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

1 Answer
reudhreghs
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.

Related questions
See all questions in Introduction to Reactions and Mechanisms
Impact of this question
35276 views around the world
You can reuse this answer
Creative Commons License