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Thorough Chemistry on Hyperconjugation

List of Topics Included

(1) Meaning of the term, “hyperconjugation”

(2) Conditions required for hyperconjugation

(3) How does the phenomenon of hyperconjugation take place? (Means mechanism of hyperconjugation)

(4) Why does hyperconjugation give stability to the organic species?

(5) Three other names of hyperconjugation

(5 A) Baker-Nathan effect

(5 B) No bond resonance

(5 C) σ-π conjugation

(6) Application of concept of hyperconjugation to understand various phenomena of chemistry

(6 A) Understanding of relative stability of various alkene hydrocarbons

(6 B) Understanding of relative stability of various carbocation intermediates

(6 C) Understanding of relative stability of various free radical intermediates

(6 D) Understanding of, "Saytzeff's rule"

(6 E) Understanding of, "Anti-Markovnikov addition" which is also known as, "Peroxide effect"

(7) References

(1) What is called, “Hyperconjugation”?

It is a phenomenon through which displacement of electrons takes place within a particular type of organic species. The species undergoing such phenomenon may be:

(a) A neutral organic molecule (like propene) or

(b) A carbocation (like tertiary butyl carbocation) or

(c) A free radical (like tertiary butyl free radical).

Due to such electronic displacement the species becomes more stable and less reactive.

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Definition of hyperconjugation:

“The release of electrons by hydrogen of an alkyl group which is attached to α-carbon atom of unsaturated system is called hyperconjugation”.

To understand the above definition, knowledge of following three terms is required.

(A) Unsaturated system

(B) α-carbon and

(C) α-hydrogen

What is called, “unsaturated system”?

The system which contains such carbon atom which is:

(a) Electron deficient or

(b) Electron rich or

(c) Having odd (means single) electron on it; is called an unsaturated system.

Following are examples of such systems.

  • A system containing carbon-carbon double bond (for example, all alkenes compounds)
  • A system containing positively charged carbon (for example, all carbocation intermediates)
  • A system containing an odd electron on carbon (for example, all free radical intermediates)

What is called, “α-Carbon”?

The first (or immediate) carbon atom which is joined with an unsaturated system is called α–carbon. One or more α–carbon atoms may be present in the given species.

What is called, “α-Hydrogen”?

The hydrogen atom which is joined with α-carbon through σ-bond is called α–hydrogen. One or more α–hydrogen atoms may be present on each α–carbon atom.

Refer the following pictures to understand more about above terms.

Some Illustrative Examples to explain meaning of: (1) Unsaturated System, (2) Alpha Carbon and (3) Alpha Hydrogen

In propene, carbon-carbon double bond is called unsaturated system and carbon atom joined with double bond is called alpha carbon. Three hydrogen atoms joined with alpha carbon are called alpha hydrogen.

In propene, carbon-carbon double bond is called unsaturated system and carbon atom joined with double bond is called alpha carbon. Three hydrogen atoms joined with alpha carbon are called alpha hydrogen.

In toluene, the benzene ring containing three carbon-carbon double bonds is called unsaturated system and carbon atom joined with benzene ring is called alpha carbon. Three hydrogen atoms joined with alpha carbon are called alpha hydrogen.

In toluene, the benzene ring containing three carbon-carbon double bonds is called unsaturated system and carbon atom joined with benzene ring is called alpha carbon. Three hydrogen atoms joined with alpha carbon are called alpha hydrogen.

In tertiary butyl carbocation, middle carbon bearing positive charge is called unsaturated system and 3 carbon atoms joined with positively charged carbon are alpha carbons. Nine hydrogen atoms joined with each alpha carbon are called alpha hydrogen.

In tertiary butyl carbocation, middle carbon bearing positive charge is called unsaturated system and 3 carbon atoms joined with positively charged carbon are alpha carbons. Nine hydrogen atoms joined with each alpha carbon are called alpha hydrogen.

In tertiary butyl free radical, middle carbon bearing odd electron is called unsaturated system and 3 carbon atoms joined with it are called alpha carbons. Nine hydrogen atoms joined with each alpha carbon are called alpha hydrogen.

In tertiary butyl free radical, middle carbon bearing odd electron is called unsaturated system and 3 carbon atoms joined with it are called alpha carbons. Nine hydrogen atoms joined with each alpha carbon are called alpha hydrogen.

(2) Conditions required for Hyperconjugation

Each and every organic species cannot show phenomenon of hyperconjugation. The organic species satisfying following three conditions at a time can only show hyperconjugation.

(1) It must contain an unsaturated system. Species like "ethane" and "pentane" having no unsaturated system cannot show hyperconjugation.

(2) Some species contain unsaturated system but do not contain any alpha carbon atom. Such system cannot exhibit the phenomenon of hyperconjugation. For example species like "ethene" and "benzene" having no alpha carbon atom cannot show hyperconjugation.

(3) Even though the given species contain unsaturated system as well as alpha carbon but does not contain any alpha hydrogen atom, it cannot show the phenomenon of hyperconjugation. For example species like "benzotrichloride" and "3,3-dichlorobut-1-ene" cannot show hyperconjugation.

This will be clear from following pictures.

Following species cannot exhibit phenomenon of hyperconjugation

Both of these species lack in unsaturated system.

Both of these species lack in unsaturated system.

Though both of these species have unsaturated system, they lack in alpha carbon.

Though both of these species have unsaturated system, they lack in alpha carbon.

Though both of these species have unsaturated system as well as alpha carbon, they lack in alpha hydrogen.

Though both of these species have unsaturated system as well as alpha carbon, they lack in alpha hydrogen.

Check your Primary Knowledge of Hyperconjugation

For each question, choose the best answer. The answer key is below.

  1. How many alpha carbon atoms are present in propene?
    • 1
    • 2
    • 3
    • zero
  2. Which of the following species contains 9 alpha hydrogen atoms?
    • Tertiary butyl carbocation
    • Tertiary butyl free radical
    • Both
    • None
  3. Due to phenomenon of hyperconjugation, a species becomes
    • More stable
    • .More reactive
    • Bigger
    • Smaller
  4. Which chemical species can exhibit the phenomenon of hyperconjugation?
    • Only inorganic species
    • Only organic species
    • Both
    • None

Answer Key

  1. 1
  2. Both
  3. More stable
  4. Only organic species

Doubts in Chemistry? Refer Dictionary

(3) How does the phenomenon of Hyperconjugation take place? (Means mechanism of hyperconjugation)

(1) In this peculiar phenomenon, the electron pair of σ–bond situated between α–carbon and α–hydrogen migrates on one of the bonded atom.

(2) This results in:

(a) No bond between α–carbon and α–hydrogen and

(b) Development of unit positive and unit negative charges on respective atoms.

(3) As like phenomenon of resonance, the electron pair and charge extends (means electron pair and charge delocalize) on adjacent atoms of the species to give intermediate structures called, “hyperconjugation structures”.

(4) More is number of hyperconjugation structures more is delocalization of electrons more is the stability gained by species.

The following illustrative example showing phenomenon of hyperconjugation in propene will help to understand this.

Picture showing Mechanism of Hyperconjugation in Propene

Here, alpha hydrogen situated above alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen above it.

Here, alpha hydrogen situated above alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen above it.

Here, alpha hydrogen situated below alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen below it.

Here, alpha hydrogen situated below alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen below it.

Here, alpha hydrogen situated at left side of alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen at its left side.

Here, alpha hydrogen situated at left side of alpha carbon releases electrons. Please note that there is no bond at all between alpha carbon and hydrogen at its left side.

(4) Why does Hyperconjugation give stability to the organic species?

The species undergoing phenomenon of hyperconjugation always gains stability.

Here it must be understood that reactivity and stability are inversely proportional to each other. Means less is reactivity more is stability.

The cause of stability for various species can be explained as follows.

(1) Cause of stability of various alkenes:

The characteristic reaction of alkene is electrophilic addition.

During this reaction, it is the electrophilic part of addendum which combines first to the carbon atom of double bond of alkene. It is obvious that high electron density between carbon-carbon double bond facilitates this reaction as it results in greater electrostatic attraction between carbon atom and electrophile.

As shown in above picture of hyperconjugation of propene, the double bond is not static betwwen two carbon atoms but it is delocalized between three carbon atoms. This results in decreased electron density between carbon-carbon double bond. This in turn decreases attraction between pi electrons of double bond and electrophile.

Thus hyperconjugation decreases reactivity and increases stability.

(2) Cause of stability of various carbocations:

Due to positive charge, carbocation immediately combines with such species present in the reaction mixture which has negative charge. Thus reactivity of carbocation depends upon intensity of positive charge on them. More is the intensity of positive charge, more is the reactivity less is stability.

The phenomenon of hyperconjugation causes dispersion of positive charge. This means intensity of positive charge of carbocation decreases due to hyperconjugation. Thus, hyperconjugation gives stability to carbocation.

This will be clear from following picture which shows hyperconjugation in ethyl carbocation.

(3) Cause of stability of various free radicals:

Free radicals are neutral species having odd electron.

The pairing of such odd electron with another electron which has opposite spin results in lowering of potential energy. It is due to this reason that free radicals are reactive species. Hyperconjugation causes dispersion of odd electron resulting in decreased electron density on them. This increases their stability.

Thus, hyperconjugation gives stability to free radicals.

This will be clear from following picture which shows hyperconjugation in ethyl free radical.

Picture showing Mechanism of Hyperconjugation in Ethyl Carbocation

Note that positive charge on carbon is not static but is distributed among 4 atoms, one carbon and 3 hydrogen atoms. This decreases intensity of positive charge. Thus stability of carbocation increases due to hyperconjugation.

Note that positive charge on carbon is not static but is distributed among 4 atoms, one carbon and 3 hydrogen atoms. This decreases intensity of positive charge. Thus stability of carbocation increases due to hyperconjugation.

Picture showing Mechanism of Hyperconjugation in Ethyl Free Radical

Note that odd electron on carbon is not static but is distributed among 4 atoms, one carbon and 3 hydrogen atoms. This decreases electron density on any 1 atom. Thus stability of free radical increases due to hyperconjugation.

Note that odd electron on carbon is not static but is distributed among 4 atoms, one carbon and 3 hydrogen atoms. This decreases electron density on any 1 atom. Thus stability of free radical increases due to hyperconjugation.

(5) Three other names of Hyperconjugation

There are three other names for the phenomenon of hyperconjugation.

These are:

  • Baker-Nathan effect
  • No bond resonance and
  • σ (Sigma) –π (pi) conjugation. These are discussed in detail in the following sections.

Do it Yourself

(5 A) Baker-Nathan Effect

Relative stability of various alkenes could not be explained by any existing theories. To resolve this problem, theory of hyperconjugation was introduced.

Baker and Nathan noticed a particular trend in the stability of various alkenes.

Since it was noticed for the first time by Baker and Nathan, it is called, “Baker-Nathan effect".

(5 B) No Bond Resonance

It can be seen in the hyperconjugation structures of propene that there is no bond between α-carbon and that α-hydrogen which is participating in the phenomenon of hyperconjugation.

This is actually very strange and also difficult to believe. However, based on this belief several problems of chemistry can be resolved. Hence there is a reason to believe that such type of no-bond situation may also be possible.

Due to bond less hyperconjugation structures and also due to resemblance of this phenomenon with that of resonance, hyperconjugation is also termed as, "no-bond resonance".

(5 C) σ (Sigma) -π (pi) Conjugation

We know that in the phenomenon of resonance, unshared pair of electron and electron of π-bond participates. However due to strong nature of σ-bond, the electron held in it never participate in resonance.

But here due to involvement of electrons of both type of covalent bonds (means involvement of σ-bond as well as π-bond), the phenomenon of hyperconjugation is also termed as: "σ-π conjugation".

(6) Application of Concept of Hyperconjugation to Understand Various Phenomena of Chemistry

By applying the concept of hyperconjugation, several phenomena of chemistry can be understood properly. These phenomena are: