Aroamticity msc chemistry sem 1 (rajasthan university)

Aromaticity organic chemistry msc sem 1

1. aromaticity Aromatic compounds :- Compounds which are cyclic, planer, conjugated and contains (4n+2)  electrons system are termed as aromatic compounds. Some examples of aromatic compounds are given below- 9

2. Antiaromatic compounds :- Compounds which are cyclic, planer, conjugated and contain (4n)  electrons system are known as antiaromatic compounds.

3. Some examples of antiaromatic compounds are given below-

4. Non aromatic compounds :- Compounds which do not follow Huckel’s Rule (i.e. cyclic/acyclic, planer/non planer,conjugated /non conjugated system and number of electron is not certain )are termed as Non aromatic compounds. Some examples of non aromatic compounds are given below -

5. Stability Order – Aromatic > Non aromatic > Antiaromatic

6. Aromaticity In Benzenoids Compounds – Benzenoid aromatic compounds are the organic molecular species either with isolated benzene rings which fused to form a more complex structure. Aromatic molecules are very stable and generally undergo electrophilic substitutions rather than addition .

7. Aromaticity In non benzenoid compounds – A compound which exibhitics an aromatic behaviour but does not contain any benzene ring. The non benzenoid aromatic compounds have one or more ring but non of these rings is benzene ring.

8. Aromaticity in non benzenoid compound – Annulene (Monocyclic non benzenoid neutral compounds) :-

9. Homoaromatic Compound – If a cyclic conjugated system can be formed by passing one or more than one SP3 hybrid carbon atom then it is termed as homoaromaticity .In comparison to the true aromatic systems, the net stabilization in homoaromatic system may be low due to poor overlapping of orbitals . Homotropylium ion ( cyclo octatrienyl cation )is the best example of homoaromaticity .In this ion an aromatic sextet is spread over seven carbons while eight carbon is SP3 carbon and it is not involved in aromaticity .

10. Alternate and non alternate hydrocarbons :- 1) Alternate Hydrocarbons – An alternate hydrocarbons is any conjugated hydrocarbon system which does not have an odd membered ring. In this system the atom can be divided into two sets such that the atoms are of one set are bonded only to atoms of the other set. It may be of 2 types -

11. Even alternate hydrocarbons – The hydrocarbons containing even number of conjugated atoms (i.e. number of one set of atoms =number of other set of atoms )are known as even alternate hydrocarbons . Example – benzene , naphthalene , 1,3 butadiene , 1,3,5 hexatriene etc. Aliphatic and aromatic even alternate hydrocarbons contain only bonding and antibonding molecular orbitals .

12. 2) Odd alternate hydrocarbons – The hydrocarbons containing odd number of conjugated atoms (i.e. number of one set of atoms  number of the other set of atoms) are known as odd alternate hydrocarbons . Example – carbocation , carboanion or free radicles and it contain all bonding , antibondingas well as non bonding molecular orbitals (BMO will always be equal to ABMO )

13. 2.) Non Alternate Hydrocarbons :- Non alternate hydrocarbons are those hydrocarbons in which the conjugated atoms , whose number may be even or odd are divided into two sets in a such a manner that two atoms of same set is connected to each other . Cyclic non alternate hydrocarbons contain at least one ring with odd number of carbon atoms . Non Alternate hydrocarbons may be aromatic , non aromatic or anti aromatic .

14. PMO (perturbation molecular orbital) – PMO approach can be used to predict the aromatic character of different aromatic compounds. According to this method , we need to observe the total energy of cyclic conjugate system becomes less than the open chain counterpart, it will be labeled as aromatic. If the energy of the cyclic conjugated system becomes greater than the open chain counterpart , it will be labeled as antiaromatic. In this approach we have to follow the following steps – Firstly we have to open a cyclic system into open chain (In carbon chain the no. of electrons must be even numbers. The open chain conjugated pi electron system having a even no. of electron can be classified itno 2 types-

15. 1- system in which the HOMO having a m symmetry and LUMO having a C2 symmetry . 2- system in which HOMO having a C2 symmetry and LUMO has m m symmetry But later GOLDSTEIN and HOFFMAN has named these two system as MODE 2 and MODE 0 repectively .