Question

Explain why alkyl group acts as an electron donor when attached to a pi electron system?

Answer 1

An alkyl group is a monovalent group ( R- ), When it is linked to some other group or part of some other molecule it has a tendency to donate electrons. It is an electron donating group.

In an alkyl group, for example, in methyl group carbon is linked to three hydrogen atoms. Carbon is more electronegative than hydrogen. So carbon pulls bonded electrons nearer to carbon and carbon accquires a slight nxtra negative charge (delta-) and correspondingly hydrogen lose hold over bonded electrons and acquires a slight positive charge (indicated by delta +). Thus carbon get a little more electronic charge which the carbon will push towards the group to which it is linked.

Thus methyl group will be electrin releasing or electron donating in nature. In ethyl group (CH3-CH2-) this effect will be more than methyl. The CH3 part releases elecrons to the next carbon plus two hydrogens also make contributions. The net electron releasing effect of ethyl group is more than that of a methyl group.