Additions to Substituted Alkenes

For alkenes that have halogen or similar substituents at the doubly bonded carbons, the same principles apply as with the simple alkenes. That is, under kinetic control the preferred product will be the one derived from the more stable of the two possible intermediate carbon cations. Consider a compound of the type Y−CH=CH2. If Y is more electron-attracting than hydrogen, then hydrogen halide should add in such a way as to put the proton of HX on the YCH= end and X on the =CH2 end. The reason is that the positive carbon is expected to be more favorably located if it is not attached directly to an electron-attracting substituent:

The addition goes as predicted, provided that the atom directly attached to the carbon of the double bond carries no unshared (nonbonding) electron pairs. For example,

Such substituents are relatively uncommon, and most of the reported H−X additions have been carried out with YY groups having unshared electron pairs on an atom connected directly to a carbon of the double bond:

These substituents usually are strongly electronegative relative to hydrogen, and this often causes diminished reactivity of the double bond toward electrophiles. Nonetheless, the preferred orientation of HX additions situates the positive charge of the intermediate carbocation next to the substituent:

The electron-attracting power of the substituent is more than counterbalanced by stabilization of the intermediate cation by the ability of the substituents to delocalize their unshared electrons to the adjacent positive carbon.