How to distinguish between carbon-carbon double bond lengths? Which of the following compounds should have the longest carbon-carbon double bond?
  
  
  $\ce{CH2=C=CH2}$
  $\ce{H3C-CH=CH2}$
  $\ce{(H3C)3-C-CH=CH2}$
  $\ce{(H3C)2C=CH2}$
  


I know that single bond > double > triple length-wise, but how do you distinguish between double bonds?
You can draw hyperconjugation structures to compare bond length. The more &alpha;-hydrogens the alkene has, the more hyperconjugation structures it has, and the higher is the degree of delocalization of the &pi;-bond and therefore the longer is the bond. When double bond character decreases, bond length increases. Therefore the answer should be (d).