What are nucleophiles and nucleophilic substitution reactions? Provide a basic overview of the nucleophilic substitution reaction of haloalkanes.

(N/A) Nucleophiles: Species that are electron-rich and donate an electron pair to an electron-deficient center are called nucleophiles. Examples include $:CN^-$,$OH^-$,$OR^-$,$NH_3$,$RCOO^-$,$H^-$,etc.
$(b)$ Nucleophilic Substitution Reaction:
$(i)$ $A$ reaction in which a nucleophile replaces an existing nucleophile (leaving group) in a substrate molecule is called a nucleophilic substitution reaction.
$(ii)$ In haloalkanes,the carbon atom bonded to the halogen is partially positively charged due to the electronegativity difference,making it susceptible to nucleophilic attack.
$(iii)$ During the reaction,the halogen atom departs as a halide ion,which is known as the 'leaving group'.
$(iv)$ The general reaction is: $R-X + Nu^- \rightarrow R-Nu + X^-$,where $R-X$ is the haloalkane,$Nu^-$ is the nucleophile,and $X^-$ is the leaving halide ion.