Hyperconjugation effect

 

Most of the delocalisation effects involve π electrons. There are only a few examples where conjugation involves σ electrons. This type of conjugation may be called extended conjugation. When a carbon containing at least one hydrogen is attached to a π bond, the σ electrons of the C-H bond become involved in delocalisation with the π electrons of the unsaturated system, i.e., this is a σ-π conjugation. This special type of conjugation is known as hyperconjugation. The prefix, hyper- of the title means excessive. Therefore, the excessive, i.e., extended conjugation involving σ orbitals generally of C-H and C-X(X = F, Cl) or C-C bond, is called hyperconjugation. Hyperconjugation also includes σ-p (partially filled or vacant) type of orbital overlapping. According to molecular orbital theory, an extended orbital is the result of such kind of σ-π or σ-p overlaps. Thus in propene, delocalisation occurs between C=C π M.O. and C-H σ M.O.s of the carbon atom to the double bonded carbon; here the extended orbital does not encompass two π bonded carbon atoms or two σ-bonded carbon atoms or σ-bonded carbon and hydrogen atoms only but it holds all four atoms together.

In the language of resonance this may be pictured as a resonance hybrid of the following resonating structure.

As the structures 2, 3 and 4 do not have any formal Bond between H⁺ ion and carbon atom, the state of molecule is known as no-bond resonance. The H⁺ iron does not leave the species. Each of these structures is hypothetical and non-existent. This sort of resonance too stabilizers the entity, though no-bond resonating structures bare isolated charges and have fewer number of covalent bonds than the main uncharged form. This is a flat contradiction of the fact that is resonance hybrid consisting of such structures should be of low stability. For this reason, nowadays this type of hyperconjugation is turned as sacrificial hyperconjugation.

In ethyl free radical (^.CH₂-CH₃), we encounter with overlap between C-H orbital and incomplete orbital of the adjacent carbon atom. Here the extended orbital encloses the two carbon and one hydrogen atoms.

Similarly, in ethyl cation (CH₃CH₂⁺) hyper conjugation is regarded as an overlap of the C-H orbital and the vacant p orbital of the positively charged carbon atom; the extended orbital formed holds the two carbon atoms and the hydrogen atom together.

Thus, in free radicals and in carbenium ions with adjacent C-H σ bonds, the resonating structures display no more odd electron or charge separation that do the main structures and all of the structures poses the same number of covalent bonds. Hence this type of hyperconjugation is termed as isovalent hyperconjugation.

The effect of hyperconjugation is that of electron release from the carbon atom adjacent to the π-bonded carbon atom or electron deficient carbon atom (i.e., a carbon atom with odd electron or positive charge). When it happens to occur in the ground state of a molecule, some of the physical and chemical properties of the molecule can be explained with its help. On the other hand, when hyperconjugation takes place in the excited state of the entity, it can then only be helpful in explaining chemical reactivity of the entity. It is here worth mentioning that the hyperconjugative effect was fast noted by Baker and Nathan in connection with the anomalous inductive electron release by p-substituted groups in benzyl bromide; this effect came to be called Baker-Nathan effect. However, hyperconjugative effect can be utilized to explain the chemical reactivity and stability of entities.

As an example we can see that in ethane the C-C bond length is 1.54 Å but in propene C-C bond length is 1.50 Å. Owing to the hyperconjugative effect in propene, C-C single bond has some double bond character as seen from the following resonating structures.

Therefore the C-C bond length should be a bit longer than C=C bond and shorter than C-C single bond. But charged structures with fewer numbers of covalent bonds are less contributing to the resonance hybrid than the uncharged structure. For this reason the shortening of bond length is not so much as expected. Hence it is very near to 1.54 Å.

In this connection we can also see propene, which is more stable than ethene due to hyperconjugation. Hyperconjugation is actively present and influence the stability of propene where as it is absent in ethene. The heat of hydrogenation value of ethene is greater than that of propene by 2.7 kcal/mole. However, the sp³-sp³ bond character also explains the stability and shorter bond length of H₃C-CH bond.

When an alkyl group is attached to a benzene ring, the C-H σ-bond of the carbon atom, which is directly joined to the benzene ring, overlaps with the π-orbital of the ring. For each C-H σ-bond three charged resonating structures are found. As a methyl group has three C-H σ-bonds, we get nine charged resonating structures in the resonance hybrid.

In an ethyl group there are two such C-H σ-bonds. We should have eight resonating structures only for ethyl benzene; six of them are charged structures and the rest to our uncharged structures. In the resonance hybrid of isopropyl benzene we find only five resonating structures, two uncharged and three charged structures. This is because of the presence of only one C-H σ bond α to the benzene ring.

tert-Butylbenzene does not have any no-bond resonating structures as there is no C-H σ-bond on the carbon atom bonded to the benzene ring.

Therefore we can say that alky groups attached to the benzene ring have electron releasing effect in the order

Me- > MeCH₂-> Me₂CH- > Me₃C-

This is quite opposite to that expected from inductive electron release.

The stability of alkylcarbenium ions and radicals is well explained by considering inductive and hyperconjugative effects and the order are found to be:

 (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃-CH₂⁺ > CH₃⁺

(CH₃)₃C^̇ > (CH₃)₂CH^̇ > CH₃-CH₂^̇ > CH₃^̇

Two effects are to be considered for explaining the given order:

Hyperconjugation effect

We know that the more the number of contributing structures of comparable energy in a resonance hybrid, the greater is the stability. In tert-butyl cation, there are nine α C-H σ-bonds and hence its resonance hybrid consists of ten resonating forms, nine of which are uncharged structures; we shall have six similar resonating forms in isopropyl cation, Three such in ethyl cation and none in methyl cation. Therefore, the order of stability of the carbenium ions is as given in the question.

Inductive effect

This can be explained by +I effect of the methyl groups. Thus, due to the +I effect of the three methyl groups attached to the positively charged carbon atom of tert-butyl cation, its charge is neutralized to a greater extent than that of isopropyl cation which possesses only two methyl groups for such an effect; this charge neutralization effect is still days in the case of ethyl cation as it contains only one methyl group and it is least in the methyl cation because it does not have any electron releasing group attached to the positively charged carbon atom.

The order for the stability of the radicals can be explained by hyperconjugative effect.

(CH₃)₃Ċ has nine resonating forms without odd electron in its resonance hybrid, (CH₃)₂CḢ has six such structures, CH₃CH₂̇ possesses only three structures without odd electron, and methyl radical has none. Therefore, the order of contributing structures in the resonance hybrid of the radicals is

Key points about hyperconjugation include:

1) Stabilization of molecules: Hyperconjugation sheds some light on stabilization and destabilization of organic molecules. This theory emphasizes on delocalization that spreads out the charge in an organic molecule, reducing its overall energy and stabilizing it in the process. For example, in the case of CH₃-CH₂⁺ carbocation, the adjacent alkyl groups can donate electron density through hyperconjugation, leading to the stabilization of the positive charge on the carbocation as mentioned previously. Similarly, hyperconjugation also stabilizes radicals. This increased stability explains several reaction intermediates and pathways which is crucial for determination of reaction mechanisms.

2) Alkenes and conjugated systems: Hyperconjugation can explain the stabilization of several alkenes and polyenes. In these systems the π-electrons are delocalized over multiple carbon atoms, and hyperconjugation further stabilizes the system by allowing the adjacent σ-bonds to interact with the π-orbitals. Such extended double involving σ-π bonds also explains the electron delocalisation.

3) Transition states and reaction rates: Hyperconjugation can influence reaction rates as well as transition states of reactions. For example in elimination reactions such as E1 and E2, hyperconjugation can stabilize the transition state leading to the formation of the double bond. This stabilization phenomenon in such molecules lowers the activation energy barrier for the reaction which makes elimination process more and reaction rate favourable.

4) Spectroscopic effects: Effects of hyperconjugation can be observed though spectroscopic methods, particularly through nuclear magnetic resonance (¹H-NMR) spectroscopy. The presence of hyperconjugation can lead to changes in the chemical shifts observed in NMR spectra. For example, in ¹H-NMR, the presence of –CH₃ group generating the hyperconjugative interactions can cause the chemical shift of a proton to deviate from its typical value, providing insights into the local electronic environment of the molecule.

5) Theoretical understanding: Hyperconjugation is a hypothetical concept. But it is supported by theoretical calculations, particularly molecular orbital theory. This theory explains how the overlap of orbitals involved in hyperconjugative interactions leads to σ-π overlapping and the stabilization of molecules. Therefore theoretically hyperconjugation is provides a strong argument which portray stabilities in several organic molecules.

Overall, hyperconjugation is a hypothetical concept in organic chemistry that gives a strong explanation of stability of various organic molecules, reaction rates and transition states, chemical shifts etc. For this reason, it is one of the most important molecular effects discussed in organic chemistry.

Reference

1) Jonathan Clayden, Nick Greeves, Stuart Warren, organic chemistry book second edition.