1. aromaticity

Basic Concepts of Aromaticity

1. POWER POINT PRESENTATION 0N STRUCTURE AND REACTIVITY BY DR. GOPINATH SHIROLE (M.Sc. SET. Ph.D.) ASSISTANT PROFESSOR DEPARTMENT OF CHEMISTRY ARTS, SCIENCE & COMMERECE COLLEGE, RAHATA A/P/TAL-RAHATA, DIST.-AHMEDNAGAR

2. Aromaticity : Th e nam e “ aromatic” wa s originate d from the “aroma” o f benzen e like compounds . Conditions for A romaticity : 1) The molecule (system) must be cyclic . 2) The molecule (system) must be planar ( all atoms in the molecule must lie in the same plane ) 3) The molecule (system) must be fully conjugated . 4) The molecule (system) has ( 4n+2 ) π e- ( n= A ny integer = 0, 1,2,3 ... etc .) 01 Aromatic Obey ( 4n+2 ) rule + Resonance (C onjugation) 02 Anti-aromatic Obey ( 4n ) rule + Resonance (C onjugation) 03 Non-aromatic Obey (4n+2) rule or (4n) rule but no resonance (Non- conjugation)

3. (4n+2) Rule (or) Huckel Rule Aromatic (n=0,1,2,3…) (4n) Rule Anti-aromatic (n=0,1,2,3…) n=0 2 π e - n=0 π e - n=1 6 π e - n=1 4 π e - n=2 10 π e - n=2 8 π e - n=3 14 π e - n=3 12 π e - n=4 18 π e - n=4 16 π e - n=5 22 π e - n=5 20 π e - n=6 26 π e - n=6 24 π e -

4. 1) Cyclopropene 2) Cyclopropenyl cation 3) Cyclopropenyl anion Planer Cyclic Planer Cyclic Planer Cyclic Non-conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=0, 4n+2= 2 π e - Obey (4n+2) rule n=0, 4n+2= 2 π e - Obey (4n) rule n=1, 4n= 4 π e - Non-aromatic Aromatic Anti-aromatic

5. 4) Cyclobutadiene 5) Cyclobutadienyl dication Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Obey (4n) rule n=1, 4n= 4 π e - Obey (4n+2) rule n=0, 4n+2= 2 π e - Anti-aromatic Aromatic

6. 6) Cyclopentadiene 7) Cyclopentadien yl cation 8) Cyclopentadien yl anion Planer Cyclic Planer Cyclic Planer Cyclic Non-conjugated system Fully conjugated system Fully conjugated system Obey (4n) rule n=1, 4n= 4 π e - Obey (4n) rule n=1, 4n= 4 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Non-aromatic Anti-aromatic Aromatic

7. 9) Cycloheptatriene 10) Cycloheptatrienyl cation 11) Cycloheptatrienyl anion Planer Cyclic Planer Cyclic Planer Cyclic Non-conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n) rule n=2, 4n= 8 π e - Non-aromatic Aromatic Anti -a romatic

8. 12) Cyclo-octatetrene Cyclo-octatetrene l ooks Planer but in actual structure it is non-planer Cyclic Fully conjugated system Obey (4n) rule n=2, 4n= 8 π e - Anti -a romatic But Non-aromatic

9. AROMATICITY IN BENZENOID COMPOUNDS The benzenoids  are a class of chemical compounds with at least one benzene ring. OR The aromatic compound which has only benzene rings in their structure is known as benzenoid  aromatic compound. 

10. 13) Cyclohexatriene i.e. Benzene 14) Naphthalene 15) Anthracene Planer Cyclic Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n+2) rule n=2, 4n+2= 10 π e - Obey (4n+2) rule n=3, 4n+2= 14 π e - Aromatic Aromatic Aromatic

11. 16) Phenanthrene 17) Chrysene Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Obey (4n+2) rule n=3, 4n+2= 14 π e - Obey (4n+2) rule n=4, 4n+2= 18 π e - Aromatic Aromatic

12. 18) Pyrene 19) Benzopyrene Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Obey (4n) rule n=4, 4n= 16 π e - Obey (4n) rule n=5, 4n= 20 π e - It disobey Huckel's rule as it has 16 π e - s instead of 14 to obey Huckel's rule. But one of the pi bond is not participating in delocalisation so it cannot be considered in the (4n+2) e - s. So it has only 14 delocalised π e - s and its  aromatic . It disobey Huckel's rule as it has 20 π e - s instead of 18 to obey Huckel's rule. But one of the pi bond is not participating in delocalisation so it cannot be considered in the (4n+2) e - s. So it has only 18 delocalised π e - s and its  aromatic . Aromatic Aromatic

13. AROMATICITY IN NON-BENZENOID COMPOUNDS  The compound which exhibits an aromatic behaviour but does not contain any benzene nucleus . The non benzenoid  aromatic compound have one or more rings fused but none of the rings is a benzene ring.

14. 20) Pyridine 21) Pyrrole 22) Furan 23) Thiophene Planer Cyclic Planer Cyclic Planer Cyclic Planer Cyclic Don't consider lone pair of N atom consider lone pair of N atom consider lone pair of O atom consider lone pair of S atom Fully conjugated system Fully conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Aromatic Aromatic Aromatic Aromatic

15. The fully conjugated monocyclic hydrocarbons are called annulenes . They can be represented by a general formula ( -CH=CH- ) n They may be called cyclic polyenes and they are named by writing the ring size followed by the word “ annulene ”. 4,6,8,10,12,14,16,18 - annulenes are available. According to Huckel’s rule, Annulenes containing (4n+2) π e- and having a coplanar ring should be aromatic in nature. AROMATICITY IN ANNULENE COMPOUNDS

16. 24) (4 - annulene ) Cyclobutadiene 25) (6 - annulene ) Cyclohexatriene i.e. Benzene 26) (8 - annulene ) Cyclo-octatetrene Planer Cyclic Planer Cyclic Cyclo-octatetrene l ooks Planer but in actual structure it is non-planer Cyclic Fully conjugated system Fully conjugated system Fully conjugated system Obey (4n) rule n=1, 4n= 4 π e - Obey (4n+2) rule n=1, 4n+2= 6 π e - Obey (4n) rule n=2, 4n= 8 π e - Anti-aromatic Aromatic Anti -a romatic But Non-aromatic

17. 27) (10 - annulene ) 28) (12 - annulene ) 29) (14 – annulene ) Planer Cyclic Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=2, 4n+2= 10 π e - Obey (4n) rule n=3, 4n= 12 π e - Obey (4n+2) rule n=3, 4n+2= 14 π e - Aromatic Anti-aromatic Aromatic

18. 30) (16 - annulene ) 31) (18 - annulene ) Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Obey (4n) rule n=4, 4n= 16 π e - Obey (4n+2) rule n=4, 4n+2= 18 π e - Anti-aromatic Aromatic

19. AROMATICITY IN FUSED RING COMPOUNDS i.e. AZULENES

20. 32) Azulenes – It is a combination of 5 memb . & 7 membered ring compounds like cyclopentadiene & cycloheptatriene . 33) Pentalene – It is a combination of two 5 membered ring compounds like cyclopentadiene . 34) Heptalene - It is a combination of two 7 membered ring compounds like cycloheptatriene . Planer Cyclic Planer Cyclic Planer Cyclic Fully conjugated system Fully conjugated system Fully conjugated system Obey (4n+2) rule n=2, 4n+2= 10 π e - Obey (4n) rule n=2, 4n= 8 π e - Obey (4n) rule n=3, 4n= 12 π e - Aromatic Anti-aromatic Anti-aromatic