16.4 Notes

                                                      Benzene

 Aromaticity.

Aromatic character or Aromaticity is the characteristic behavior of aromatic compounds apparently contains alternate double & single bonds in a cyclic planner structure, resemble with benzene in chemical behavior and undergo substitution rather addition reaction.

Critereria For Aromaticity.

The compounds which have the following three properties are called as aromatic compounds.

i) An aromatic compound is cyclic and planar.

ii) An aromatic compound contains alternate double and single bonds. Which are delocalized?

iii) The cyclic molecular orbital formed by overlap of p orbitals must contain (4n+2) pi electrons where n=0,1,2,3……

This is known as HUCKEL RULE.

Reactivity Of Benzene.

Benzene like an alkene has p electron system but they do not undergoes addition because the benzene ring with delocalize p electron has low energy then p electron system of alkane. Therefore they undergoes electrophillic substitution reaction. In these reactions of the benzene ring is retained the electrophilic substitution reactions of benzene and other aromatic take place through the formation of a highly resonance stabilized complex.              

 

Kekule’s Structure.

In 1865, August KEKULE suggested that benzene consisted of a cyclic planar structure of six carbons, with alternate double single bonds all contained in a regular hexagonal frame work with one hydrogen at each carbon.

 

MODERN CONCEPT OF THE STRUCTURE OF BENZENE.

Molecular Orbital Structure of Benzene.

The structure of benzene is best described in terms of modern molecular orbital theory.

According to this theory in benzene each carbon atom is bonded to only three other atoms ( i.e. two carbons and one hydrogen atom ), hence 2s and two of the three 2p orbital are hybridized to get three equivalent, co-planer sp² hybrid orbitals at an angle of 120^o. Two of these hybrid orbital are utilized in forming two sigma bonds with two adjacent carbon atoms due to sp² - sp² over lapping and the third one is forming a sigma bond with hydrogen due to sp² – 1s overlap.

There remains one unhybridized p-orbital of each carbon atom, oriented perpendicular to the plane of hexagonal ring structure. This gives the following picture of benzene molecule.

Effect of substituent on further substitution.

The substituent group already present on further substitution.

Directive  effects.

This first substituent may direct the next incoming substituent^- (E⁺) to ortho-para or meta position depending on the nature of the first substituent. This is called the “directive or the orientation effect”.

The groups have two types of directing effect

a) Ortho - para directing effect.

The substituted groups which direct the second substituent to ortho - para position simultaneously are called ortho - para directors.

                   It was observed that the groups in which high electronegative element directly attached to the benzene ring direct the incoming group to ortho - para positions . For example -OH,-NH₂, -NHR, -NR₂, -F, -Cl, -Br, -I, and also alkyl groups such as -CH₃, -C₂H₅ etc.

b) Meta Directing Effects.

The substituent which direct the second incoming substituent primarily to meta position are called Meta directors.

                                                                   It was observed that groups in which lightly electronegative element in directly attached with the benzene ring are meta directors for example.

          -NO₂, -SO₃H , -COOH, CHO, -CO-R, CN, -⁺NH₃ here -⁺NH₃ is not according to above assumption.

Activating effect.

The substituent already present may activate or deactivate the benzene ring towards further substitution. There effects are called activity effect.

a) Activating groups.

A substituent which activates the aromatic ring to further substitution , is called an activating substituent or ring activator. For example.

          Toluene   (C₆ H₅ - CH₃ ) is nitrated 25 times faster than benzene.

          -OH, -OR   strangely activating.

          -NH₂, -NHR, -NR₂ , -R(CH₃ , C₂ H₅ , etc) weakly activating.

b) Deactivating groups.

A substituent which deactivates the aromatic ring to further

Substitution is called a Deactivating group or ring - deactivator. For example

The rate of nitration of Chlorobenzene (C₆ H₅ -Cl ) is 30 times less than benzene.

-NO₂ , CHO, - COR , -SO₃ H , -CN, COOH strongly Deactivating -F, -Cl, -Br,-I