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.

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