Question

The nucleophilicities of $R_3{C}^{-}$, $R_2{N}^{-}$, $R{O}^{-}$ and $F^{-}$ decrease in the order:

• A. $R_3{C}^{-}>R_2{N}^{-}>R{O}^{-}>F^{-}$
• B. $F^{-}>R{O}^{-}>R_2{N}^{-}>R_3{C}^{-}$
• C. $R{O}^{-}>F^{-}>R_2{N}^{-}>R_3{C}^{-}$
• D. $R_3{C}^{-}>R{O}^{-}>F^{-}>R_2{N}^{-}$

Answer 1

The correct option is A $R_3{C}^{-}>R_2{N}^{-}>R{O}^{-}>F^{-}$

Moving across a period in periodic table nucleophilicity decreases as the ability of a species to donate that pair should be inversely proportional to electronegativity. As electronegativity increases, nucleophilicity decreases $(C>N>O>F)$. The presence of alkyl groups increases the basicity due to $+I$ effect, therefore the nucleophilicity increases as $3^o>2^o>1^o$. The correct order is $R_3{C}^{-}>R_2{N}^{-}>R{O}^{-}>F^{-}$ (Other factor to be considered is steric hindrance in case of bulkier nucleophiles).