Category: Aromatic Substitution

Nucleophilic attack can also occur at a position already occupied by a substituent, the ipso position. Such ipso substitutions are not common, but they are industrially useful. An example is ipso nitration by displacement of a sulphonic acid group. A proton can also displace the sulphonic acid group, with benzenesulphonic acid being converted into benzene. Nucleophilic ipso substitution reactions also occur. The more deactivating…

Nucleophilic substitution is not a typical reaction of aromatic compounds. However, appropriately substituted aromatic compounds do undergo nucleophilic substitution. There are two distinct and major mechanisms by which a nucleophile can be introduced into the aromatic ring. In one, the nucleophile attacks at a ring carbon atom and this type is covered in detail below.…

The relative ability of substituents in an aromatic ring to donate or withdraw electrons is indicated qualitatively by Hammett substituent constants. It was observed that a plot of the logarithms of the rate constants for the alkaline hydrolysis of esters of benzoic acid against the pKa values of the corresponding acids, XC6H4CO2H, was linear; that is,…

In general, the effects of two substituents on the orientation and rate of electrophilic substitution are additive. The best product selectivity occurs when the two substituents are working together, but, unfortunately, this is not always the case. In general, resonance effects take predominance over inductive effects. There are several guiding principles that help to decide…

It is expected that in monosubstituted benzene if the substituent is ortho, para–directing, the ortho to pora ratio should be 2:1. But usually in ortho, para-directing systems, the ortho substitution occurs to lesser extent than expected. This difference may be due to many factors, that is, polar effects of the substituent, solvent, temperature, size of the substituent, and the entering group (steric factors).…

It is well known that, in comparison to hydrogen, alkyl groups donate electrons. It is therefore to be expected that toluene and other alkyl benzenes will react with electrophiles rather more easily than benzene. This is certainly the case, with toluene reacting with mixed acid at room temperature. The canonical forms that contribute to the…

Withdrawal of electrons through a σ bond is called inductive electron withdrawal. Groups such as trifluoromethyl (CF3), trialkylammonium (R3N+), PMe3+ and AsMe3+ are unable to interact with the π-system, but withdraw electrons as a result of the electronegativity of the fluorine atoms and the positively charged nitrogen, phosphorus and arsenic, respectively. A study of the canonical forms for…

Substituents that fall into this category include NO2, CO2R, COR, CN and SO3R. All are characterized by the atom attached to the ring being linked to a more electronegative atom by a multiple bond, and may be represented by X=Y, where Y is more electronegative than X. Electrons are therefore attracted towards Y, making X…

Groups in which the atom attached to the benzene ring possesses a lone pair of electrons can interact with the aromatic ring, promoting ortho and para attack. The ring becomes more electron rich and, so, the reaction with electrophiles is facilitated. In order to assess the influence that substituents have on the reactivity of aromatic molecules, it is important…

In benzene, the six hydrogen atoms are chemically equivalent and an electrophilic attack would give rise to a single product only. But if a group is already present on the ring, the hydrogens become non-equivalent and the incoming electrophile can attack either the ortho, meta or para positions to yield a mixture of products. It is believed that electrophiles…