Identify the reaction type

The question asks: The reaction is fastest when $X$ is ... with options $X = OCOR,\, OC_2H_5,\, NH_2,\, Cl$

This standard organic chemistry question refers to nucleophilic acyl substitution of an acid derivative, where the rate depends mainly on the leaving group ability of $X$.

General principle

For acyl derivatives of type $RCOX$ the reactivity order is governed by how easily $X^-$ leaves.

Better leaving group $\Rightarrow$ faster reaction.

The usual reactivity order is: $RCOCl > (RCO)_2O > RCOOR' > RCONH_2$

That corresponds to leaving groups: $Cl^- > RCOO^- > RO^- > NH_2^-$

Compare the given groups

• A: $OCOR$ corresponds to an acid anhydride derivative.
• B: $OC_2H_5$ corresponds to an ester.
• C: $NH_2$ corresponds to an amide.
• D: $Cl$ corresponds to an acid chloride.

Reason using leaving group basicity

The weaker the base, the better the leaving group.

• $Cl^-$ is a weak base $\Rightarrow$ very good leaving group
• $RCOO^-$ is less good than $Cl^-$
• $RO^-$ is a stronger base, poorer leaving group
• $NH_2^-$ is a very strong base, worst leaving group

Hence the rate is maximum for the acid chloride derivative.

Conclusion

Therefore, the reaction is fastest when $X = Cl$

So the correct option is D.