Directing Effects on Electrophilic Nitration

Electrophilic nitration of a substituted benzene may lead to ortho, meta or para products, depending on the substituent. According to the Hammond Postulate, the kinetic product will be that which follows from the most stable intermediate benzenium ion, i.e. 

Draw a Lewis structure (or a series of Lewis structures) for nitrobenzenium ion. Where is the positive charge Examine the electrostatic potential map for nitrobenzenium ion. Where would you expect electron-donor substituents to have the greatest stabilizing effect (consider meta and para positions only) Which is the more stable, meta or para-nitrotoluenium ion (intermediates in nitration of toluene) Compare electrostatic potential maps to that for nitrobenzenium ion. Does your result suggest that methyl acts as an electron donor  

Compare energies fox meta andpara-nitroanilinium ions (intermediates in nitration of aniline). Are these differentiated to a lesser or greater extent than the intermediates in toluene nitration Examine electrostatic potential maps. What do these suggest about the relative electron-donor strengths of methyl and amino groups  

Compare energies for meta and para-dinitrobenzenium ions (intermediates in nitration of nitrobenzene). Is the ordering the same as those observed for intermediates in toluene and aniline nitration Examine electrostatic potential maps. What does your result suggest about the electron donor/aceptor properties of the nitro substituent  

Predict the products of electrophilic nitration of toluene, aniline and nitrobenzene. 

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Partial rate factors (PRFs) are a quantitative way of describing directing effects in electrophilic aromatic substitution. If we take the data from Table 12.3, together with the fact that nitration of toluene proceeds 24 times faster than nitration of benzene, we can obtain an estimate of the relative reactivity of a specific site on toluene relative to a single site on benzene. Benzene has six identical protons that can be replaced, whereas toluene has two ortho-, two meta-, and one para-position. 

Comparison of the partial rate factors for nitration of toluene with those for chlorination and bromination (above) show that these differ, both absolutely and relatively, with the attacking electrophile in other words relative directive effects in C6H5Y do depend on E as well as on Y. We notice above that the absolute values of the partial rate factors, i.e. fcY//cH, increase in the order,... 

This activating influence by the azido group was predicted on electronic grounds by Miller and Parker in 1958. Both the susceptibility of aromatic azides to electrophilic substitution and the orthopara directing effect could perhaps be inferred from earlier qualitative investigations of the nitration of aromatic azides which are summarized in Table 5. These conclusions have since been placed on a... 

The observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

As the TP nucleus is a polycyclic hydrocarbon, electrophilic aromatic substitutions can be easily carried out on it. Electrophilic aromatic substitution in parent hydrocarbon 1 is directed by steric and electronic effects. Because of steric effect, substitution at -position or 2-position is favored compared to a- or 1-position [46]. Bromination of TP is reported to yield mainly 2,3,5,6,10,11-hexabromo-TP [58], however, the electronic effect plays a major role in the nitration of TP. 

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