Question

Statement-I: Basic strength of $R{S^ - } > R{O^ - }$
Statement-II: Nucleophilicity of $R{S^ - } > R{O^ - }$
A. Both I and II are correct
B. Both I and II are Incorrect
C. I is correct and II is incorrect
D. I is incorrect II is correct

Answer 1

Hint: RSH is the stronger acid than ROH. Since conjugate state is more stable, stronger the acid, the weaker is the conjugate base. Down a group, basicity and nucleophilicity are inversely proportional to each other.

Complete answer:
A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction. Nucleophilicity measures the ability of a nucleophile to react at an electron-deficient center. The basic strength is determined by the ability of an ion or molecule to accept a proton. Nucleophilicity and basicity reflect the same property, namely the ability to share the lone electron pair (or pairs) with an electron acceptor. Since, electronegativity decreases down the group, thus nucleophilic strength increases down the group, Hence, RS- is more nucleophilic than RO- due to less electronegativity of S than O.
$R{O^ - }$ is a stronger base than $R{S^ - }$. Thus, the basic strength of the central atom decreases down the group. Hence, $R{S^ - }$ is more nucleophilic but less basic than $R{O^ - }$
Basicity $R{S^ - }$< $R{O^ - }$
Nucleophilicity $R{S^ - }$ >$R{O^ - }$
In the above statements the statement I is incorrect and statement II is correct.

Therefore the correct option is option (D).

Note:
Within a group, the order of nucleophilicity is opposite to the order of basicity. The order of nucleophilicity is related to the polarizability of the nucleophile. Nucleophilicity is determined at kinetically controlled conditions whereas basicity is determined at conditions controlled thermodynamically.