Hyperconjugation

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HYPERCONJUGATION

HYPERCONJUGATION IN CARBOCATIONS

The hyperconjugation is a way to stabilized carbocations. When we have a carbocation, the H of the C beside the carbocation, give the electron density of the C-H  bond to the carbocation. This interaction occur if the H and the 2p empty orbital of the carbocation are in the same plane In other words, the filled  bond overlaps with the pempty orbital of the carbocation.


Picture 1. Hyperconjugation in ethyl cation.

Between two carbons the new bond is not enough strong like a double bond but is stronger than a simple bon, so the bond length of this bond is between the single bond and the double bond. Remember that the double bond is forming by 2p-2p overlap and in hyperconjugation; the overlap is formed by 2p-sp3 and thisis not as strong as 2p-2p overlap.

• The single C-C bond length is 1.54Å
• The double C-C bond length is 1.35Å
• The bond length in is 1.46Å (because the hyperconjugation)


The next diagram shows the energy levels in the hyperconjugation:


Diagram 1. Energy diagram of hyperconjugation in a carbocation.

As we can see, after the hyperconjugation the electrons are in a lowerenergy state, so with the hyperconjugation we obtain a more stable system.

The stability increase if we have more C-H bonds in the carbocation, because increases the possibility of the hyperconjugation. In other words, the more highly substituted the carbocation, is more stable.


Picture 2. Hyperconjugation in tert-butil carbocation.

The stability order: tBu > Pri > Et > Me

Also aprimary carbocation has only two hyperconjugative forms, and a tertiary has a hyperconjugative forms per H has the C beside the carbocation.







Picture 3. Hyperconjugation forms in a) primary carbocation; b) tertiary carbocation.

In the other hand, in case of vinyl and phenyl carbocations are not stabilized as well as the similar aliphatic molecules because the hyperconjugationin this case is between sp2 and the p orbitals, and sp2 orbitals are stronger than the sp3 orbitals, so the sp2 orbitals are worse electron donors.


Picture 4. Hyperconjugation in vinyl carbocation.

Furthermore, if the overlap is between a sp2 orbital and 2p orbital of the carbon, the C-C-H angle is 120º more than the sp3 2p overlap (109.5º), and orbitals are farther.

HYPERCONJUGATION INC=C BONDS
To have hyperconjugation is not necessary to have a carbocation; if we have double bonds, also we have hyperconjugative forms with this alkene.


Picture 5. Hyperconjugaation forms in the case of double bond.

DIPOLE MOMENT

If we have hyperconjugative forms, the dipole moment low, we can see that for example comparing next molecules:µ=2.72


µ=3.67
Picture 6. Dipole moments in two similar molecules to compare the effect of the resonance and the hyperconjugation effect.

The resonance effect is stronger than hyperconjugative effect. As we can see, in the resonance form, the charges aremore separate, than in the hyperconjugative form, is the reason for increases the dipole moment in resonance forms.

In the case of the different substitution, the dipole moment increase with increasing the substitute groups, we can see that in the next table:

PhCH3 PhCH2CH3 PhCH(CH3)2 PhC(CH3)3
μ 0.37 0.58 0.65 0.70
Table 1. Dipole moment of similar phenyl compounds.

This is because,if the sustituient is only a methyl group, it has more hyperconjugation forms, so the positive and negative charges are deslocalized in the molecule, so the dipole moment is smaller.

Picture 7. Hyperconjugation forms in methyl sustituited phenyl group.

But in the other hand in tert-butil sustituient, it has few hyperconjugation forms, so, the negative and positive charges are more...
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