Additions to C=C and C≡C — formula sheet

Addition: a reaction in which two atoms or groups add across a multiple bond, with no atom leaving. A π bond is destroyed and two σ bonds are made. Because σ bonds are stronger than π bonds, addition to an alkene is almost always exothermic; the interesting questions are never “does it go?” but where and from which face.

Syn addition: both new groups add to the same face of the former π system.Anti addition: the two new groups add to opposite faces.Stereospecific: each stereoisomer of the starting alkene gives a different stereoisomer of the product. This is a statement about a mechanism, and it is much stronger than ‘stereoselective’, which only says one product predominates.

Markovnikov selectivity (mechanistic statement): in a two-step electrophilic addition, the electrophile bonds to whichever alkene carbon leaves the more stable carbocation on the other carbon. The nucleophile then goes wherever the positive charge is. The regiochemistry is decided in the first step, by the energy of the intermediate, and by nothing else.

Bridged (onium) ion: a cyclic cation in which an electrophilic atom bearing a lone pair — Br, Cl, I, S, Hg — bridges the two former alkene carbons. The positive charge sits largely on the bridging atom. One face of the carbon–carbon unit is now physically blocked, so the nucleophile has no choice but to attack from the opposite face: anti addition, enforced by geometry.

Hydroboration: addition of a B–H bond across C=C through a concerted, four-centre transition state. No intermediate of any kind is formed. Consequences, all of them forced: the addition is syn; there is no rearrangement; and boron ends up on the less substituted carbon, so the hydrogen ends up on the more substituted one — the anti-Markovnikov pattern.

The peroxide effect (Kharasch effect): in the presence of a radical initiator, HBr adds to an alkene by a radical chain mechanism rather than an ionic one. Bromine, not hydrogen, adds first, and it adds to the carbon that leaves the more stable radical. The result is the anti-Markovnikov bromide.

1,2-Addition (direct addition): the nucleophile attacks the carbonyl carbon. The C=O is destroyed; the C=C survives. Product: an allylic alcohol.1,4-Addition (conjugate addition): the nucleophile attacks the β-carbon. The C=C is destroyed; the C=O survives (after the enol tautomerises). Product: a β-substituted carbonyl compound.The numbers count atoms along the conjugated system starting from oxygen: O(1)=C(2)–C(3)=C(4).