Aromatic Hydrocarbon Class 11 NEB Chemistry Notes | Nepali Educate

Aromatic Hydrocarbon

Introduction

The term aromatic is derived from the Greek word aroma which means fragrance or pleasant smell. Aromatic hydrocarbons are benzene and compounds that resemble benzene in chemical behavior. They are also called arenes. (i.e. aromatic alkenes) They have the general formula CnH2n -6y where n is the number of carbon and y is the number of rings. Some examples are:

• Benzene
• Napthelene
• Anthracene

Characteristics of aromatic compounds

1. They are cyclic compound having planar ring.
2. They have delocalization of π electrons (conjugated system having alternate single and double bonds.
3. They show slight unsaturation. They do not give addition reactions easily.
4. They easily give electrophilic substitution reactions.
5. The percentage of carbon in the aromatic compounds is higher than in the aliphatic compound. So, they burn with a sooty flame.
6. They follow Huckel’s (4n+2)π rule.
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Huckel's Rule:

Huckel’s rule states that a cyclic, planar and conjugated molecule is aromatic if it contains 4n+2 delocalized π electrons, where n = 0, 1, 2, 3, 4, etc.
when n=0, (4n+2)π =2π eg. cyclopropenyl cation
when n=1, (4n+2)π =6π eg. benzene, thiophene, furan, pyrrole, pyridine, etc
when n=2, (4n+2)π =10π eg. naphthalene
when n=3, (4n+2)π =14π eg. anthracen

Some aromatic compounds

Structure of Benzene

Kekule, a German scientist proposed the structure of benzene for the first time. According to Kekule, all the 6 carbon atoms of benzene molecule are joint to each other by alternate single and double bond forming a hexagonal ring and a hydrogen atom is bonded to each carbon atom.Benzene is stabilized by resonance. Resonance is the phenomenon in which a compound can be represented by more than one structure. In benzene C-C bond length is 1.39A which is intermediate between C-C single bond length (1.50A) and C=C double bond length (1.34A).

Molecular orbital structure of benzene

1. Each carbon atom in benzene has to join to three other atoms( one H and two C) and doesn’t have enough unpaired electrons to form the required number of bonds, so it needs to promote one of the 2s² electron into empty 2p_z orbital. This is called excited state configuration.
2. Because each carbon is only joining to three other atoms, carbon atom is sp² hybridized using one 2s electron and two 2p electrons leaving one 2p electron unchanged.
3. Two of the sp² hybrid orbitals of each of the six carbon atoms overlap with each other to form a hexagonal ring, while rest of the hybrid orbital overlaps with 1s orbital of a hydrogen atom. The unhybridized 2p_z orbital of each carbon atom undergoes sidewise overlapping below and above the ring to form π bonds:
   

Nomenclature of benzene derivatives

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