Chapter 18.13, Problem 32CC

Interpretation Introduction

Interpretation:

Considering the following reaction as example –

                    

• The lowering of temperature in aromatic nucleophilic substitution reaction when there is increase in number of nitro groups has to be justified.
• The impact caused by the placement of the fourth nitro group in the above compound in lowering the reaction temperature of aromatic nucleophilic substitution reactions has to be analyzed.

Concept Introduction:

• Benzene and its compounds undergo a unique type of reactions namely Aromatic nucleophilic substitution reactions designated as ${\text{S}}_{\text{N}}\text{Ar}$.
• A nucleophile, unlike an electrophile is a species that is electron rich and always attracted towards electron deficient site. A nucleophile is mobile unlike electrophile. Thus in aromatic nucleophilic substitution reactions the aromatic compound behaves as electron deficient species. The aromatic compound must be deficient of electrons so that it acquires positive charge and attacked by nucleophile.
• In the mechanism of ${\text{S}}_{\text{N}}\text{Ar}$ reaction the nucleophile attacks the position that is bonded to leaving group and forms a resonance stabilized intermediate called Meisenheimer complex.
• The criteria for aromatic nucleophilic substitution reaction to occur are –
• The aromatic ring must contain a strong electron withdrawing group like nitro group.
• The aromatic ring must contain a leaving group.
• The leaving group must be posited either ortho or para position to the strong electron withdrawing group.
• In aromatic compounds the site in the ring where there is deficient of electrons is attacked by the nucleophile. A new $\text{C-Nu}$ bond is formed where ‘Nu’ is the nucleophile and ‘C’ is aromatic ring Carbon. In this process the bond between aromatic Carbon and the substituent bonded with it is broken.
• Let the substituent be ‘X’ that then $\text{C-X}$ bond gets broken and  $\text{C-Nu}$ bond is formed. The substituent ‘X’ is termed as ‘Leaving group’. A good leaving is large in size with high electron affinity that they are capable to grabbing the bond pair electron and leaving form the aromatic C atom. Halogens are known to be excellent leaving group.
• The overall reaction can be represented as follows –

                    

• Activation energy is the minimum amount of energy that the reactant must possess to undergo chemical reaction. If the reactant molecules do not possess the minimum energy then the required energy can be supplied externally in the form of heat or some other means.