ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- AROMATICITY OF FUSED RINGS

ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- AROMATICITY OF FUSED RINGS

**A fused aromatic ring is one which shares one side of the ring. These are monocyclic rings that shares their connecting bond.

** Fused ring systems arc classified into two categories depending on the number of atoms and bonds shared by the rings. 

Orho-fused rings-If the two rings of the fused system that have only two atoms and one bond is common between the rings. Ortho and peri fused rings-If three atoms are common between the first ring and the other two rings then it is said to be Ortho and pert fused rings. 

**The criteria for the aromaticity in the fused rings also follows the same rule as applied to the monocyclic systems. 

1. It is cyclic, planar and has continuous delocalization of π electrons (electrons in p orbitals) with or without the participation of lone pair(s)/- charge/ + charge (Le., having electrons or vacant p orbital). 

2. The delocalized π-electron cloud must contain a total of (4n+2) π electrons, where n is a whole number (i.e., n =0.1,2,3 and so on) 

**Aromaticity of some benzenoid fused rings like naphthalene, anthracene and phenanthrene. 

**The resonance energy of fused system is generally less than of the number of benzene ring present in that system. 

**Aromaticity of some non-benzenoid fused rings like azulene and oxaazulanones. **Aromaticity of some way ergy of fused system is generally less than of the number of benzene special molecules like phenalene, ferrocene and benzo cyclobutadiene.

 

ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- ANNULENES

ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- ANNULENES

**Annulenes are the completely conjugated monocyclic hydrocarbons containing an even number of carbon atoms. They have the general formula CnHn (when n is an even number) or Cn.Hn+1 (when n is an odd number) 

**The first 3 members of the series, (4]-, [6]-, and [8]-annulene but you must have used their name as 1,3- cyclobutadiene, benzene and 1,3,5,7- cyclooctatetraene. 

**Annulenes could be aromatic, anti aromatic or non-aromatic.

#[4] Annulene: anti-aromatic 

#[6] Annulene: aromatic 

#[81 Annulene: non-aromatic

#[10] Annulene: non-aromatic 

#[12] Annulene anti-aromatic 

#[14] Annulene: anti-aromatic.

#[18] Annulene: aromatic. 

**The following bridgehead [10] annulenes are aromatic: 

#9,10 methane [10] annulene 

#9,10 oxa [10] annulerne 

#9,10 aza [10] annulene 

**Preparation of [14] annulene, [18] annulene and 9,10-methane [10] annulene from simple and easily available molecules.

 

ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- AROMATICITY –PART-1

ORGANIC CHEMISTRY- I (NATURE OF BONDING AND STEREOCHEMISTRY)- AROMATICITY –PART-1

These compounds exhibit significantly high diamagnetic susceptibility. Cyclic electron delocalization also results in bond length equalization, abnormal chemical shifts and magnetic anisotropies, as well as chemical and physical properties which reflect energetic stabilization. On the other hand, compounds with high paramagnetic susceptibility may be antiaromatic. 

**Huckel's rule can be used to determine whether a molecule is aromatic or anti-aromatic by broadly two conditions. 

* It is cyclic, planar and has continuous delocalization of π electrons (electrons in p orbitals) with or without the participation of lone pair(s)/- charge/ + charge (i.e having electrons or vacant p orbital). 

* The delocalised π-electron cloud must contain a total of (4n+2)π electrons, where n is a whole number (i.e., n= 0,1,2,3 and so on) **If in the second condition there are 4n π electrons, it is anti-aromatic.

** If the first condition is not fully met,it is aliphatic. 

**The term non-aromatic is applicable to both aliphatic as well as anti-aromatic. **Aromaticity is the state of stability that any molecule would thrive to achieve. These molecules are cyclic, planar and have (4n+2)π electrons continuously delocalised in a cyclic manner.