Aromatic Chemistry
Aromaticity
Structure of benzene
Elemental analysis and molecular weight determine the molecular formula of benzene is C6H6
Open Chain Structure
Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene.
Drawbacks of open chain structure
The open chain structure for benzene was rejected due to the following reasons:
1. Addition reactions usually given by alkenes and alkynes are not given by benzene.
2. Benzene forms only one kind of mono-substituted product.
3. An open chain structure however, can form more than one kind of monosubstituted product.
4. The open chain compounds do, not give reactions such as FriedelCraft reaction, nitration, sulphonation.
5. On reduction with hydrogen in the presence of Ni at 200°C, actually a cyclic compound cyclohexane is obtained.
Elemental analysis and molecular weight determine the molecular formula of benzene is C6H6
Open Chain Structure
Based upon observable facts given above and the tetravalency of carbon, the following open chain structures were proposed for benzene.
Drawbacks of open chain structure
The open chain structure for benzene was rejected due to the following reasons:
1. Addition reactions usually given by alkenes and alkynes are not given by benzene.
2. Benzene forms only one kind of mono-substituted product.
3. An open chain structure however, can form more than one kind of monosubstituted product.
4. The open chain compounds do, not give reactions such as FriedelCraft reaction, nitration, sulphonation.
5. On reduction with hydrogen in the presence of Ni at 200°C, actually a cyclic compound cyclohexane is obtained.
Ring Structure
In 1858, August Kekule had proposed that carbon atoms can join to one another to form chains and in 1865, he gave the structure of benzene as:
1. All the carbon-to-carbon bonds in benzene are equivalent.
2. The molecule is unusually stable.
3. Chemists often represent benzene as a hexagon with an inscribed circleThe inner circle indicates that the valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized, or spread out, over all the carbon atoms).
4. Each corner of the hexagon is occupied by one carbon atom, and each carbon atom has one hydrogen atom attached to it.
5. Any other atom or groups of atoms substituted for a hydrogen atom must be shown bonded to a particular corner of the hexagon.
6. The six bond lengths are identical and they are one-and-a half bonds and their length, 1.39 A or 139 picometer, is intermediate between the lengths of single and double bonds (is shorter than typical single-bond lengths, yet longer than typical double-bond lengths).
In 1858, August Kekule had proposed that carbon atoms can join to one another to form chains and in 1865, he gave the structure of benzene as:
2. The molecule is unusually stable.
3. Chemists often represent benzene as a hexagon with an inscribed circleThe inner circle indicates that the valence electrons are shared equally by all six carbon atoms (that is, the electrons are delocalized, or spread out, over all the carbon atoms).
4. Each corner of the hexagon is occupied by one carbon atom, and each carbon atom has one hydrogen atom attached to it.
5. Any other atom or groups of atoms substituted for a hydrogen atom must be shown bonded to a particular corner of the hexagon.
6. The six bond lengths are identical and they are one-and-a half bonds and their length, 1.39 A or 139 picometer, is intermediate between the lengths of single and double bonds (is shorter than typical single-bond lengths, yet longer than typical double-bond lengths).
Electrophilic Substitution Reactions in Benzene
Directive Influence of Functional Groups and Orientation
Benzene undergoes typical electrophillic substitution reactions forms mono-substituted benzene as the product. When this mono-substituted benzene is subjected to further electrophillic substitution, it forms three possible disubstituted products. The major product depends on the reactivity of the mono-substituted benzene.
When mono substituted benzene undergoes an electrophilic attack, the rate of reaction and the site of attack vary with the functional group already attached to it.
Electron donating groups increase the reactivity of benzene ring and are known as activating groups while electron withdrawing group decrease the reactivity of benzene ring are known as deactivating groups.
The electron donating groups direct the incoming group to ortho and para positions are called ortho and para directing groups, as the electron density is more on ortho-and para-positions. Hence the electrophilic substitution takes place mainly at these two positions.
The electron withdrawing groups direct the incoming group to meta- positions are called meta directing groups, as the electron density is more on meta-positions. Hence the electrophilic substitution takes place mainly at this position.
Benzene undergoes typical electrophillic substitution reactions forms mono-substituted benzene as the product. When this mono-substituted benzene is subjected to further electrophillic substitution, it forms three possible disubstituted products. The major product depends on the reactivity of the mono-substituted benzene.
Electron donating groups increase the reactivity of benzene ring and are known as activating groups while electron withdrawing group decrease the reactivity of benzene ring are known as deactivating groups.
The electron donating groups direct the incoming group to ortho and para positions are called ortho and para directing groups, as the electron density is more on ortho-and para-positions. Hence the electrophilic substitution takes place mainly at these two positions.
The electron withdrawing groups direct the incoming group to meta- positions are called meta directing groups, as the electron density is more on meta-positions. Hence the electrophilic substitution takes place mainly at this position.
Methods of Preparation of Phenol
Methods of Preparation of Aniline
Important Reactions of Aniline
Methods of Preparation of Benzaldehyde
Important Reactions of Benzaldehyde
Methods of Preparation of Acetophenone
Important Reactions of Acetophenone
Methods of Preparation of Benzoic Acid
Important Reactions of Benzoic Acid
