JEE Main & Advanced Chemistry GOC Attacking Reagents

The fission of the substrate molecule to create centres of high or low electron density is influenced by attacking reagents. Most of the attacking reagents can be classified into two main groups.             

Electrophiles or electrophilic reagents and Nucleophiles or nucleophilic reagents.            

(1) Electrophiles : Electron deficient species or electron acceptor is an electrophile.            

It can be classified into two categories :            

(i) Charged electrophiles : Positively charged species in which central atom has incomplete octet is called charged electrophile.

All cations are charged electrophiles except cations of IA, IIA group elements, \[A{{l}^{+++}}\] and \[\overset{\oplus \ \ \ \ }{\mathop{N{{H}_{4}}}}\,\]            

(ii) Neutral electrophiles : It can be classified into three categories,            

(a) Neutral covalent compound in which central atom has incomplete octet is neutral electrophile,

\[BeC{{l}_{2}},\,B{{H}_{3}},\,ZnC{{l}_{2}},\,Al{{X}_{3,\,}}Fe{{X}_{3}},\,\overset{.\ \ \ \ \ }{\mathop{C{{H}_{3}}}}\,,\,\overset{.\,.\ \ \ \ }{\mathop{C{{H}_{2}}}}\,,\,\overset{.\,.\ \ \ \ }{\mathop{C{{X}_{2}}}}\,\]            

(b) Neutral covalent compound in which central atom has complete or expended octet and central atom has unfilled -d-sub-shell is neutral electrophile,            

\[SnC{{l}_{4}},\,SiC{{l}_{4}},\,PC{{l}_{5}},\,S{{F}_{6}},\,I{{F}_{7}}\]            

(c) Neutral covalent compound in which central atom is bonded only with two or more than two electronegative atoms is called neutral electrophile.

\[BeC{{l}_{2}},\,B{{X}_{3}},\,Al{{X}_{3}},\,Fe{{X}_{3}},\,SnC{{l}_{4}},\,PC{{l}_{3}}\];             

\[PC{{l}_{5}},\,\,\,\,N{{F}_{3}},\,\,\,\,\overset{.\,.}{\mathop{C}}\,{{X}_{2}},\,\,\,C{{O}_{2}},\,S{{O}_{3}},\,\,\,C{{S}_{2}},\]         

\[C{{l}_{2}},B{{r}_{2}}\] and \[{{I}_{2}}\] also behave as neutral electrophiles.                

Electrophiles are Lewis acids.            

(2) Nucleophiles : Electron rich species or electron donors are called nucleophiles. Nucleophiles can be classified into three categories :            

(i) Charged nucleophiles : Negatively charged species are called charged nucleophiles.              

\[\overset{\Theta }{\mathop{H}}\,,O\overset{\Theta }{\mathop{H}}\,,R-\overset{\Theta }{\mathop{O}}\,,C{{\overset{\Theta }{\mathop{H}}\,}_{3}},\overset{\Theta }{\mathop{X}}\,,\overset{\Theta }{\mathop{S}}\,H,R-\overset{\Theta }{\mathop{S}}\,\]            

(ii) Neutral nucleophiles : It can be classified into two categories :            

(a) Neutral covalent compound, in which central atom has complete octet, has at least one lone pair of electrons and all atoms present on central atom should not be electronegative, is neutral nucleophile.

\[\overset{.\,.}{\mathop{N}}\,{{H}_{3,\,}}R-\overset{.\,.}{\mathop{N}}\,{{H}_{2}},\,{{R}_{2}}\overset{.\,.}{\mathop{N}}\,H,\,{{R}_{3}}\overset{.\,.}{\mathop{N}}\,,\,\overset{.\,.}{\mathop{N}}\,{{H}_{2}}-\overset{.\,.}{\mathop{N}}\,{{H}_{2}}\](Nitrogen nucleophile)              

\[H-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{O}}\,}}\,-H,\,R-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{O}}\,}}\,-H,\,R-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{O}}\,}}\,-R\]  (Oxygen nucleophiles)            

\[H-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{S}}\,}}\,-H,\,R-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{S}}\,}}\,-H,\,R-\overset{.\,.}{\mathop{\underset{.\,.}{\mathop{S}}\,}}\,-R\] (Sulphur nucleophiles)

\[\overset{.\,.}{\mathop{P}}\,{{H}_{3}},\,R\overset{.\,.}{\mathop{P}}\,{{H}_{2}},\,{{R}_{2}}\overset{.\,.}{\mathop{P}}\,H,\,{{R}_{3}}\overset{.\,.}{\mathop{P}}\,\]    (Phosphorus nucleophiles)            

(b) Organic compound containing carbon, carbon multiple bond/ bonds behaves as nucleophile.            

Alkenes, Alkynes, Benzene,            

\[C{{H}_{2}}=CH-CH=C{{H}_{2}},\,C{{H}_{2}}=CH-C\equiv CH\]            

(iii) Ambident nucleophiles : Species having two nucleophilic centres out of which, one is neutral (complete octet and has at least one lone pair of electrons) and the other is charged (negative charge) behaves as ambident nucleophile                                

\[\overset{\Theta }{\mathop{C}}\,\equiv \overset{.\,.}{\mathop{N}}\,,\,\overset{\Theta }{\mathop{O}}\,-\overset{.\,.}{\mathop{N}}\,=O,\,\overset{\Theta }{\mathop{O}}\,-\underset{\underset{O}{\mathop{\downarrow }}\,}{\overset{\overset{O}{\mathop{\uparrow }}\,}{\mathop{S}}}\,-OH\underset{x\to \infty }{\mathop{\lim }}\,\]         

• Organometallic compounds are nucleophiles.            
• Nucleophiles are Lewis bases.            

Organic compounds which behave as an electrophile as well as a nucleophile : Organic compound in which carbon is bonded with electronegative atom (O, N, S) by multiple bond/bonds behaves as electrophile as well as nucleophile :

\[R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-H,\,R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-R,\,R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-OH,\,R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-Cl\],

\[R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-OR,\,R-\overset{O}{\mathop{\overset{||}{\mathop{C}}\,}}\,-N{{H}_{2}},\,R-C\equiv N,\,R-\overset{\oplus }{\mathop{N}}\,\equiv \overset{\Theta }{\mathop{C}}\,\]          

• During the course of chemical reaction electrophile reacts with nucleophile.            
• Strong Lewis acid is stronger electrophile \[C{{O}_{2}}<\overset{\oplus }{\mathop{N}}\,{{O}_{2}}<\overset{\oplus }{\mathop{S}}\,{{O}_{3}}H\]. Stronger is an acid, weaker is its conjugated base or weaker is the nucleophile.            

Examples :

\[\frac{\begin{align}   & HF>{{H}_{2}}O>N{{H}_{3}}>C{{H}_{4}} \\  & {{F}^{-}}<O{{H}^{-}}<NH_{2}^{-}<CH_{3}^{-} \\ \end{align}}{Increasing\text{ }order\text{ }of\text{ }nucleophilicity.}\]