What is retention and inversion of configuration? Explain with suitable examples.

(N/A) Retention of configuration: If in a chemical reaction,no bonds to the stereocentre (chiral centre) are broken,the product will have the same general spatial arrangement of groups around the stereocentre as that of the reactant. This is called retention of configuration.
Example: The reaction of $(-)-2-$methylbutan$-1-$ol with concentrated $HCl$ proceeds with retention of configuration at the chiral centre:
$(-)-CH_3CH_2CH(CH_3)CH_2OH + HCl \xrightarrow{\Delta} (+)-CH_3CH_2CH(CH_3)CH_2Cl + H_2O$
Inversion of configuration: If the incoming nucleophile attacks the chiral centre from the side opposite to the leaving group,the spatial arrangement of the groups around the chiral centre in the product is opposite to that of the reactant. This is known as Walden inversion or inversion of configuration.
Example: The $S_N2$ reaction of $(R)-2-$bromobutane with $OH^-$ leads to the formation of $(S)-$butan$-2-$ol:
$R-C(C_2H_5)(CH_3)(C_6H_5)Br + OH^- \rightarrow HO-C(C_2H_5)(CH_3)(C_6H_5) + Br^-$