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I'm going through basic tutorials about hybridisation, but I am having trouble understanding the reason for drawing a triple bond for C2H2. It seems to me that a double bond is enough for the octet rule. What dictates the triple bond between the two carbons?

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    $\begingroup$ Could you show the Lewis structure with a double bond that satisfies the octet rule? $\endgroup$ – Karsten Theis Feb 6 at 22:23
  • $\begingroup$ Maybe it does not have to satisfy the octet rule. Cyclopropenyl cation gets along well without it, and molecular oxygen is just weird. Acetylene forms a triple bond because it has the right molecular orbitals to make that bonding most stable. $\endgroup$ – Oscar Lanzi Feb 7 at 3:09
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Firstly, nothing "dictates" the triple bond between 2 carbon atoms. The hybridisation of the carbon atom determines the number of pi bonds it has, and a carbon with 3 pi bonds and 1 sigma bond will have 3 unhybridised p orbitals and hence 2 regions of electron density, thus it is sp hybridised. For example, an alkyne can be formed from the two-fold elimination of either a geminal or vicinal dihalide. I think u may be having a misconception here, hybridisation of carbon atoms do not simply change and alkenes do not interconvert to form alkynes; strictly speaking, it is an oxidation from alkane to alkene to alkyne since oxidation state of carbon atom increases.

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