Electrophilic substitution reaction

Electrophilic substitution reaction

1. Molecular Orbital Diagram of Benzene & Electrophilic Substitution Reaction Mr. Yashvant V. Khandare

2. Molecular Orbital Diagram of Benzene Molecular Formula of Benzene is C6H6. All the carbon atoms are present in sp2 hybridization state. C-C bonds --  sp2-sp2 overlaps C-H bonds -- sp2-s overlaps All bonds lie in a same plane at an angle 120  

3. s

4. Molecular Orbital Diagram of Benzene Each carbon contains one unhybridized p-orbital with single electron. P-orbitals perpendicular to sigma bonds and parallel to each other. These overlap sidewise on either to form delocalized Containing electron cloud of six delocalized electrons which are revolving above and below the plane.  

5. Molecular Orbital Diagram of Benzene s

6. Substitution reaction A chemical reactions in which an atom, ion, or group of atoms or ions in a molecule is replaced by another atom, ion, or group.

7. Electrophilic Substitution Reaction Definition :- Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not always, a hydrogen atom.

8. Definition :- Replacement of hydrogen atom of benzene nucleus by nitro (NO2) group is called nitration of benzene. Ex. Benzene on heating with mixture (conc,HNO3 & conc. H2SO4) gives nitrobenzene. Nitration

9. Nitration

10. Mechanism : It consist of following three steps a) Formation of electrophile : Nitrating mixture gives nitronium ion (NO2+) as electrophile. b) Formation of complex: Nitronium ion (NO2+) attacks on benzene to give intermediate carbonium ion called complex. c) Formation of final product : In this step, there is removal of proton by attack of base HSO4- to give nitrobenzene. It is fast step.  

11. s HSO4- H2SO4

12. Halogenation & Friedel Craft Reaction

13. Halogenation Replacement of hydrogen atom of benzene nucleus by halogen is called halogenation of benzene. For example benzene reacts with chlorine in presence of catalyst anhydrous AlCl3 to give chlorobenzene.

14. Chlorination of Benzene

15. Mechanism Step-I : Formation of Electrophile Chlorine reacts with AlCl3 to give chloronium (Cl+) as electrophile. Cl 2 + AlCl 3 ---  Cl + + AlCl 4 - Chloronium ion

16. Step-II : Formation of σ -Complex Chloronium ion attacks on benzene to give intermediate carbonium ion called sigma complex. It is stabilized by resonance. It is slow & rate determining step.

17. Resonating Structures

18. Step-III :- Formation of Final Product In this step, there is a removal of proton by attack of base AlCl 4 - to give chlorobenzene. It is fast step. AlCl 4 - AlCl 3

19. Friedel Craft Reactions a)Friedel Craft Reaction: Replacement of hydrogen atom of benzene nucleus by alkyl group is called alkylation of benzene. For example, benzene on heating with alkyl halide in presence of catalyst anhydrous AlCl 3 or Fe Cl 3 gives alkyl halide.

20. Friedel Craft Reaction Alkylation & Acylation

21. Friedel Craft Reactions a)Friedel Craft Reaction: Replacement of hydrogen atom of benzene nucleus by alkyl group is called alkylation of benzene. For example, benzene on heating with alkyl halide in presence of catalyst anhydrous AlCl 3 or Fe Cl 3 gives alkyl halide.

22. Friedel Craft Reaction Acylation

23. Friedel Craft Acylation Replacement of hydrogen atom of benzene nucleus by acyl (R-CO) group is called acylation of benzene. Example :- Benzene on heating with acetyl chloride in presence of anhydrous AlCl3 gives acetophenone.

24. Mechanism It consist of following three steps: Step-I : Formation of Electrophile Acetyl Chloride reacts with AlCl 3 to give acetyl carbonium ion as electrophile.

25. Step-II : Formation of σ -Complex Acetyl Carbonium ion attacks on benzene to give intermediate carbonium ion called sigma complex. It is stabilized by resonance. It is slow & rate determining step.

26. Step-III :- Formation of Final Product In this step, there is a removal of proton by attack of base AlCl 4 - to give Acetophenone. It is fast step.