See image — Hydrocarbons Chemistry Question

💡 Solution & Explanation

Concept: Diastereomers are stereoisomers that are not mirror images of each other. For a reaction to produce a pair of diastereomers, the product must have two stereocenters (or another source of defined stereochemistry) such that the reaction creates a new stereocenter while an existing one is retained, yielding two products that differ in relative configuration at one center but not the other. Step 1 - Analyze option (a): The starting material has a carbon with two identical methyl groups (geminal dimethyl) and a terminal alkene. Addition of HBr would create a new stereocenter, but because the two methyl groups are identical, the carbon bearing them is not a stereocenter. The product would have only one new stereocenter, giving enantiomers (a racemic mixture), not diastereomers. Step 2 - Analyze option (b): The starting material has an existing stereocenter (carbon bearing H3C on wedge, H on dash, ethyl, and a chain -CH2CH2CH=CH2). The terminal alkene is remote (homoallylic or further). Addition of HBr to the terminal double bond (Markovnikov) creates a NEW stereocenter at the internal carbon of the former alkene. Now the molecule has TWO stereocenters: the original pre-existing one (configuration fixed/retained) and the newly formed one. Because the original stereocenter is fixed (the starting material is a single enantiomer), addition of HBr can give (R,R)/(R,S) at the two centers — these are diastereomers. The two possible configurations at the new stereocenter, combined with the fixed original stereocenter, produce a pair of diastereomers. Step 3 - Analyze option (c): The starting material has an existing stereocenter and a vinyl group directly attached. Under radical conditions (ROOR, UV), HBr adds anti-Markovnikov. The new stereocenter formed is at the terminal carbon of the vinyl group (now a CH2Br). However, terminal -CH2Br carbon has two H's; it is not a stereocenter (two identical substituents). So only one stereocenter exists in the product — the original one — giving only one product (no new stereocenter created), not a pair of diastereomers. Step 4 - Analyze option (d): Hydroboration-oxidation of the vinyl group gives an alcohol at the terminal carbon (anti-Markovnikov, syn addition). The terminal carbon after hydration becomes -CH2OH, which again bears two hydrogens and is not a stereocenter. Product has only the original stereocenter; no new stereocenter is generated, so no diastereomers are formed. Conclusion: Only option (b) generates a new stereocenter while retaining a pre-existing fixed stereocenter, resulting in the formation of a pair of diastereomers. Therefore, the correct answer is B.