σ(C-H) --> π*(C=X) interaction makes it little stronger.

http://en.wikipedia.org/wiki/Hyperconjugation

@Dragan.....Can you explain bit more...Although you have given your link, I would love to read your analysis...

It is a hyperconjugation effect. Hyperconjugation is the stabilising interaction that results from the interaction of the electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a π-orbital to give an extended molecular orbital that increases the stability of the system.

Hyperconjugation is a factor in explaining why increasing the number of alkyl substituents on a carbocation or radical centre leads to an increase in stability.

Dear Hirbod

If you wanna get more details about the hyper conjugation phenomena, try to perform some NBO analysis on this matter. This calculations show the molecular orbitals interactions which lead to decreasing the total energy (and amount of this value).

Thanks everybody. . .

I shall answer by a simple example:

single bonds to sp2-hybridized carbon are somewhat shorter than single bonds to sp3-hybridized carbon. The carbon-carbon single bond of propene, for instance, is derived from the overlap of a carbon sp3 orbital of the -CH3 group with a carbon sp2 orbital of a carbon. A carbon-carbon bond of propane is derived from the overlap of 2 carbon sp3 orbitals. Because the electron density of an sp2 orbital is somewhat closer to the nucleus than the electron density of an sp3 orbital, a bond involving sp2 orbital, such as the one in propene, is shorter (i.e. stronger) than one involving only sp3 orbitals, such as the one in propane.

Carbon SP2 carries less hydrogen atoms therefore is less negative than carbon SP3. So, the charge difference between an SP2 carbon and an SP3 carbon is higher than two SP3 carbon, That means there is less coulombic repulsion between an SP2 carbon and an SP3 carbon atom than between two SP3 carbon atoms

Similar to π-π-orbital over lap in 1,3-dienes leading to resonance structures like...-C=C-C=C- -C+-C=C-C-- , called conjugative effect leading to a ‘double bond character’ for the bond between C(2) –C(3) , In a system with -CH-C=C- type of bonding, there is a σ(C-H) orbital overlaps with the π-orbital of the adjacent C=C , called ‘hyperconjugative effect’, leading to resonance structure like H+ -C=C-C-- making the C(1)-C(2) bond to acquire a double bond character….obviously double bond character makes the bond stronger.....

Prof. Charles

Normally, s-orbitals have lower energy than p orbitals. Hence an sp3-sp2 bond has more s-s contribution than na sp3-sp3 bond. This might explain the stronger bond.