Why does alkyne and alkene undergo addition reaction whereas alkane does not?

Apr 8, 2016

Alkanes are already bonded strongly, while alkenes and alkynes have weak $\pi$-bonds. They want to undergo addition reactions to turn $\pi$-bonds to stronger $\sigma$'s and become more stable.

Explanation:

Alkenes and alkynes are unsaturated - they have $\pi$-bonds, so don't have the full number of hydrogen that they could have.

This means that they are more unstable than alkanes, since $\pi$-bonds aren't as strong as $\sigma$-bonds. The alkenes and alkynes want to form more $\sigma$-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 $\pi$-bonds have to be taken away and used as $\sigma$-bonds with the new atoms, rather than the $\sigma$-bonds already there being reattached - it's easier to break $\pi$ than $\sigma$.

Alkanes do not undergo this reaction because they already only have single $\sigma$-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.