A substitution reaction means the replacement of one or more hydrogen atoms in an alkane by another atom or group. It does not introduce a double bond into the product. The product retains single bonds between the species present.
For instance;
CH4 + Cl2 -------> CH3Cl +HCl
is a substitution reaction
Dehydrogenation is the process of removing two hydrogen atoms from a saturated compound compound thereby yielding an unsaturated product.
E.g
C2H6 --------->C2H4 + H2
The key difference is that substitution reaction yields a saturated product while dehydrogenation yields an unsaturated product.
