Write a note on nucleophiles.

Explanation

Nucleophiles are negatively charged or neutral species that is capable of donating electrons. These species are electron deficient centres attracting molecules.
Nucleophiles not only donate an electron pair to an electron deficient centre but also forms bond with that atom. Whereas bases donate an electron pair to `H^(+)`.

Examples of Nucleophile are :
(i) Charged nucleophiles : `""^(-)CN, I^(-), Br^(-), Cl^(-), OH^(-), ""^(-)SH, ""^(-)OR, ""^(-)CH_(3),""^(-)NH_(2), ""^(-)H, ""^(-)C-=CH` etc.
(ii) Neutral nucleophiles : `H_(2)overset(..)(O):,H_(2)overset(..)(S):, CH_(3)overset(..)underset(..)(O)H, (CH_(3))_(2)overset(..)(O):` etc.
Nucleophilicity is defined as the ability of a nucleophile to react with an electron deficient centre. Nucleophilicity is kinetically controlled whereas basicity is thermodynamically controlled.
Certain anionic nucleophiles have negative charge delocalised by resonance over two different atoms or to non - equivalent atoms. These nucleophiles are known as ambident nucleophiles. Ex : `""^(-)CN, NO_(2)^(-)`, etc. The cyanide group is a hybrid of two contributing structures and therefore can act as a nucleophile in two different ways :
`""^(ө)C-=N harr :C=N^(ө)`
The linking of cynide group through carbon atom results in alkyl cyanides and through nitrogen atom results in isocyanides. Similarly nitrite ion also represents an ambident nucleophilicity with two different atoms of linkage `[""^(ө)O-overset(..)(N)=overset(..)(O):]`
The linkage through oxygen results in alkyl nitrites while through nitrogen atom, it leads to nitroalkanes.
The nucleophilicity of a nucleophile is affected by :
(i) Size of a nucleophile
(ii) Electronegativity of electron donor atom
(iii) Electron density on electron donor atrom
(iv) Presence of solvents.