Since there are two pi bonds in alkynes - are they more reactive? Is it correct? I heard that acetylene is less reactive than ethylene.
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The reactivity order depends on the process(as mentioned in the comment).
1) In acid-base reactions, alkynes are the most reactive followed by alkenes and alkanes. This is due to the stability of conjugate base of alkyne by sp hybridized carbon atom.
2)In electrophilic substitution, alkynes are less reactive than alkenes. It is because of relative stability of carbocation formed in the case of alkenes.
3)In case of hydrogenation reactions, alkynes are more reactive than alkenes.
The reactivity is:
Alkynes > Alkenes > Alkanes.
Alkynes have two pi-bonds between two (or more) carbon atoms, along with a sp-sp hybridised orbital bonding (sigma bond). The pi-bonds can be easily broken to free the valence shell electrons for combining with other atoms. Hence they are the most reactive.
Alkenes have one pi-bond between two (or more) carbon atoms, along with a sp2-sp2 hybridised orbital bonding (sigma bond). The pi-bond can be easily broken to free the valence shell electrons for combining with other atoms, but as the number of pi bonds is lesser than Alkynes, they are less reactive.
Alkanes have no pi-bonds between carbon atoms. They only have a sp3-sp3 hybridised orbital bonding (sigma bond). For Alkynes to react, first the sigma bonds with Hydrogen (s-sp3 sigma bonds) have to be broken, and only then can other atoms combine. This entails more energy, so Alkanes are the least reactive among the three.
This also explains why Alkynes and Alkenes undergo addition reactions, while Alkanes only undergo substitution reaction.