Engineering Chemistry III Prof. K. M. Muraleedharan Aromatic electrophilic substitution (Ar-SE) Reactions The special reactivity of aromatic systems towards electrophiles arises mainly from two factors: the presence of ? electron density above and below the plane of the ring – making it nucleophilic, and the drive to regain the aromatic character by opting for substitution as opposed to a simple addition reaction. Preference towards addition reactions in the case of alkenes and substitution in the case of aromatic compounds becomes evident if we analyze the energy profiles of these reactions (Figures 1 and 2).
Figure 1. Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan Figure 2. Note: consider all the resonance structures of the wheland complex The mechanism of electrophilic aromatic substitution involves an initial rate determining interaction of the ? system with the electrophile to give a benzenonium ion intermediate (? complex or wheland complex), which undergoes a rapid de-protonation by the base in the second step to restore aromaticity (Figure 3). E H E H + E+ E H fast E + HB+ B Figure 3.
Some common electrophilic aromatic substitution reactions are: halogenation, nitration, sulfonation, Friedel-Crafts Acylation and Friedel-Crafts alkylation. These differ only in the Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan nature and mode of generation of electrophiles, but in general follow the same two-st...
ep mechanism described above. Reagent combinations that lead to the generation of electrophiles in these reactions are shown in Figure 4. Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan
Reaction Electrophile Generation of electrophiles Cl2 + FeCl3 Cl3Fe Cl Cl First step Chlorination Cl+ bromination Br+ Br2 + FeBr3 Br3Fe Br Br Iodination I+ I2 + 2Cu2+ 2 I+ + 2Cu+ Nitration O=N=O HO NO2 + H OSO3H H2O NO2 + HSO4 O=N=O + H2O Sulfonation O S O O O HO S OH + O O H O S OH O O HO S OH2 O + O O S OH O O O S O Cl Alkylation eg. 1 + H3O O HO S O + H2O + AlCl3 + AlCl4 eg. 2 H3C H C C H CH3+ HF H3C ? H C C H2 CH3 + F OH eg. 3 + H2SO4 O + O S OH + H2O O O Acylation H3C H3C O Cl + AlCl3 H3C AlCl3 O R O R O H3C acylium ion O R R O O + AlCl3 R O O + AlCl3(RCO2) O + AlCl4 Figure 4
Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan Note: since the product of acylation is a ketone which can complex with AlCl3, two equivalents of the Lewis acid is necessary to bring about the conversion efficiently. The complex can later be hydrolyzed using water Among the reactions mentioned above, Friedel crafts alkylation suffers from two main draw backs: a) the possibility of multiple substitutions due to ring activation on mono-alkylation and b) the formation of products arising from rearranged electrophiles (carbocations) to more stable ones (Figure 5).
Multiple substitutions can be avoided by using a large excess of the substrate (aromati
system) to ensure the collision of the electrophile with an un-substituted substrate and not a mono-substituted one. H3C AlCl3 + CH3CH2CH2CH2Cl 0oc 65% 35% H3C CH3 CH3 CH3 H3C CH3 100% 0% CH2 H3C H3C 1,2-methyl shift CH2 CH3 CH3 CH2 H2 C CH3 H2 C H CH2 H2 C CH3 CH H2C C H2 H3C H3C 1,2-hydride shift C H2 H2C CH3 H3C CH2Cl CH3 AlCl3 CH3 C H3C C CH3 H2C + Figure 5.
Since acylium ions do not undergo rearrangement, an acylation-reduction strategy can be conveniently used to introduce alkyl groups which are prone to rearrangement as demonstrated by the example below (Figure 6) Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan O + CH3CH2CH2-C Cl O 1. AlCl3 2. H2O C H2 C C H2 CH3 Zn(Hg), HCl ? Clemmensen reduction H2C H2 C C H2 CH3 Figure 6. Reversibility in Ar-SE Reactions Sulfonation of aromatic compounds is reversible and generally follows the following path (Figure 7). Similar equilibration is possible with electrophilic species such as HSO3+ as well.
HSO4 + SO3 SO3 H SO3 H3O SO3H consider all resonance forms H2SO4 H2O Figure 7. To drive the equilibrium forward, we can use large excess of the sulfonating agent with minimum amount of water at lower temperature. Likewise, the reverse reaction will be facilitated by using dilute acids at higher temperatures with excess of water or passing superheated steam through the reaction mixture to remove the volatile de-sulfonated product. Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan Problem: How would you bring about the following transformation CH3 ? CH3 D
As per the principle of microscopic reversibility, the desulfonation should follow the same path in the reverse direction and energies of transition states and intermediates should remain the same. That is, the rate determining step here would involve the loss of +SO3H from the protonated intermediate as shown below (Figures 8 and 9). SO3H H+ SO3 H RDS + + SO3H consider all resonance forms Figure 8. Indian Institute of Technology Madras Engineering Chemistry III Prof. K. M. Muraleedharan RDS for the forward and reverse reactions Free Energy If Deuterium labeled SO3 D ——-SO3 H consider all resonance ———————-+ + SO3H ——-SO H 3 + H+ Progress of the Figure 9. Note: The energy barrier in the second step (deprotonation) would be slightly higher for deuterated benzenonium ion intermediate compared to protonated one. This leads to a moderate isotope effect, where H+ is displaced almost twice as fast compared to D+ from the corresponding analogs. Forward reaction in the case of protonated species becomes more feasible due to relatively lower energy barrier, where as larger fraction of deuterated ones revert back to the starting material, thus decreasing the overall rate.
Reactivity of substituted aromatics Electron donating substituents usually activate the ring towards substitution where as electron withdrawing groups deactivate it. Compounds with activating groups react faster compared to benzene where as the ones with deactivating substituents react slower. This trend can be explained if we analyze the energy profile of the reaction and compare the stabilities of Indian Institute of Technology Madras
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