The false statement about SN1 (Substitution Nucleophilic Unimolecular) reaction mechanism is: (D) It is a single-step mechanism.
The SN1 reaction mechanism is a two-step process. In the first step, the leaving group departs, generating a carbocation intermediate. In the second step, the nucleophile attacks the carbocation, leading to the formation of the substitution product.
The correct statement regarding SN1 reaction mechanism is:
(A) Racemization takes place if the reaction is carried out at a chiral carbon in an optically active substance. This is true because the carbocation intermediate formed during the reaction is planar and can be attacked by nucleophiles from either side, resulting in the formation of both enantiomers, leading to racemization.
(B) Intermediate formed during the reaction is a carbocation. This is true as the leaving group departure creates a carbocation intermediate that is stabilized by the surrounding electrondonating groups or the solvent.
(C) Concentration of nucleophile does not affect the rate of the reaction. This is generally true for $\mathrm{SN} 1$ reactions because the rate-determining step is the formation of the carbocation intermediate, which does not involve the nucleophile. The rate is primarily dependent on the stability of the carbocation and the leaving group ability.