Title 16. CARBON-CARBON DOUBLE BOND AS SUBSTITUENTS.pdf ✅

PHARMD GURU Page 1  We saw the double bond as a place in the alkene molecule where reaction occur: electrophilic or free-radical addition.  Besides providing a site for addition, the double bond exerts powerful effects on certain reactions taking place elsewhere on the molecule.  Although suffering no permanent change itself, the double bond plays an essential role in determining the course of reaction.  It is this part of alkene chemistry that we shall take up in this chapter: the carbon- carbon double bond, not as a functional group, but as a substituent.  A double bond, exerts its effect differently from an alkyl group, and, as a result, its effects are often more powerful. Most of these effects stem from the structural feature called conjugation: the location of the π orbital in such a way that it can overlap other orbitals in the molecule.  To implement our discussion of conjugation we shall make use of the structural theory called resonance. FREE-RADICAL HALOGENATION OF ALKENES: SUBSTITUTION VS. ADDITION & FREE RADICAL SUBSTITUTION IN ALKENES: Let us look at the structure of the simple alkene, propylene. CH2=CH—CH3 Propylene It contains a carbon–carbon double bond, where the same addition reactions that are characteristic of ethylene take place. With hydrogen chloride, for example, propylene undergoes electrophilic addition; with hydrogen bromide in the presence of peroxides, it undergoes free-radical addition. CARBON-CARBON DOUBLE BOND AS SUBSTITUENTS More readily than for ethylene