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Which of the following compounds should have the longest carbon-carbon double bond?

  1. $\ce{CH2=C=CH2}$
  2. $\ce{H3C-CH=CH2}$
  3. $\ce{(H3C)3-C-CH=CH2}$
  4. $\ce{(H3C)2C=CH2}$

I know that single bond > double > triple length-wise, but how do you distinguish between double bonds?

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You can draw hyperconjugation structures to compare bond length. The more α-hydrogens the alkene has, the more hyperconjugation structures it has, and the higher is the degree of delocalization of the π-bond and therefore the longer is the bond. When double bond character decreases, bond length increases. Therefore the answer should be (d).

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