Arenium Ion Mechanism in Aromatic Electrophilic Substitution

Lokesh Jangid semester 1

1. ARENIUM ION MECHANISM IN AROMATIC ELECTROPHILIC SUBSTITUTION 2020-21 SUBMITTED BY : LOKESH JANGID M.Sc. CHEMISTRY(SEM 1) DEPARTMENT OF CHEMISTRY S.P.C. GOVERNMENT COLLEGE, AJMER

2. CONTENT :- Introduction Why Aromatic Compounds Undergoes Electrophilic Substitution Reactions Rather Than Addition Reactions? Examples Of Aromatic Electrophilic Substitution Reactions Arenium Ion Mechanism Steps Involved In Arenium Ion Mechanism Evidence Of Arenium Ion Mechanism Energy Profile Diagram Of The Arenium Ion Mechanism. Summary

3. Introduction In aromatic compounds substitution may take place by electrophilic , nucleophilic , or free radical mechanism. There is a pi-electron cloud above and below the aromatic ring . These π -electrons are loosely held as compared to σ -electrons and are available to electrophiles . Thus, electrophilic substitutions are characteristic reactions of aromatic compounds. On the other hand, the electron cloud above and below the plane of an aromatic ring shielded the ring carbon from the attack of a nucleophile .

4. Why Aromatic Compounds Undergoes Electrophilic Substitution Reactions Rather Than Addition Reactions ? One of the characteristics of benzene derivatives is that they tend to undergoed substitution at aromatic carbon rather than addition reaction (to the double bonds). This property aromatic compounds is mainly due to their ‘ aromaticity ’.

5. Examples Of Aromatic Electrophilic Substitution Reactions

6. Arenium Ion Mechanism The most widely accepted mechanism of aromatic electrophilic substitution reaction is biomolecular and involves arenium ion Intermediate. Some time this mechanism is called as SE2 Mechanism, as it is biomolecular .

7. Steps Involved In Arenium Ion Mechanism The mechanism aromatic electrophilic substitution is known as the arenium ion mechanism and has two main steps Step 1 : The initial step is the attack of an electrophile creating a resonance stabilized carbocation or intermediate called arenium ion, which is also known as the Wheland Intermediate.Although the Wheland Intermediate or sigma-complex or now popularly known as arenium ion is stabilised by resonance (with charge dispersal over the carbon ortho and para to the site of attachment of the electrophile ).

8. This step is accompanied by loss of aromaticity , so the energy of activation is high. This is also the rate-determining step of the reaction because of the disruption of aromaticity and reaction follow second order kinetics. The attacking electrophile may be a positive ion or a dipole. Resonance stabilised arenium Resonance stabilised arenium ion intermediate

9. Step 2: In the second step the leaving group departs . This leads to regeneration of aromatic stabilization. The second step is nearly always faster than the first.

10. Evidence For The Arenium Ion Mechanism Isolation of arenium ion intermediates : A very strong evidence for the arenium ion mechanism is the isolation of arenium ion intermediates in many cases. For example, the arenium ion 1 was heated in the following reaction and when it was heated, the normal substitution product 2 was obtained. Arenium ions have also been detected spectrally

11. 2. Isotope effects : A difference in the rate of a reaction due to a diffrence in the isotope present in the reaction system is called isotope effect. If the proton is lost before the arrival of the electrophile (SE1 mechanism) or if the loss of the proton and the arrival of the electrophile are simultaneous then there should be a substantial isotope effect(i.e. , deuterated substrates should undergo substitution more slowly than undeuterated substrates) because, in each case, the C-H bond is not broken in rate- determining step, hence no isotope effect should be found. As expected, isotope effect has not been observed in most aromatic electrophilic substitutions. For example, the rate of nitration of deuterio and tritiobenzene are same as the rate of benzene.

12. This clearly shows that aromatic electrophilic substitutions involve two-steps and that the loss of proton is not the rate-determining step. The above facts are incompatible with either SE1 or the following concerted(simultaneous) mechanism resembling SN2 mechanism.

13. Energy Profile Diagram Of The Arenium Ion Mechanism A more detailed picture of the arenium ion mechanism may be presented by its energy profile.

14. The energy diagrame of this reaction shows that step 1 is highly endothermic and has a large Δ G(1) The first step Requires the loss of Aromaticity of very stable benze ring, which is highly unfavourable. The first step is a slow step, is rate-determining. Step 2 is highly exothermic and has a small Δ G(2) The ring regains its aromatic stabilization, which is highly favourable process. The rate of reaction at ortho , para or meta positions depends on the height of the energy barrier ( Δ G ) b/w the reactants and the T.S .

15. Summary Arenium ion/ Wheland intermediate/ σ -complex is the name of the intermediate involved in the electrophilic substitution of aromatic compounds. Arenium ion mechanism has two main steps. The first step is rate determining step, involves the attack of electrophile and loss of aromaticity . The second step involves departure of the leaving group leading to regain of aromaticity . The isotope effect and the isolation of arenium ion using various techniques and their spectral studies form the main evidence for the arenium ion mechanism. The first step is highly endothermic and has a large Δ G while the later step is highly exothermic and has a small Δ G.

16. Refrences : Advanced Organic Chemistry-Reactions, Mechanism and Structure, Jerry March, John Wiley. epgp.inflibnet.ac.in (e-PG Pathshala - A Gateway to all Post Graduate Courses).

17. Thank You