Arenes and aromaticity

This presentation has been reported for its great application in Organic synthesis and effects of various substituents. It will be useful in understanding of many organic reaction mechanisms.

1. Dr. Arun Sharma M.Sc. (Gold medalist), Ph.D . (DRDE) Assistant Professor Arenes and Aromaticity

2. Aromatic hydrocarbons are called “Arenes”. Benzene was the first isolated compound. It was isolated by Michael Faraday in 1825 who determined C:H ratio to be 1:1. Aromatic compounds are cyclic compounds having ring structure. These are highly stable compounds. Generally, these compounds are insoluble in water. Aromatic hydrocarbons follow the Huckle’s Rule: (When the number of π -electrons = 4n+2) Where, n= 0, 1, 2, 3,…..are called “Aromatic compounds”. These are cyclic, planar and completely conjugated. Introduction

3. Resonance structure of Benzene & MO-picture Kekulé structure of Benzene Kekule benzene: Two forms are in rapid equilibrium Resonance hybrid 6  -electron delocalized over 6 carbon atoms Localized π -electrons Delocalized π -electrons Resonance Delocalization of π -electron cloud in Benzene Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. =>

4. Aromaticity Aromatic compounds are cyclic, planar, stable, completely conjugated and follow the Huckle’s Rule i.e. contains (4n+2) π electrons. Anti-aromatic compounds are cyclic, planar, completely conjugated and follow the 4n π electrons. These compounds are generally unstable. e.g. cyclobutadiene. Non-aromatic compounds are the compounds that lack one or more of the four requirements to be aromatic or anti-aromatic. Buckminsterfullerene—Is it Aromatic ? C 60 is completely conjugated, but it is not aromatic since it is not planar .

5. Aromatic Electrophilic Substitution Electron deficient species are called “ Electrophiles ”. Represented by E + Electrophiles loves to bind with the electron rich molecules. “ Nucleophiles ” are electron rich species. Represented by Nu - Nucleophiles loves to bind with the electron deficient molecules. Benzene is an electron rich compound. Therefore, it can easily gives the electrophilic substitution reaction. Mechanism: Step 1. Attack of electrophile E + Slow Resonance stabilization Step 2. Removal of proton

6. Examples of Aromatic Electrophilic Substitution (Nitration) (Nitrobenzene) (Sulfonation) (Benzene sulfonic acid) (Bromination) (Bromo benzene) HNO 3 H 2 SO 4 H 2 SO 4 SO 3 (Ethyl benzene) (Alkylation) CH 3 CH 2 Cl AlCl 3

7. Mechanism of Nitration Formation of electrophile: Nitronium ion Nitric acid Step 1. Attack of electrophile i.e. Nitronium ion Slow Intermediate cation Step 2. Removal of proton Nitrobenzene

8. Mechanism of Sulfonation Formation of electrophile: Sulfur trioxide  is formed (or already present). It is a powerful  electrophile , i.e. electron pair acceptor because of the effect of the three very electronegative oxygen atoms bonded to the central sulfur atom. Step 1. Attack of electrophile i.e. SO 3 Slow Resonance stabilization Step 2. Proton transfer to give the Benzene sulfonic acid.

9. Mechanism of Bromination This complex is more electrophilic than Br 2 alone Formation of electrophile: Step 1. Attack of electrophile i.e. Br 2 -FeBr 3 complex Resonance stabilization Step 2. Removal of proton

10. Mechanism of Alkylation Step 2. Removal of proton Step 1. Attack of electrophile i.e. CH 3 CH 2 + Formation of electrophile: CH 3 CH 2 Cl + AlCl 3 CH 3 CH 2 + + AlCl 4 -

11. Activating and Deactivating groups Activating or Ortho-Para (o/p) Directing Groups Rx n of an electrophile at the para position of anisole

12. Rx n of an electrophile at the meta position of anisole The charge on the meta-derived intermediate cannot be delocalized onto the -OCH 3 group Electrophilic Aromatic Substitution Reactions of Substituted Benzenes

13. Deactivating or meta (m)-Directing Groups

14. Activating and Deactivating Effects Activating group : A group that causes the substituted benzene ring to react more rapidly than benzene itself All ortho-para directing groups except Halogens Deactivating group : A group that causes the substituted benzene ring to react more slowly than benzene itself All meta directing groups Inductive/Polar effect +I effect - I effect

15. Electron-withdrawing (-I effect) Electron-releasing (+I effect) Resonance effect: Electron withdrawing by resonance

16. Electron donation by resonance Inductive and resonance effects can work in the same or opposite directions

17. Summary of Substituent Effects Activating and Deactivating Effects Table: Substituent Effects in Aromatic Electrophilic Substitution

18. Use of Aromatic Electrophilic Substitution in Organic Synthesis

19. Thanks………………………