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Conjugation

The word "conjugation" is derived from a Latin word that means "to link together". In organic chemistry terms, it is used to describe the situation that occurs when π systems (e.g. double bonds) are "linked together".

Definition: “Conjugation is the interaction/overlapping of one p-orbital with another across an intervening sigma (single) bond in conjugated systems/structures.”

It may also be defined as “Special stability provided by electron delocalization in three or more adjacent, parallel, overlapping p-orbitals.”

Main Components of this second definition:

Special Stability = Lower electron energy.
Electron delocalization = Electron density is not fixed to one place, it is spread across multiple atoms.
Three p-orbitals = minimum requirements for conjugation.
Adjacent, parallel overlapping p-orbitals = p orbitals that are parallel and next to each other. This usually results from (but is not limited to) alternating single and double bonds, which result in partial pi bond behavior in sigma bonds as well.

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Explanation:

If two double bonds are separated by a single bond, their orbitals will interact and become conjugated.
If the double bonds are separated by two sigma bonds, these atoms are non-conjugated.

Example:

Conjugated system is a molecular entity/system of connected p-orbitals with delocalized electrons in molecules with alternating single and multiple bonds.
In a multiple bond, there is one sigma bond and one or two pi ponds.

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Pi bonds are made up by overlapping p orbitals.

The electrons in the p-orbitals are located perpendicular to the plane of the molecule.

So when there are pi bonds in alternating bonds, all the electrons are delocalized throughout the conjugated system.

In other words, we call it an electron cloud.

Since electrons are delocalized, they belong to all the atoms in the conjugated system, but not for only one atom.

This lowers the overall energy of the system and increase stability.

Note: The atoms that contribute to conjugation cannot be sp³hybridized. They can either be sp² (more common) or sp (not as common).

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A simple  p  orbital  overlap  between  two  molecules is shown below.  Notice  that  the  sp² orbitals  forms  a  sigma  bond,  while  the  p  orbital  overlap  results in  a  pi  bond.

 A  conjugated  system with “delocalized”  pi electrons.  The “sharing” of  electron  density  among  the  p 

orbitals  

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Conjugation in Simple Cyclic compounds

Cyclic compounds can be partly or completely conjugated.

For example, benzene is conjugated.

In the structure of benzene all 6 six carbons are sp² hybridized which means that each of them have a p orbital.

The adjacent p orbital’s allow for the delocalization of electron density making the molecule more stable by lowering the overall energy of the molecule.

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Types of Conjugation

There are four types of conjugation which include:

[1]. Homoconjugation: A form of conjugation in which a non-conjugating atom or group is interposed between the conjugating bonds. It is an overlap of two π-systems separated by a non-conjugating group, such as -CH₂.

For example, the molecule CH₂=CH–CH₂–CH=CH₂ (1,4-pentadiene) is homoconjugated because the two C=C double bonds (which are π-systems because each double bond contains one π bond) are separated by one CH₂ group.

[2]. Hyperconjugation: It is conjugation between σ- and π-bonded segments (detail will be discussed in the next topic).

[3]. σ-Conjugation: Conjugation between σ-bonded segments (least discussed).

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[4]. π-Conjugation: π-Conjugation, or usually just conjugation, is the strongest and most studied conjugation type.

For optimal π-conjugation there needs to be good orbital overlap between adjacent local π-orbitals.
Example:

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Relation Between Conjugation and Resonance

Resonance and conjugation are interrelated.

If there is conjugation in a molecule, we can draw resonance structures to it by alternating the pi bonds.

Since the pi electrons are delocalized in the whole conjugated system, all the resonance structures are valid for such molecule.

Resonance allows a conjugated system to delocalize electrons.

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Consequences & Applications of conjugation

Conjugated compounds are common in nature. Conjugation has a profound effect on the properties of the entire molecule. Some of the effects are as follows:

[1]. A molecule with a conjugated double bond is more stable than a molecule with the same number of non-conjugated double bonds. It is due to electron delocalization over a molecule. More  extensive  conjugation leads to greater  stability of the molecule .

[2]. Conjugation leads to electron delocalization over a molecule. Delocalization of electrons results in shortenings of chemical bonds.

Foe example, length of the central single bond is shorter than non-conjugated similar molecule. This bond is shorter because the two conjugated double bonds interact with each other.

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[3]. The presence of conjugation can be significant in many cases with different electronic, optical and structural properties as well as different chemical reactivity when compared to non-conjugated analogous compounds.

[4]. Presence of conjugation allows the molecules to act as chromophores which can absorb light in the ultraviolet region of the spectrum.

Extended conjugation leads to absorption of visible light, producing color.

Example: Conjugated hydrocarbon with many double bonds are polyenes (e.g. Lycopene is responsible for red color in tomatoes).

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Hyperconjugation

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Hyperconjugation

Definition: “The delocalization of σ-electrons or lone pair of electrons into adjacent π-bond or p-orbital is called hyperconjugation.”

"Hyperconjugation is defined as the interaction (usually stabilizing) of a σ - bond with an adjacent vacant or partially filled orbital (usually a p- or π -orbital).“

In the structure on the right there is no bond between a C and H due to migration of the sigma bond. Hence hyperconjugation is also called no bond resonance.
This does not mean the hydrogen atom is completely detached from the structure. It indicates some degree of ionic character in the C-H bond and some single bond character between the carbon-carbon double bond.

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Salient Features of Hyperconjugation:

Hyperconjugation is a permanent effect. This concept was introduced by Baker and Nathan.

It is a “mild or partial sort of conjugation” and it explains how saturated σ-bonded segments interact with π-bonded segments i.e. a double or triple bond. It is also called as delocalization of sigma electrons.

The sigma bond involved in hyperconjugation is usually a C-C or C-H bond.

Greater the number of alkyl groups attached to doubly bonded C atoms, greater is the number of contributing structures and greater is the stability.

It is also known as no bond resonance or bond sacrificial resonance or “Baker-Nathan effect” or “sigma-pi (σ-π) conjugation”.

Phenomena of hyperconjugation occurs in alkene, alkynes, free radicals (saturated type), carbonium ions (saturated type) and nitro compounds with α-hydrogen.

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The magnitude of effect of hyperconjugation is smaller than resonance, so it is also called secondary resonance.

Important Condition for hyperconjugation:

There must be at least one α-CH group (more precisely an α-hydrogen) or a lone pair of electrons on the atom adjacent to sp² hybridized (double bonded) or sp hybridized (triple bonded) carbon or carbocation or free radical etc.

For example, in case of the following alkene containing a tert-butyl group on doubly bonded carbon, the hyperconjugation is not possible.

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Illustration of Hyperconjugation:

Example 1: Hyperconjugation is possible in CH₃-CH₂⁺(ethyl cation) in which the positively charged carbon atom has an empty p-orbital.

One of the C-H bonds of the methyl group can align in the plane of this empty p-orbital and the electrons constituting the C-H bond in plane with this p-orbital can then be delocalized into the empty p-orbital as follows:

Note: This type of overlap stabilizes the carbocation because electron density from the adjacent σ-bond helps in dispersing the positive charge.

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It is also called no bond resonance shown with the structures of ethyl cation below:

Example 2: In propene, the σ-electrons of C-H bond of methyl group can be delocalized into the π-orbital of doubly bonded carbon as follows:

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In above examples structures ii, iii, iv are hyperconjugative structures (H-structures).

Note: Number of hyperconjugative structures = number of α-Hydrogen.
All the three hydrogen atoms on the methyl group participate one by one in the hyperconjugation.

In terms of structures, hyperconjugation in propene may be represented as follows:

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In the contributing structures: (II), (III), & (IV) of propene, there is no bond between an α-carbon and one of the hydrogen atom.

Hence the hyperconjugation is also known as “no bond resonance”.

This type of hyperconjugation is also referred to as sacrificial hyperconjugation since one bond is missing.

Resulting Parameters of Propene due to Hyperconjugation:

These equivalent contributing structures i.e., (II), (III) & (IV) are also polar in nature and hence are responsible for the dipole moment of propene (0.36D or 0.36 debyes).

The C-C bond lengths in propene are equal to 1.48 A^o(148 pm).

Its value is in between 1.54 A^o (154 pm) of C-C bond length and 1.34 A^o (134 pm) of C=C bond length.

It is so because C-C bonds show partial double bond character due to hyperconjugation.

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Negative/ Reverse Hyperconjugation:

Definition: “Negative or reverse hyperconjugation is the donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital resulting in building π-character into bonds that normally possess only σ-character.”

In negative hyperconjugation, the electron density flows in the opposite direction (from π- or p-orbital to empty σ*-orbital) than it does in the more common hyperconjugation (from σ-orbital to empty p-orbital). In other words, there is a movement of electron density towards the sigma bond.

Example: In case of α-halo alkenes, the delocalization of electrons occurs towards halogen group through reverse hyperconjugation mechanism.
The dipole moments of α-halo alkenes are amplified (increased) due to this phenomenon.

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Strength Comparison

However, propene is more stable than 1-butene. This is because there are three hydrogens on α-methyl group involved in hyperconjugation. Whereas, in 1-butene there are only two hydrogen atoms on –CH₂ group that can take part in hyperconjugation.
Therefore, stability of alkenes increases with number of hyperconjugative structures.

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Consequences & Applications Of Hyperconjugation

[1]. Stability of Alkenes: A general rule is that, the stability of alkenes increases with the increase in the number of alkyl groups (containing hydrogens) on the double bond. Increasing order of stability of alkenes is as follows:

It is due to increase in the number of contributing hyperconjugated structures. For example, 2-butene is more stable than 1-butene. This is because in 2-Butene, there are six hydrogens involved in hyperconjugation whereas there are only two hydrogens involved in case of 1-butene. Hence the contributing structures in 2-butene are more and is more stable than 1-butene.

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[2]. Stability of carbocations (Carbonium ions):

The ethyl carbocation, CH₃-CH₂⁺ is more stable than the methyl carbocation, CH₃⁺.

This is because, the σ-electrons of the α-C-H bond in ethyl group are delocalized into the empty p-orbital of the positive carbon center and thus by giving rise to ‘no bond resonance structures’ as shown below.

Whereas hyperconjugation is not possible in methyl carbocation and hence is less stable.

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Relative stability of carbocations:

In general, greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the cation.

Thus, we have the following relative stability of carbocations :

Note:

Number of hyper-conjugating structures = number of α-hydrogens.
Greater the no. of hyper-conjugating structures, more is the stability of the system.

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[3]. Stability of Free Radicals: The stability of free radicals is influenced by hyperconjugation in the same fashion as in case of carbonium ions.

The σ-electrons of the α-C-H bond can be delocalized into the p-orbital of carbon containing an odd electron.
Due to hyperconjugation, the stability of free radicals also follows the same order as that of carbonium ions. i.e. methyl < primary < secondary < tertiary.

[4]. Dipole Moment & Bond Length: The dipole moment of the molecules is greatly affected due to hyperconjugation since the contributing structures show considerable polarity.

The bond lengths are also altered due to change in the bond order during hyperconjugation. The single bond may get partial double bond character and vice versa. Example - 1: The observed dipole moment of nitro methane i.e. 3.15 is greater than the calculated value i.e. 2.59 due to hyperconjugation. The observed C-N bond length is also less than the expected value due to same reason.

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Example - 2: The C-C bond lengths in propene are equal to 1.48 A^o(148 pm).

Its value is in between 1.54 A^o (154 pm) of C-C bond length and 1.34 A^o (134 pm) of C=C bond length.
It is so because C-C bonds show partial double bond character due to hyperconjugation.

Propene

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[5]. Reactivity & Orientation of Electrophilic Substitution on Benzene Ring:

Example: In Toluene, the methyl group releases electrons towards the benzene ring partly due to the inductive effect and mainly due to hyperconjugation.

Thus the reactivity of the ring towards electrophilic substitution increases and the substitution is directed at ortho and para positions to the methyl below.
The no bond resonance forms of toluene due to hyperconjugation are shown below:

From the above diagram, it can be seen clearly that the electron density on benzene ring is increased especially at ortho and para positions.

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