Question

Which is more stable, 1,3-butadiene or 1,4-pentadiene? Why?

Answer 1

There is no such things as 1,4-butadiene. The numbering tells you whic carbon the double bond starts at. So 1 tells you the first double bond is between carbons 1 and 2 (doesn’t matter which end of the four-carbon chain you start counting at: the answer will be the same). The next double bond can’t start at the 2 position (this would be an allene not a diene) which means the only place left for the second double bond is between the 3 and 4 carbons, i.e it starts at the 3 position. Hence 1,3-butadiene, or just butadiene as there isn’t any choice.