Write a note on nucleophiles.

(N/A) Nucleophiles are electron-rich species that possess at least one lone pair of electrons and are capable of donating them to an electron-deficient center.
Nucleophiles form a covalent bond with the electron-deficient center. In contrast,bases specifically donate an electron pair to $H^+$.
Example of base behavior: $HO^- + H^+ \rightleftharpoons H_2O$
Example of nucleophilic behavior: $HO^- + CH_3-Cl \longrightarrow HO-CH_3 + Cl^-$
Examples of nucleophiles:
$(i)$ Charged nucleophiles: $CN^-, I^-, Br^-, Cl^-, OH^-, -SH, -OR, -CH_3, -NH_2, -H, -C \equiv CH$ etc.
$(ii)$ Neutral nucleophiles: $H_2\ddot{O}:, H_2\ddot{S}:, CH_3\ddot{O}H, (CH_3)_2\ddot{O}:$ etc.
Nucleophilicity is the kinetic measure of a nucleophile's ability to react with an electron-deficient center,whereas basicity is a thermodynamic measure.
Ambident nucleophiles are species that possess two or more nucleophilic sites. For example,$CN^-$ can link through carbon to form alkyl cyanides or through nitrogen to form isocyanides. Similarly,the nitrite ion $[^-O-\ddot{N}=\ddot{O}:]$ can link through oxygen to form alkyl nitrites or through nitrogen to form nitroalkanes.
Factors affecting nucleophilicity include:
$(i)$ Size of the nucleophile
$(ii)$ Electronegativity of the donor atom
$(iii)$ Electron density on the donor atom
$(iv)$ Nature of the solvent