, hydration of alkenes to alcohols. Alkynes, with two pi bonds available, can in principle undergo addition twice, first converting a triple bond to a double bond and then, with a second equivalent of reagent, converting that double bond to a single bond — though the intermediate alkene product can often be isolated by controlling the reaction conditions.