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HYperconjugation

HYPERCONJUGATION (or) NO BOND RESONANCE (or) BAKER-NATHAN EFFECT

The delocalization of σ-electrons or lone pair of electrons into adjacent  π-orbital or p-orbital is called hyperconjugation. 

The interactions between the electrons of π systems (multiple bonds) and the adjacent σ bonds ( single H-C bonds) of the substituent groups in organic compounds.

It occurs due to overlapping of σ-bonding orbital or the orbital containing a lone pair with adjacent π-orbital or p-orbital.
It is also known as "no bond resonance" or "Baker-Nathan effect".
Conditions for hyperconjugation
* There must be an α-CH group or a lone pair on atom adjacent to sp² hybrid carbon or other atoms like nitrogen, oxygen etc.
E.g., Alkenes, alkyl carbocations, alkyl free radicals, nitro compounds with α- hydrogen

ILLUSTRATION OF HYPERCONJUGATION

The displacement of  σ-electrons towards the multiple bond occurs when there are hydrogens on the α-carbon (which is adjacent to the multiple bond). This results in the polarization of the multiple bond.
E.g. In propene, the σ-electrons of C-H bond of methyl group can be delocalized into the π-orbital of doubly bonded carbon as represented below.

In the same way, the other hydrogens on the methyl group also participate in the hyperconjugation. This is possible due to free rotation of C-C bond so that the other C-H bonds can also participate in the hyperconjugation. Thus the propene molecule can show following resonance structures, which confer stability to it.

 

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".

These equivalent contributing structures i.e., (II), (III) & (IV) are also polar in nature and hence are responsible for the dipole moment of propene (0.36 D).
The C-C bond lengths in propene are equal to 1.48. Its value is in between 1.54 (of C-C) and 1.34 (of C=C). It is because the bond order of C-C bonds is approximately 1.5 due to hyperconjugation. 
This type of hyperconjugation is also referred to as sacrificial hyperconjugation since one bond is missing.

CONSEQUENCES & APPLICATIONS OF HYPERCONJUGATION

1) Stability of alkenes:
A general rule is that, the stability of alkenes increases with increase in the number of alkyl groups (containing hydrogens) on the double bond. It is due to increase in the number of contributing no bond resonance 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.

The increasing order of stability of alkenes with increases in the number of methyl groups on the double bond is depicted below.

This order is supported by the heat of hydrogenation data of these alkenes. The values of heats of hydrogenation decrease with increase in the stability of alkenes.
Also the heats of formation of more substituted alkenes are higher than expected.
However it is important to note that the alkyl groups attached to the double bond must contain at least one hydrogen atom for hyperconjugation. For example, in case of the following alkene containing a tert-butyl group on doubly bonded carbon, the hyperconjugation is not possible.

It is also important to note that the effect of hyperconjugation is stronger than the inductive effect. 
For example, the positive inductive effect of ethyl group is stronger than that of methyl group. Hence based on inductive effect, 1-butene is expected to be more stable than propene. 
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.

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.

In general, the stability of carbonium ions increases with increase in the number of alkyl groups (containing hydrogen) attached to the positively charged carbon due to increase in the number of contributing structures to hyperconjugation.
Note: This type of hyperconjugation can also referred to as isovalent hyperconjugation since there is no decrease in the number bonds in the no bond resonance forms.
Thus the increasing order of stability of carbocations can be given as: methyl < primary < secondary < tertiary as depicted below:

3) Stability of free radicals:
The stability of free radicals is influenced by hyperconjugation as in case of carbonium ions. The 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 follow 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.
E.g. The observed dipole moment of nitro methane is greater than the calculated value due to hyperconjugation. The observed C –N bond length is also less than the expected value due to same reason.

The same arguments can be applied to shortening of C-C bond adjacent to -C≡N in acetonitrile and also the C-C bond adjacent to the -C≡C in propyne. Also note that the observed dipole moments are again different from their expected values.

5) Reactivity & orientation of electrophilic substitution on benzene ring :
In Toluene, the methyl group releases electrons towards the benzene ring partly due to 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 postions to the methyl group. 
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.
Since the hyperconjugation overpowers the inductive effect, the substitution (e.g. nitration) on the following disubstituted benzene occurs ortho to the methyl group. In the tert-butyl group, there are no hydrogens on the carbon directly attached to the benzene ring. Hence it cannot involve in hyperconjugation.

Also note that the tert-butyl group is bulky and hinders the approach of electrophile.

 ORGANIC CHEMISTRY

Organic chemistry is not only an important branch of chemical science but very much contributes to the modern industrialized civilization and has a profound effect on the habits of modern society.
Though the word "organic" refers to the living things, the modern organic chemistry is defined as the study of compounds containing carbon which are either originated from living organisms or prepared synthetically in the laboratories. Most of the compounds in the nature like carbohydrates, proteins, enzymes, vitamins, lipids and nucleic acids contain carbon and the reactions that take place in biological systems are organic reactions. However, not only those present in the nature, but millions of the organic compounds are synthetically prepared. These include synthetic polymers like synthetic fabrics, synthetic rubbers, plastics; medicines; organic dyes etc.
Organic chemistry deals with the study of structure, properties: both chemical and physical, synthesis and applications of organic compounds, which always contain carbon as one of the element.

WHAT IS ORGANIC CHEMISTRY?

Let us start with the question "What is Organic chemistry?".
The simple answer is: It is the chemistry of carbon containing compounds, which are otherwise known as organic compounds.
So it is pretty easy to recognize that we should start our journey of organic chemistry by exploring the chemical nature of carbon.

WHAT IS CARBON?

So the next question is: What is carbon?
    * Carbon is an element with atomic number (Z) = 6.
    * Its ground state electronic configuration can be represented as: 1s²2s²2p² (or) 1s²2s²2p_x¹2p_y¹2p_z⁰

   

    * It is the first element in Group-14 of Long form of Periodic table.
    * It is a non metal.
    * On Pauling's scale, its electronegativity value is around 2.5.
    * It usually forms covalent bonds.
    * Its valency is 4 since there are four electrons in the outer shell i.e., it can form four covalent bonds with other atoms.

WHY THERE ARE MILLIONS OF ORGANIC COMPOUNDS? 

It is well known that there are millions of organic compounds around, which are either originated from the nature or prepared synthetically.
Examples of organic compounds include carbohydrates, proteins, enzymes, vitamins, lipids, nucleic acids , synthetic polymers, synthetic fabrics, synthetic rubbers, plastics, medicines, drugs, organic dyes and so on.
Now the immediate question is: Why the carbon atom is so special and forms millions of compounds?
To answer this question, we should know about catenation.
Catenation is the ability of atoms of same element to bond covalently among themselves and form long chains or rings.
Carbon has a stronger tendency to catenate since it is a smaller atom and can form stronger covalent bonds with other carbons. The C-C bonds are stronger due to effective overlapping of atomic orbitals.

It also forms stronger bonds with other elements like hydrogen, oxygen, nitrogen, halogens, sulfur, phosphorus etc.
The organic compounds can also exhibit isomerism due to different structural and spatial arrangement of atoms or groups leading to formation of huge array of compounds.
These arguments explain why the carbon can form millions of compounds and organic chemistry is flourishing like nothing.

HOW DOES CARBON FORM CHEMICAL BONDS?

LEWI'S DOT MODEL 

Carbon is an appreciably electronegative element and tends to form four covalent bonds by using all the four electrons in its valence shell i.e., the second shell for which the electronic configuration can be written as 2s²2p². Hence the combining power or the valency of carbon is 4. It can form 4 bonds with other atoms.
It is possible to understand the bonding in carbon compounds by using Lewi's dot model. According to this model, each atom participating in the bonding contributes one electron to form an electron pair which is shared between the two contributing atoms. Thus a covalent bond is formed. If atoms share two electron pairs, a double bond is formed. And a triple bond is formed when three electron pairs are shared.
The purpose of participating in bond formation is to get the nearest inert gas configuration and thus by getting stability. Most of the atoms try to get eight electrons or octet configuration in the valence shell. This is also called as octet rule.
The structures of some simple organic molecules are explained as shown below.
Methane, CH₄: The carbon atom contributes four valence electrons to make four bonds with hydrogen atoms. Each hydrogen also contributes one electron for the bond formation.
Thus there are 4 C-H bonds in the methane molecule and carbon gets octet configuration in the valence shell.
Note that the valency of hydrogen atom is one. It can form only one bond since there is only one electron in this atom. It also gets Helium's configuration during bond formation.
Also note that in Lewi dot models, only the valence electrons are shown. The bond pairs can also be shown by lines.

Ethane, C₂H₆: In Ethane molecule each carbon forms 4 bonds again. Among them three are C-H bonds, while the fourth one is a C-C bond.

Ethylene, C₂H₄: In this molecule, there is a double bond between two carbon atoms due to sharing of two pairs of electrons. Each carbon also forms two bonds with hydrogen atoms.

Acetylene, C₂H₂: There is a triple bond between two carbon atoms in acetylene molecule. It is formed due to sharing of three electron pairs. Each carbon also forms a single bond with hydrogen atom.

 Methyl fluoride, CH₃F: Since there are 7 electrons in the valence shell of Fluorine, it require one electron to complete octet. Hence it contributes one electron for bond formation with carbon as shown below.
Note that the only the bond pairs are shown as lines in the second representation.

There are three lone pairs and one bond pair around fluorine atom. The bond pair is shown as a line.

In the same way, carbon atom forms bonds with other halogen atoms.
Formaldehyde, CH₂O: There are six electrons in the valence shell of oxygen. It forms two bonds by contributing two of its valence electrons and thus by completing the octet. In formaldehyde, the oxygen atom forms two bonds with a carbon atom.

Methyl alcohol, CH₃OH: However, the oxygen atom can also form just one bond with the carbon as in case of methyl alcohol. It forms the second bond with hydrogen.

Hydrogen cyanide, HCN: The carbon atom can also form bonds with nitrogen. In hydrogen cyanide there is a triple bond between carbon and nitrogen. The nitrogen atom contributes three of its valence electrons for the formation of this triple bond.

Methyl amine, CH₃NH₂: In this molecule, the carbon and nitrogen atoms are sharing only one pair of electrons.

However this model could not explain the exact geometry of organic molecules. For example, methane molecule is tetrahedral, whereas ethylene is a planar molecule. These structures with exact bond angles cannot be explained by this model. Therefore it is necessary to understand the structures of these molecules by using valence bond theory as explained in the next section.

VALENCE BOND THEORY 

According to valence bond theory, four unpaired electrons are required to form four covalent bonds. But there are only 2 unpaired electrons in the valence shell of carbon in the ground state.
However it is possible to get 4 unpaired electrons by transferring one of the electrons from 2s orbital into the empty 2p orbital. This process is called excitation and carbon is said to be in the excited state. Now the electronic configuration of carbon in the excited state becomes 2s¹2p³.
A small amount of energy, which is available during the chemical bond formation, is sufficient for a carbon atom to undergo excitation.

It is now possible for carbon atom to form 4 bonds in the excited state.
However, carbon undergoes hybridization before forming actual chemical bonds with other atoms.
Hybridization is the process of intermixing of two or more pure atomic orbitals of almost same energy to form same number of identical and degenerate new orbitals known as hybrid orbitals.
The carbon atom can undergo three types of hybridizations i.e., sp³ or sp² or sp.

SP³ HYBRIDIZATION OF CARBON

In sp³ hybridization, one 2s and three 2p orbitals of excited carbon intermix together and form 4 hybrid orbitals which are oriented in tetrahedral geometry in space around the carbon atom. Each sp³ hybrid orbital is occupied by one electron.

Each of these sp³ orbitals can form σ-bond with other atom. Thus carbon is forming four single bonds with other atoms in tetrahedral geometry. The bond angles are usually equal to or nearer to 109^o28'.
E.g. In methane molecule, CH₄, the carbon atom undergoes sp³ hybridization and forms four σ-bonds with hydrogen atoms.

Note that whenever carbon atom undergoes sp³ hybridization, it forms 4 σ-bonds i.e., 4 single bonds.

SP² HYBRIDIZATION OF CARBON

In sp² hybridization, there is intermixing of one 2s and two of the 2p orbitals of carbon in the excited state to form three hybrid orbitals. These are oriented in trigonal planar geometry. Each sp² hybrid orbital is occupied by one electron. The remaining pure 2p orbital with one electron lies at right angle to the plane of hybrid orbitals.

 The sp² hybrid orbital form 3 σ-bonds in trigonal planar geometry. Thus the bond angles are about 120^o. The remaining pure 'p' orbital will form a π-bond. Thus carbon forms total four bonds i.e., three σ-bonds and one π-bond.
E.g. In ethylene molecule, C₂H₄, each carbon atom undergoes sp² hybridization. Each carbon forms 2 σ-bonds with hydrogens and one σ-bond with another carbon. The remaining pure 'p' orbitals on two carbons overlap sidewise to form a π-bond. Thus there is a double bond between two carbons.

Note that whenever carbon atom undergoes sp² hybridization, it forms 3 σ-bonds and 1 π-bond i.e., two single bonds and one double bond.

SP HYBRIDIZATION OF CARBON

In sp hybridization, one 2s and one 2p orbitals of excited carbon intermix to form two sp-hybrid orbitals in linear geometry. Each sp hybrid orbital is occupied by one electron. The remaining pure 2p orbitals ( for our convenience, let us say: 2p_y and 2p_z) orient at right angles to the sp-hybrid orbitals. These are also occupied by one electron each.

The two sp hybrid orbitals form 2 σ-bonds in linear geometry. Thus the bond angle will be about 180^o. The remaining pure 'p' orbitals will form two π-bonds. Thus carbon again forms total four bonds i.e., two σ-bonds and two π-bonds.
E.g., In acetylene molecule, C₂H₂, each carbon undergoes sp hybridization and forms one σ-bond with a hydrogen atom and one σ-bond with another carbon. The two carbon atoms also form two π-bonds with each other due to sidewise overlapping of pure p-orbitals. Thus a triple bond is formed between two carbon atoms in acetylene molecule.

Note that whenever carbon atom undergoes sp hybridization, it forms 2 σ-bonds and 2 π-bonds. It may either form one triple bond as in case of acetylene or two double bonds e.g. allenes.

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