Question

Two isomers (A) and (B) with Molar mass 184 g/mol and elemental composition C, 52.2%; H, 4.9 % and Br 42.9% gave benzoic acid and p-bromobenzoic acid, respectively on oxidation with KMnO₄. Isomer ‘A’ is optically active and gives a pale yellow precipitate when warmed with alcoholic AgNO₃. Isomers ‘A’ and ‘B’ are, respectively.

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is C

The problem requires identifying two isomers (A) and (B) based on their chemical behavior and given conditions. Let's analyze the information given step by step:

1. The molar mass of the compounds is 184 g/mol, with the elemental composition of C = 52.2%, H = 4.9%, and Br = 42.9%.
2. When isomers were oxidized with KMnO₄, isomer 'A' gave benzoic acid while isomer 'B' gave p-bromobenzoic acid.
3. Isomer 'A' is optically active and forms a pale yellow precipitate when warmed with alcoholic AgNO₃, indicating the presence of an alkyl or aryl halide group susceptible to AgNO₃.

Let's determine the compounds:

• Given the conditions, isomer 'A' must contain an optical center (a chiral carbon), making it optically active.
• The formation of benzoic acid upon oxidation of isomer 'A' indicates that the rest of the molecule could be derivative of a substituted toluene, which upon oxidation results in benzoic acid.
• Isomer 'B' results in p-bromobenzoic acid, suggesting that it must have a para substitution of bromine on the benzene ring.
• Given the optical activity and the precipitation with AgNO₃, isomer 'A' likely has a chiral center next to the bromine group, and isomer 'B' will be the para-bromo derivative.
• Furthermore, the presence of an agostic interaction or easily dissociable bromine makes both susceptible to precipitation with AgNO₃.

Based on these characteristics, isomer 'A' is most likely to be a chiral brominated toluene, and isomer 'B' is para-bromotoluene. The correct pair that satisfies these conditions can be analyzed from the provided images.

The compounds in the image above represent the correct structures for isomers 'A' and 'B'. Isomer 'A' is optically active methyl 2-bromobutane, and isomer 'B' is para-bromotoluene, consistent with the oxidation and precipitation reactions mentioned.