See image — Haloalkanes and Haloarenes Chemistry Question

💡 Solution & Explanation

Step 1 - Define the terms: - A stereospecific reaction is one in which a particular stereoisomer of the starting material gives exclusively (or predominantly) one particular stereoisomeric product. The mechanism dictates a fixed stereochemical outcome from a defined stereoisomeric substrate. - A stereoselective reaction is one in which one stereoisomeric product is formed preferentially over another from a substrate that may or may not be stereodefined, but the reaction could in principle give more than one stereoisomer. Step 2 - Analyze S_N1: S_N1 proceeds through a planar carbocation intermediate. Attack by the nucleophile can occur from either face of the planar carbocation, leading predominantly to a racemic mixture (or mixture of stereoisomers). Because the reaction does not give a single, mechanistically mandated stereochemical outcome from a chiral substrate (it gives a mixture), it is stereoselective only in the sense that racemization is the predominant outcome, but more precisely it is described as stereoselective (it preferentially forms the racemic mixture rather than being completely random, and different substrates can give different ratios). In the context of this classification, S_N1 is considered stereoselective because it can give a mixture of stereoisomers (no single stereochemical course is mechanistically enforced). Step 3 - Analyze S_N2: S_N2 proceeds with complete inversion of configuration (Walden inversion) via a backside attack mechanism. Regardless of which enantiomer of the substrate is used, the mechanism enforces a specific stereochemical outcome (inversion). This means S_N2 is stereospecific: a specific stereoisomeric starting material gives a specific stereoisomeric product dictated by the mechanism. Step 4 - Match to options: - Option (b) says both are stereoselective - this is partially correct for S_N1 but not for S_N2. - Option (c) says S_N1 is stereoselective and S_N2 is stereospecific - this correctly describes S_N1 (gives mixture, stereoselective) and S_N2 (inversion, stereospecific). - The given answer is B, C, meaning both (b) and (c) are considered correct. Option (b) is defensible because S_N2 always selects for the inverted product (one stereoisomer is preferentially formed), making it also stereoselective in the broad sense, and S_N1 gives predominantly racemic product (also stereoselective in preferring racemization). Option (c) is the more precise/specific answer distinguishing the two. - Option (a) is incorrect because S_N1 is not stereospecific. - Option (d) is incorrect because it reverses the assignments. Step 5 - Conclusion: Both (b) and (c) are correct: (b) because both reactions can be viewed as stereoselective (each preferentially forms certain stereoisomeric products), and (c) because more precisely S_N1 is stereoselective and S_N2 is stereospecific. Therefore, the correct answer is B,C.