Hyperconjugation - Electromeric Effect

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Introduction

The displacement of an electron in an organic molecule can take place because of the presence of an appropriate attacking reagent. Such kind of electron displacement leads to bond polarization. A few effects that feature this electron displacement can be given as hyperconjugation and electromeric effect, where a brief explanation is provided below:

Electromeric Effect and its Types:

The electromeric effect is a temporary effect, which is experienced primarily in the presence of a vicinity's attacking reagent of an organic compound containing the multiple bonds (either a double or triple bond). In this effect, the complete transfer of the mutual pair of π-electrons to one of the atoms, along with several bonds, occurs at the request of the attacking reagent. Also, this effect ceases soon as the attacking reagent is removed from the reaction's domain. The electromeric effect is primarily categorized into 2 categories, which are listed below.

  • Positive Electromeric Effect (The +E Effect):

The positive electromeric effect can be described as the transfer of the π−electrons of the multiple bonds to the atom, where the reagent gets attached.

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  • Negative Electromeric Effect (The –E Effect):

The positive electromeric effect can be defined as the transfer of several bonds to the atom, where the reagent is not attached.

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Mesomeric Effect

The mesomeric effect is described as a property of substituents or functional groups, that are present in a chemical compound.

The mesomeric effect or Negative resonance can be denoted by -M or -R.


Hyperconjugation Definition

To define hyperconjugation, it can be described as a permanent effect. The C-H bond's localization of σ electrons of an alkyl group is directly attached either to an unsaturated system's atom, or an unshared p orbital's atom takes place.


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From the above representation, we can observe that one of the 3 C-H bonds of the methyl group can align in the plane of an empty 'p' orbital, and the electrons that are constituting the C-H bond in a plane with this particular 'p' orbital can then be delocalized within the empty 'p' orbital.


Often, we will see that hyperconjugation stabilizes the glucose so it allows spreading of the positive charge. Therefore, we can define that the greater the alkyl group number, which is attached to a positively charged carbon atom, the greater is the stabilization and hyperconjugation interaction of the carbonation. The relative stability based on the hyperconjugation can be given as follows.


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Cause of Hyperconjugation

Hyperconjugation is described as the stabilizing interaction, results from the interaction of the electron in a σ-bond (in general, C-C or C-H) with either an adjacent partially filled or empty p-orbital or a π-orbital to form an extended molecular orbital, which increases the stability of the system.


Depending on the bonding valence bond model, hyperconjugation is described as "double bond - no bond resonance." It is not what we would "generally" call resonance, though the similarity is perhaps useful.


Resonance vs. Hyperconjugation

Hyperconjugation is a crucial reason to understand that there is an increase in the number of alkyl substituents either in the radical center or in the carbohydrate, which leads to an increase in stability. Let us consider how a methyl group involves in the hyperconjugation with a carbocation center.


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First, we are supposed to draw it to represent the C-H σ-bonds.

It should also make a note that the empty 'p' orbital, which is associated with the positive charge at the center of the carbocation, is in a similar plane (it means coplanar) with one of the C-H σ-bonds (as shown in blue).


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This geometry is meant by the σ-bond's electrons can be stabilized with an interaction using the carbocation center’s empty p-orbital.

(this diagrammatical representation represents the similarity with resonance, and the structure on the right side has the character of "double bond - no bond").


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  • This stabilization arises due to the orbital interaction leads to the electrons being in the orbital of lower energy.

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  • Also, the C-C σ-bond allows free to rotate, and as it performs so, in turn, each of the C-H σ-bonds undergoes the stabilizing interaction.

  • The ethyl cation holds 3 C-H σ-bonds that are involved in the hyperconjugation.

  • The more hyperconjugation there exists, the greater the system's stabilization.

  • For suppose, the t-butyl cation contains 9 C-H σ-bonds that are involved in hyperconjugation.

  • Thus, (CH3)3C+ is more stable compared to CH3CH2+.

  • The effect is not only limited to the C-H σ-bonds, where, in addition, but the appropriate C-C σ-bonds are also allowed to be involved in the hyperconjugation.

Applications of Hyperconjugation

Hyperconjugation is used to rationalize various chemical phenomena, including the gauche effect, the anomeric effect, the beta-silicon effect, the rotational barrier of ethane, the relative stability of the substituted carbocations & substituted carbon centered radicals, the vibrational frequency of the exocyclic carbonyl groups, and finally, the thermodynamic Zaitsev's rule for alkene stability. Also, more controversially, hyperconjugation is proposed by the quantum mechanical modeling to give a better explanation for the staggered conformation preference rather than the traditional notion of steric hindrance.


Did You Know?

Why the effect of resonance is more powerful than hyperconjugation?

In resonance, pi electrons are transferred when in hyperconjugation, the sigma bond splits, and the electron is moved. However, we are already aware that the pi bond is weaker than that of sigma. So, it is easy to displace pi bonded electrons than that of sigma, and pi-electron easily involve in the resonance. Thus, the resonance takes place easily compared to hyperconjugation. Hence, resonance is more powerful than hyperconjugation.

FAQ (Frequently Asked Questions)

1. What is the Resonance Effect?

Answer: The resonance effect defines the polarity that is formed in a molecule by the interaction between either a pi bond or the lone pair electron or the interaction of 2 pi bonds in the adjacent atoms. In general, it is found in molecules with conjugated double bonds or in molecules having at least 1 lone pair and 1 double bond. Understanding the topic, resonance is important in understanding the compound's stability, including its energy state.

2. Explain How the Hyperconjugation Increases Stability.

Answer: The stability of free radical or carbocation will increase with an increase in the alpha Hydrogen. Alpha Hydrogen is hydrogen, which is connected to either sp3 hybridized carbon or active carbon.

3. Give an Example of Hyperconjugation?

Answer: One of the Hyperconjugation examples can be given as follows.

The CH3-CH=CH2 is more stable and has a higher heat if the hydrogenation than CH2=CH2, due to propylene's hyperconjugation stabilization.