# How does hyperconjugation affect bond lengths in alkenes?

How does the number of hyperconjugates affect the $\ce{C=C}$ bond length in the compounds propene and 3,3-dimethylbut-1-ene?

In general, is bond length directly or inversely proportional to the number of hyperconjugates?

• Have a look here for a detailed explanation on how hyperconjugation works on a frontier-orbital level. In propene and 3,3-dimethylbut-1-ene you have 3 bonds that can take part in hyperconjugation: in propene it's the 3 $\ce{C-H}$ bonds and in the other compound it's the 3 $\ce{C-C}$ bonds. So, the premise of your question seems to be wrong: The number of "hyperconjugates" is the same, only their types differ. Concentrating on the type, I would say the $\ce{C-C}$ bonds should be a bit more effective at hyperconjugation than the $\ce{C-H}$ bonds. – Philipp Nov 22 '15 at 13:40