A bond between sp3 and sp2 is stronger than a bond between sp3 and sp3 because sp2 hybridized orbitals contains 33.33% s-character while sp3 contains 25% s-character. A general rule, the more s-character in a hybridized orbital, the stronger a bond it will form. Thus a bond between sp3 and sp2 with 33.33% + 25% % s-character will be more strong than a bond between sp3 and sp3 with 25% + 25% s-character.
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).
A bond between sp3 and sp2 is stronger than a bond between sp3 and sp3 because sp2 hybridized orbitals contains 33.33% s-character while sp3 contains 25% s-character. A general rule, the more s-character in a hybridized orbital, the stronger a bond it will form. Thus a bond between sp3 and sp2 with 33.33% + 25% % s-character will be more strong than a bond between sp3 and sp3 with 25% + 25% s-character.
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.
s electrons are closer to the nuclei than the p electrons (while in same energy level), so the bonds made up of s electrons are shorter than those made up of p electrons, extrapolate from this, the more s componet in the spx hybrid orbital, the shorter the bond. a shorter bond often means a stronger bond
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