Question

Alkanes are considered as paraffins. So they undergo substitution reactions but not addition reactions. Explain with suitable example?

Answer 1

Alkanes are already bonded strongly, while alkenes and alkynes have weak
π
-bonds. They want to undergo addition reactions to turn
π
-bonds to stronger
σ
's and become more stable.
Explanation:
Alkenes and alkynes are unsaturated - they have
π
-bonds, so don't have the full number of hydrogen that they could have.
This means that they are more unstable than alkanes, since
π
-bonds aren't as strong as
σ
-bonds. The alkenes and alkynes want to form more
σ
-bonds and have a structure more like an alkane, so they undergo addition reactions.
Addition reactions are where more atoms are added to the molecule, not swapped or taken away. This means that the
π
-bonds have to be taken away and used as
σ
-bonds with the new atoms, rather than the
σ
-bonds already there being reattached - it's easier to break
π
than
σ
.
Alkanes do not undergo this reaction because they already only have single
σ
-bonds, and so they cannot become more stable or stronger structurally - they are already at the peak, and so can only swap things around in substitution reactions.