See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Concept: Conformational isomers vs. configurational stereoisomers. Normally, gauche and anti butane are conformational isomers that interconvert freely by rotation around the C2-C3 single bond. However, the problem states that on Pluto (extremely cold), rotations around single bonds are frozen — meaning these conformations cannot interconvert and must be treated as distinct, isolable stereoisomers. Step 1: Identify the relationship between gauche and anti butane when rotation is frozen. - Butane has the structure CH3-CH2-CH2-CH3. The key bond is C2-C3. - In the anti conformation, the two CH3 groups are 180° apart (dihedral angle = 180°). - In the gauche conformation, the two CH3 groups are ~60° apart. Step 2: Determine if these frozen conformers are stereoisomers. - When rotation is frozen, these two forms have the same molecular formula and connectivity (they are both butane), but differ in the spatial arrangement of atoms. They are therefore stereoisomers. Step 3: Classify the type of stereoisomers. - Enantiomers are non-superimposable mirror images of each other. The anti form has a plane of symmetry (it is achiral — the Newman projection shows a symmetric arrangement). The gauche form exists as two mirror-image gauche conformers (gauche+ and gauche-), but compared to anti, they are not mirror images of each other. - Since gauche and anti are stereoisomers that are not mirror images of each other, they are diastereoisomers. - More precisely: the anti form is achiral (has an internal plane of symmetry), while the gauche form is chiral. Stereoisomers that are not enantiomers of each other are by definition diastereoisomers. Step 4: Why other options fail. - (a) Enantiomers: Wrong — enantiomers must be non-superimposable mirror images of each other. Anti and gauche butane are not mirror images. - (c) Meso compounds: Wrong — meso applies to a single compound with stereocenters but an internal plane of symmetry, not to a pair of compounds. - (d) Wrong — in butane there are no traditional stereocenters (no carbon with four different substituents); the stereoisomerism here arises from restricted rotation, not from sp3 stereocenters. Therefore, the correct answer is B.