Question

Why double bonds are present only in alkenes y not in alkanes n alkynes (cbse 2016) .Proper and relevant answer required

Answer 1

because 3 electrons are shared whereas in alkane 1 and in alkene 2 are shared

Answer 2

Double bond is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that is between two carbon atoms, can be found in alkenes.
The number of hydrogen atoms in an alkene is double the number of carbon atoms.

For example, the molecular formula of ethene is C2H4, while for propene it is C3H6.


And the simplest reason is that alkenes are unsaturated thus it contain double bond.

hope this helps :-)