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Which of the following molecules contains the shortest C=C double bond?

1. CH2=C=CH2
2. CH3-CH=CH2
3. CH3-C(CH3)2-CH=CH2
4. CH3-C(CH3)=CH2

Correct answer: CH2=C=CH2

Solution

Greater s-character in the hybrid orbitals pulls bonding electrons closer to the nuclei, shortening the bond. In allene (CH2=C=CH2) the central carbon is sp hybridised (50% s-character) while the terminal carbons are sp2. The C=C bonds in allene are therefore shorter than the sp2-sp2 C=C bonds in the ordinary alkenes (options 2, 3, 4). Hence allene has the shortest C=C bond.

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