B.Sc. Sem-I Unit-IV Aromatic, antiaromatic and non aromatic compounds by Dr Pramod R Padole

Aromaticity and Huckel’s rule Aromatic, antiaromatic and non-aromatic systems

1. 1 B.Sc. First year Students B.Sc. Semester – I Unit-IV Aromaticity and Huckel’s rule Aromatic, antiaromatic and non-aromatic systems by Dr Pramod R Padole Professor in Chemistry Department of Chemistry Shri Shivaji Science College, Amravati

2. Our Inspiration 2

3. Unit IV- Aromatic Hydrocarbons Orientation: Effect of substituent groups. Activating and deactivating groups. Theory of reactivity and orientation on the basis of inductive and resonance effects (-CH 3 , -OH, -NO 2 and – Cl groups). D Nomenclature and Isomerism of Aromatic Compounds: Structure of Benzene: Kekule structure and Molecular orbital structure . A Aromaticity and Huckel’s rule Aromatic, antiaromatic and non-aromatic systems B Mechanism of Electrophilic Aromatic Substitution: Nitration, Friedal Craft Alkylation and Acylation.Nuclear and Side Chain Halogination , Birch Reduction C

4. By Dr Pramod R Padole Aromaticity & Huckel’s Rule:

5. Aromaticity & Huckel’s Rule: Aromaticity & Huckel’s (4n+2) π e - s Rule: Or Characteristics of Aromatic Compound: E- Huckel (German scientist) put forward a satisfactory explanation for aromaticity . Q.1) Explain Aromaticity in the right of Hukel’s Rule. (W-06, 2 Mark) Q.2) State Huckel’s rule for aromaticity . (S-10 & S-11, 2 Mark) Q.3) Explain Huckel’s rule with examples. (W-11, 4 mark) Q.4) What are the characteristics of aromatic compounds? (S-13 & S-15, 4 Mark) Q.5) (4n+2) electron rule is called __________. (S-14 & W-16, ½ Mark) Q.6) Explain the criteria of aromaticity . (W-14, 4 Mark) Q.7) Give two examples of Aromatic compounds. (W-19, 2 Mark)

6. Aromaticity & Huckel’s Rule: Aromatic compounds possess the following characteristics: Compounds should be cyclic and planar. The ring C-atoms have unhybridised p-orbital’s (because each C-atom is sp 2 hybridised ). These unhybridised p-orbital’s are perpendicular to the plane of the molecule and must be parallel to each other, so that continuous overlapping is possible around the ring.

7. Aromaticity & Huckel’s Rule: Compound must have a conjugated electron system. Cyclic molecular orbital shall have, (4n+2); π e - s. Where, n is the integer number & n = 0,1,2,3,……, etc Thus, the conjugated cyclic aromatic compounds containing 2, 6, 10, 14, ….., etc, π e - s possess aromatic character. This is known as Huckel’s Rule. Or A cyclic, planar, completely conjugated compound with (4n+2); π e - s, is known as aromatic compounds . Aromatic compounds give substitution reaction rather than addition reaction. The aromatic compounds are stabilized by resonance energy.

8. Aromaticity & Huckel’s Rule: Examples of Aromatic compounds:

9. Antiaromatic Compound or System: Q.1) What are the characteristics of antiaromatic compunds ? ( S-14, W-16 & W-19, 4 Mark) Q.2) Which of he following compounds is antiaromatic ? (S-16 & W-18, ½ Mark) a ) Benzene b ) Cyclobutadiene c ) Cyclodecapenene d) Cyclo-octatetraene Q.3) Give two examples of non-aromatic compounds. ( W-19, 2 Mark)

10. Antiaromatic Compound or System : Antiaromatic molecules are cyclic systems containing alternating single and double bonds, where the electron energy of antiaromatic compounds is higher than that of its open chain analogue. Therefore, antiaromatic compounds are unstable and highly reactive . By Dr Pramod R Padole

11. Antiaromatic Compound or System : Antiaromatic compounds possess the following characteristics: Compounds should be cyclic and planar. The ring C-atoms have unhybridised p-orbital’s (because each C-atom is sp 2 hybridised ). Compound must have a conjugated electron system. Cyclic molecular orbital shall have, ( 4n ); π e - s. Where , n is the integer number & n = 0,1,2,3,……, etc Thus, the conjugated cyclic anti-aromatic compounds containing 0, 4, 8, 12, ….., etc, π e - s possess anti-aromatic character . By Dr Pramod R Padole

12. Antiaromatic Compound or System : Antiaromatic compounds possess the following characteristics : Or A cyclic, planar, completely conjugated compound with (4n); π e - s, is known as anti-aromatic compounds. Examples: By Dr Pramod R Padole

13. Aromatic & Antiaromatic Compound Q.1) Identify which of the following compounds are aromatic and antiaromatic : ( S-16, 4 Mark ) -----*****----- Q.2) Predict the following compounds as aromatic or antiaromatic with explanation: ( S-17, 4 Mark) By Dr Pramod R Padole

14. Non aromatic Compound or System: Q.1) Explain why, Cyclo-octatetraene , is non-aromatic compounds. (W-13, 2 Mark)

15. Non aromatic Compound or System : Those compounds which are neither aromatic nor anti-aromatic are known as non-aromatic compounds . For example - 1: Cyclo-octatetraene is cyclic with conjugated system, does not follow Huckel’s rule, (4n+2); π e - s while it fallows 4n electrons required for anti-aromatic character, but its plane is distorted and transforms into stable non planar Tub shape structure . So, it is non aromatic.

16. Non aromatic Compound or System : Those compounds which are neither aromatic nor anti-aromatic are known as non-aromatic compounds . For example - 2: Cyclo-decapentaene is cyclic with conjugated system and has 10 electrons so follows Hucke’s rule but the planar structure is strained and transforms into stable non planar structure, therefore, it is non aromatic. So, it is non aromatic.

17. Q.1) Classify the following as aromatic and anti-aromatic: (S-18 , 4 Mark ) ---------*****--------- Q.2 ) Identify which of the following compounds are aromatic and antiaromatic : ( W-18, 4 Mark)

18. Q.3) Identify the aromatic character for the following compounds:- 4n+2 = 6; π e - s 4n = 6-2 n = 4/4 n = 1, integer no. So, Benzene is aromatic compound.

19. www.themegallery.com Thank You ! Aromaticity and Huckel’s rule by Dr Pramod R. Padole Stay Home. Take Care