Electrophilic Substitution Reaction in Aromatic Compounds

Kanika Khandelwal sem 1

1. Government College , Ajmer Electrophilic Substitution Reaction On Aromatic Compounds Presented by: Kanika Khandelwal MSc. Chem Sem -1

2. Contents Electrophilic Substitution reaction Energy curve for E.S.R Nitration Sulphonation Halogenation Friedel craft Alkylation Friedel craft Acylation Reaction Activating and Deactivating Groups Introduction of third group in Substituted Ring Orientation in other Ring System

3. Electrophilic Substitution Reaction An Electrophilic Substitution reaction is  a chemical reaction in which a Hydrogen atom attached to an aromatic ring is replaced by an electrophile ( E + ) . Mechanism :-

4. Energy curve for typical E.S.R :- This curve is plotted b/w energy and progress of the reaction. This curve involve 2 hills since the E.S.R involves 2 steps for completion. Rate of substitution at any position depends on height of energy barrier i.e. ( G). 1 st hill represents T.S 1 st (pi complex) and 2 nd hill represents T.S 2 nd (sigma complex ), the gap b/w the 2 hills represents the arenium ion . E.S.R –Electrophilic substitution reaction T.S - Transition state Delta G – Gibbs free energy

5. PIE COMPLEX SIGMA COMPLEX Pie complex Sigma complex This forms first in a reaction. This does not involve actual chemical bonding but held near pie electron cloud of aromatic ring. The pie complex converts into the sigma complex. This involves the actual chemical bonding between electrophile and the carbon atom.

6. Nitration Reaction In this type of reaction , the H atom of the aromatic ring is replaced by NO 2 group . Nitrating reagent used :- mix acid (HNO 3 +H 2 SO 4 ). Active nitrating reagent :- NO 2 + ion. Other reagent that could be used :- i)Conc./fuming sulphuric acid. ii) HNO 3 acid in organic solvent (acetic acid , nitromethane). iii)Nitronium salts (NO 2 + ,BF 4 - , N 2 O 5 ). Reaction involved in reagent :- H 2 SO 4 +HO-NO 2 HSO 4 - +H 2 O + -NO 2 H 2 O + -NO 2 H 2 O +NO 2 + H 2 O + H 2 SO 4 H 3 O + + HSO 4 - overall reaction :- 2H 2 SO 4 +HNO 3 NO 2 + + H 3 O + +2HSO 4 - The above reaction is an acid base reaction where nitric acid acts as a base.

7. Reaction of Nitration :- Mechanism :- Step 1:-formation of arenium ion carbocation . In this step , aromatic ring reacts with the NO 2 + & forms an arenium ion carbocation which is stabilised by resonance .

8. Step2 :- Formation of the substituted product by removal of a proton from the arenium carbocation formed in the step 1 . This step is fast step ,and aromaticity of the compound regained in this step .

9. Sulphonation Reaction In this type of reaction , the H atom of the aromatic ring is Substituted by SO3 group . Reagent used in this reaction :- fuming sulphuric acid ( oleum ) concentrated H2SO4 . 2H2SO4 SO3 + H3O + + HSO4 – Active electrophile :-SO3 Reaction of sulphonation :-

10. Mechanism :- Step 1:- In this step , the electrophile (SO 3 ) attacks the benzene /aromatic ring to give the resonance stabilised arenium ion intermediate . This step is slow since aromaticity of the ring is disturbed in this reaction and thus is the rate determining step. Step 2:- Removal of a proton from arenium ion by a base to give substitution product i.e . Benzenesulphonic acid .

11. Reverse Sulphonation Reaction :- This reaction is a reversible reaction especially at high temperature. For eg :- Mechanism for reverse Sulphonation Reaction :-

12. Halogenation Reaction In this reaction , H atom of the aromatic ring is substituted by halogen atom . Active electrophile is either a halogen L.A complex or a positive halogen . Aromatic rings can be chlorinated / brominated with Cl /Br in presence of lewis acid (L.A) such as FeCl 3 , FeBr 3 , AlCl 3 etc. Cl-Cl +FeCl 3 Cl-Cl + -Fe - Cl 3 Cl + +FeCl 4 - (L.A complex ) Reaction of Halogenation :-

13. Mechanism:- Step 1:- Lewis acid complex or the positive halogen group itself attacks the aromatic ring to form resonance stabilised arenium ion intermediate . Step 2:-Proton is removed from the arenium ion intermediate by a base to give the halogenated product.

14. Friedel Crafts Alkylation Reaction In this reaction alkyl group substitutes the H atom of aromatic ring . Reaction is brought about by R-X , alkenes , alcohols in presence of lewis acid catalyst . Example :- AlCl3 , AlBr3 , HF , H2SO4 , ZnCl2 etc. Order of reactivity [R-X] :- R-F > R- Cl > R-Br > R-I Active electrophile :- alkyl halide – lewis acid complex (1:1) or alkyl carbocation . Reaction of F.C Alkylation

15. Mechanism of reaction Step 1 :- Formation of Carbocation :- a) From Alkyl halides :- R- Cl +AlCl 3 R + +AlCl 4 - b) From Alcohol and Lewis acid :- R-OH +AlCl 3 ROAlCl 2 R + + OAl - Cl 2 c) From alcohols and proton acids :- R-OH + H + R + OH 2 R + + H 2 O d) from alkene :- In this ,proton is always required . C C + H + C + C H Step 2:- Attack of carbocation on aromatic ring :-

16. Step 3 :- Removal of the proton to form the. substituted product. Limitations of F.C Alkylation :- Polysubstitution :- Since in these reaction, the entering group is an activating group , di and polyalkylation occurs frequently. Aryl halides cannot be used instead of alkyl halides. Aromatic compound containing meta directing group do not undergoes this reaction. The activating groups such as OH ,NH2, OR etc do not facilitate this reaction. Rearrangement may occur in alkylating group , this is possible even with Lewis acid complex therefore usually its not possibe to add primary alkyl group (except Me &Et).

17. Friedel Crafts Acylation Reaction This reaction involves introduction of acyl group into aromatic ring This is most important method in preparation of aryl ketones . Reagent used :- Acyl halides , Carboxylic acids , anhydride , Ketenes in Lewis acid catalyst. More than 1 mol. Of catalyst is required since the first mole co. ordinates with the Oxygen of reagent . Order of reactivity : RCOI > RCOBr > RCOCl > RCOF Active electrophile :-Carbocation , acylium ion , acyl halide – lewis acid complex (1:1).

18. Mechanism of Reaction Step 1 :- Generation of an electrophile Step 2 :- Attack of electrophile to the aromatic ring , this is a slow step and hence R.D.S. Step 3 :- removal of proton and formation of substituted product .

19. Activating or o/p directing group These groups are more reactive then benzene towards E.S.R . They direct the incoming electrophile towards o & p position. Eg :- O- ,NR2 , NHR , NH2 , OH , OR ,NHCOR , OCOR , SR ,Alkyl & aryl groups. Explanation for orientation and reactivity :- If atom present on key atom has lp electrons . Electron density increase at o/p position due to +R /+M . Electrophile will bind at o / p position. Overall electron density of benzene increases. Therefore activates the aromatic ring.

20. If R(alkyl ) group is present , electron density increases by +I or hyperconjugation effect . If aryl group is present on the aromatic ring , then also it shows ortho para directing effect.

21. META DIRECTING OR DEACTIVATING GROUP These groups are less reactive then benzene . They direct the incoming electrophile towards meta position . Eg :- NO2 , CF3 ,CN ,SO3H , CHO , COR , COOH , COOR , CONH2 , CCl3 etc . Key atom attached to more E.N atom by = /triple bond. Attracts electron from o & p position due to –R/ -M effect . Meta position will have relatively higher electron density Attacking carbocation attaches at ,meta position Hence overall electron density of the ring decreases . Ring deactivates.

22. Meta directing groups with positively charged key atoms ( eg :- N + R 3 , N + H 3 etc) attracts electron from o/p position through strong (-I) effect.

23. Curious case of Halogens Halogens are ortho para directing yet deactivating . Do you know ??? Here two effects (+R) &(-I) both come into play and since these both groups are opposing each other . -I group withdraw electron from the ring more strongly then they donate electron by resonance hence deactivated the ring. Deactivation is partial at o/p position due to electron donation by resonance (+R) but meta position cannot be benefited therefore they are ortho para directing .

24. Introduction of third group in Benzene Case 2:- If both groups present are meta directing then third group will bind acc. to less powerful group. Order of directing power :- Me3N >NO2 >CN . SO3H >CHO > COMe >COOH Case 3 :-If one group is o/p and other is meta directing ,orientation reinforce each other , 3 rd group will bind at 1 st position .

25. Case 4 :- If both groups present are o/p and meta directing but o/p group have more directive influence then para orientation of o/p group would be preferred Case 5 :-If both groups occupy meta position with respect to each other , 3 rd group would least likely to enter between these both groups due to steric hinderance .

26. Orientation in other ring system In fused ring system ,the position are not equivalent and there are preferred orientation even in unsusbstituted hydrocarbons . Eg :- In Naphtalene , the alpha position is more preferred then beta , since at alpha position intermediate would be stabilised by 2 resonating structures whereas in beta attack stabilised by only 1 resonating structures .

27. Sulphonation of naphtalene At 80 degree Reaction doesn’t reach equilibrium product :- alpha naphtalene sulphonic acid , less stable because of steric hinderance by SO3H and H group. At 160 degree Reaction obtained equilibrium here. Product :-beta naphtalene sulphonic acid , thermodynamically more stable. Naphtalene , anthracene , phenanthrene etc are more reactive then benzene in E.S. R due to extensive delocalisation of positive charge . Orientation predicted on basis of stability of carbocation formed by attack of electrophile . E g :- Furan , Thiophene , Pyrrole substitutes at 3 rd position which are more reactive then benzene . Pyridine substitution occurs at 3 rd position .

28. References Advanced organic chemistry - Jagdamba Singh & L.D.S Yadav March’s Advanced Organic Chemistry Organic chemistry by G. Marc Loudon

29. Thankyou