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"
(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.
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
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
(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
Check your Primary Knowledge of Hyperconjugation
For each question, choose the best answer. The answer key is below.
- How many alpha carbon atoms are present in propene?
- Which of the following species contains 9 alpha hydrogen atoms?
- Tertiary butyl carbocation
- Tertiary butyl free radical
- Due to phenomenon of hyperconjugation, a species becomes
- More stable
- .More reactive
- Which chemical species can exhibit the phenomenon of hyperconjugation?
- Only inorganic species
- Only organic species
- More stable
- 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
(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
Picture showing Mechanism of Hyperconjugation in Ethyl Free Radical
(5) Three other names of Hyperconjugation
There are three other names for the phenomenon of hyperconjugation.
- 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:
(A) Relative stability of various alkene hydrocarbons
(B) Relative stability of various carbocation intermediates
(C) Relative stability of various free radical intermediates
(D) Justification for Saytzeff's rule and
(E) Justification for Anti-Markovnikov addition (which is also known as, "peroxide effect").
The detailed understanding of each of above is given in the following sections.
(6 A) The Concept of Hyperconjugation Explains Relative Stability of various Alkene Hydrocarbons
What is meant by stability of akenes?
There are various alkene hydrocarbons like: ethene, propene, but-1-ene, but-2-ene, pent-1-ene etc. It is observed that all of these alkenes are not equally stable. This means some alkenes react more readily and vigorously while others do not. The alkene which reacts more readily is regarded as less stable while that reacting less readily is regarded as more stable.
Stability of alkenes is examined in terms of their heat of hydrogenation values.
What is meant by heat of hydrogenation value of alkene?
It is the measure of heat evolved when one mole of alkene undergoes addition reaction with one mole of hydrogen (at S. T. P.) to yield a saturated hydrocarbon.
As this is an exothermic reaction, it is denoted by symbol, "ΔH" with negative sign.
For example, when one mole of ethene reacts with one mole of hydrogen to yield one mole saturated hydrocarbon ethane, the amount of heat evolved during this reaction is 32.8 kilo calories. This is called heat of hydrogenation of ethene and expressed as, "ΔH = -32.8 kilo calories/mol.
The heat of hydrogenation values of some alkene hydrocarbons is given in the following table.
What is common in heat of hydrogenation values of alkenes?
The common thing is that the reaction is same for all alkenes. The sequence of reaction is as follows:
(1) One σ (sigma) bond between two hydrogen atoms is broken first
(2) One π (pi) bond between two carbon atoms of alkene is broken and
(3) Two new sigma bonds are formed between two carbon atoms of alkene and two hydrogen atoms.
As such, it is assumed that heat of hydrogenation values of different alkene hydrocarbons should be either same or should be nearly same.
However this is not true.
What is uncommon in heat of hydrogenation values of alkenes?
As seen from the table, values of heat of hydrogenation of different alkenes are not only different but they show a particular pattern. The alkene having more number of alpha hydrogen atoms has lesser value of heat of hydrogenation and vice a versa.
How does the concept of hyperconjugation explain this?
The concept of hyperconjugation explains this on the basis of number of hypeconjugation structures. More is the number of hyperconjugation structures of given alkene hydrocarbon, lesser is its reactivity, more is its stability.
It is obvious from the table that increasing order of stability of various alkenes is:
Ethene < 3-Methylbut-1-ene < But-1-ene < Propene < 2-Methylbut-1-ene < trans-But-2-ene < 2-Methylbut-2-ene < 2,3-Dimethylbut-2-ene.
Values of Heat of Hydrogenation of some well known alkene hydrocarbons
|Name of alkene||Value of heat of hydrogenation (in kilo calorie/mole)||Number of alpha hydrogen atoms|
(6 B) The Concept of Hyperconjugation Explains Relative Stability of various Carbocation Intermediates
We know that carbocations are such intermediates which have unit positive charge on one of the carbon atom.
It is also obvious that more is the intensity of positive charge on carbon atom more is its reactivity and lesser is its stability.
As discussed above, the phenomenon of hyperconjugation decreases the intensity of positive charge. It is due to this reason that more is the number of α-hydrogen atoms on carbocation more will be the number of hyperconjugation structures and more will be the stability.
This suggests that relative stability of various carbocations can be determined by number of α-hydrogen atoms they possess.
The following table shows details of α-hydrogen atoms present on various carbocations.
It is clear from the table that various carbocations can be arranged in increasing order of their stability as shown below:
+CH3 < +CH2-CH3 < +CH(CH3)2 < +C(CH3)3
Details of Alpha Hyhdrogen atoms present in Various Carbocations
|Name of carbocation||Its formula||Number of alpha hydrogen|
Tertiary butyl carbocation
(6 C) The Concept of Hyperconjugation Explains Relative Stability of various Free Radical Intermediates
Stability of various free radicals can also be explained in the same way.
This means higher the number of α-hydrogen atoms present on given free radical, higher will be the number of hyperconjugation structures higher will be its stability.
Based on above discussion, the stability order of various free radicals can be written as:
.CH3 < .CH2-CH3 < .CH(CH3)2 < .C(CH3)3
(6 D) The Concept of Hyperconjugation Provides Justification of Saytzeff's Rule
In 1875 a scientist, "Alexander Saytzeff" working in University of Kazan, USSR proposed that, "during dehydrohalogenation of alkyl halide, the alkene having greater number of alkyl groups attached to the doubly bonder carbon atom is produced in higher proportion”.
This is known as, "Saytzeff's rule".
For example, during dehydrohalogenation of sec-butyl bromide (means 2-bromobutane) a mixture of but-1-ene and but-2-ene is formed. However proportion of but-2-ene is found to be about 80% which is much higher than that of but-1-ene which is about 20%.
[Note: This is also observed during dehydration of monohydric alcohols. For example, during dehydration sec-butyl alcohol (means butan-2-ol) with concentrated sulphuric acid at high temperature, a mixture of but-1-ene and but-2-ene is formed. Here also proportion of but-2-ene is found to be much higher than that of but-1-ene.
The explanation of Saytzeff's rule can be given by concept of hyperconjugation as follows:
As alkene having greater number of alkyl groups attached to the doubly bonder carbon atom carries more number of α-hydrogen atoms it is more stable hence produced in higher proportion.
(6 E) The Concept of Hperconjugation provides Justification for Anti-Markovnikov addition (which is also known as, "peroxide effect")
It is well known that addition of hydrogen halide to alkene takes place in accordance with Markovnikov's rule. However, in 1933 two scientists: "M. S. Kharasch" and "F. R. Mayo" found that addition of HBr to alkene in presence of suitable peroxide gives product which is opposite to Markovnikove's rule.
This is known as, "Anti-Markovnikov addition".
As this kind of reaction takes place only in the presence of peroxide it is also known as, "peroxide effect".
Further, as it was identified first by two scientist it is also known as, "Kharasch and Mayo effect".
The explanation of Anti-Markovnokov addition can be given by concept of hyperconjugation as given below.
Due to the use of peroxide, bromine free radical forms which attacks given alkene to produce such free radical intermediate which is more stable. Due to this bromine gets attached to terminal carbon of alkene. This gives product which is opposite to Markovnikov addition.
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