Chemistry AROMATIC HYDROCARBONS-NOMENCLATURE , BENZENE STRUCTURE AND AROMATICITY

Electrophilic Aromatic Substitution Reactions :

Aromatic hydrocarbons are known generally as arenes. An aryl group is one derived from an arene by removal of a hydrogen atom and its symbol is Ar -. Thus, arenes are designated `ArH` just as alkanes are designated `RH`. The most characteristic reactions of benzenoid arenes are the substitution reactions that occur when they react with electrophilic reagents. These reactions are of the general type shown below.

`ArH+E^(+)-> Ar-E+H^(+)`

The electrophiles are ei ther a positive ion (`E^(+)`) or some other electron -deficient species with a large partial positive charge. For xample, benzene can be brominated when it reacts with bromine in the presence of `FeBr_3`. Bromine and `FeBr_3` reacts to produce positive bromonium ions, `Br^(+)`. These positive bromonium ions act as electrophiles and attack the benzene ring replacing one of the hydrogen atoms in a reaction that is called an electrophilic aromatic substitution (`EAS`). Electrophilic aromatic substitutions allow the direct introduction of a wide variety of groups into an aromatic ring and because of this they provide synthetic routes to many important compounds. The five electrophilic aromatic substitutions that we shall study in this package are outlined in fig. All of these reactions involve the attack on the benzene ring by an electron-deficient species - (by an electrophile). Later we shall learn what the electrophile is in each instance

Mechanism for Electrophilic Aromatic Substitution

`p` and `s` complexes: It might be expected that the first phase of reaction would be interaction between the approaching electrophile and delocalised `p` orbitals leading to p complexes (a weakly bonded charge transfer complex which exists in solution only and is formed by the association of an electrophilic species (`E^(+)`) and one electron-donating species) e.g. toluene form a `1:1` complex with `HCl` at `-78^(o)C`, the reaction being readily reversible. DCI also forms p complex with toluene. This complex on decomposition does not form deuterium substituted toluene. Formation of complex leads to a solution that is a non-conductor of electricity. When an electrophile reacts with an arene usually in presence of a catalyst. a salt is formed. This salt is composed of an anioin and a complex, resonance stabilized carbenium ion (arenium ion) in which only two of the total p electrons of arene are utilized to form a s bond between a particular C atom of the ring and the electrophile, known as s complex (also called Wheland intermediate).

The evidence for the two step mechanism are :
i) Detection and isolation of s complex: A large number of s complexes as intermediates have been detected, some of them have also been isolated.
ii) Displacement of an `H` (or `D`) atom of `C_6H_6` (or `C_6D_6`) by a nitronium ion `NO_2^ +` (formed by the action of concentrated `H_2SO_4` on concentrated `HNO_3`) is an aromatic electrophilic substitution reaction. If the `C - H` bond breaking is the rate determining step, then the reaction will exhibit a primary kinetic isotope effect. By contrast `C_6H_6` and `C_6O_6` are found to undergo nitration at essentially the same rate, thus `C - H` bond breaking cannot be involved in the rate determining step, that means s complex formation is the rate determining step .

Effect of Substituents on Reactivity & Orientation :

When substituted benzenes undergo electrophilic attack, groups already on the ring affect both, the rate of the reaction and the site of attack. We say, therefore, that substituent groups affect both reactivity and orientation in electrophilic aromatic substitutions. We can divide substituent groups into two classes according to their influence on the reactivity of the ring. Those that cause the ring to be more reactive than benzene itself are called activating groups. Those that cause the ring to be less reactive than benzene are called deactivating groups.
We also find that we can divide substituent groups into two classes according to the way they influence the orientation of attack by the incoming electrophile. Substituents in one class tend to bring about electrophilic substitution primarily at the positions ortho and para to themselves. We call these groups ortho - para directors because they tend to direct the incoming group into the ortho and para positions.
Subst ituents in the second category tend to direct the incoming electrophile to the meta position. We call these groups meta directors. Several examples will illustrate more clearly what we mean by these terms .