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Q.
Among the following the strongest nucleophile is
AIIMSAIIMS 2005Organic Chemistry – Some Basic Principles and Techniques
Solution:
Nucleophile is a species which has high electron density on it and it attack on the site of less electron density. More the electron density on a species better nucleophile it is. Nucleophiles may be neutral (but with an atom with lone pairs of electron) or it can be negatively charged. $C _{2} H _{5} SH$ is the weakest acid so it readily lose a proton to form strong conjugate base $C _{2} H _{5} S ^{-}$which is a strong nucleophile ( $+I$ effect of $C _{2} H _{5}$ group).