In the classical internal mechanism characterizing the uncatalyzed conversion of an aliphatic alcoho — Haloalkanes and Haloarenes Chemistry Question
Question
In the classical internal $S_Ni$ mechanism characterizing the uncatalyzed conversion of an aliphatic alcohol to an alkyl chloride using $SOCl_2$, exactly how many distinct atoms are formally and explicitly incorporated into the structural ring of the cyclic transition state during the ultimate collapse of the alkyl chlorosulphite intermediate?
💡 Solution & Explanation
The $S_Ni$ mechanism proceeds via the collapse of the alkyl chlorosulphite ester intermediate: $R-O-S(=O)-Cl$. During the rate-determining decomposition step, the bond between the alkyl carbon and oxygen begins to break, while simultaneously, the bond between sulfur and chlorine begins to break. The departing chlorine atom swings around to form a new bond intimately with the front face of the alkyl carbon, and the oxygen forms a double bond with sulfur to expel $SO_2$. This concerted rearrangement creates a square-like cyclic transition state precisely containing the Carbon atom, the Oxygen atom, the Sulfur atom, and the Chlorine atom. Total atoms = 4.