# how to distinguish between carbon-carbon double bond lengths

Which of the following should have the longest c=c?

\begin{align} \text{a)}\ \ \ \ &\ce{CH2=C=CH2}\\ \\ \text{b)}\ \ \ \ &\ce{CH3 -CH=CH2}\\ \\ \text{c)} \ \ \ \ &\ \ \ \ \ \ \ \ \ \ \ \ce{CH3} \\ &\ \ \ \ \ \ \ \ \ \ \ \ | \\ &\ce{CH3 -C -CH=CH2}\\ &\ \ \ \ \ \ \ \ \ \ \ \ | \\ &\ \ \ \ \ \ \ \ \ \ \ \ce{CH3} \\ \\ \text{d)}\ \ \ \ &\ce{CH3 -C=CH2} \\ &\ \ \ \ \ \ \ \ \ \ \ \ | \\ &\ \ \ \ \ \ \ \ \ \ \ \ce{CH3} \\ \end{align}

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

• I edited your post to replace the picture with typeset structures. – Ben Norris Dec 22 '15 at 19:19
• how do you make structures like this? – Prakhar Dec 24 '15 at 4:32
• The basics are on this help center page, with some more advanced stuff at this meta post. The use of the {align} environment and control of spacing to create multi-line structures took some practice. – Ben Norris Dec 24 '15 at 12:26

You can draw Hyper conjugating Structures to compare bond length. More the $\alpha$ Hydrogen of alkene, more are the Hyper conjugating structures, more is the delocalization of π-bond and more is the bond length (since double bond character decreases, bond length increases) Therefore answer should be (d). Hope it helps !