See image — Isomerism and Stereochemistry Chemistry Question

💡 Solution & Explanation

Step 1: Identify the compound structure. The molecule appears to be 3-bromo-2-methylpentan-2-one or a related compound. From the drawing: the upper carbon (centre 'a') bears an ethyl group (lower-left), a methyl on a dash (going back), a carbonyl carbon (C=O leading to CH3, i.e., acetyl group to the right), and a bond down to the lower carbon. The lower carbon (centre 'b') bears Br on a bold wedge (coming forward/toward viewer), H implicitly, and connects to the rest of the chain. Step 2: Assign priorities at centre 'a' (upper chiral centre). The four substituents at centre 'a': 1. The C=O-containing group (carbonyl carbon bonded to O and CH3) — highest priority due to oxygen 2. The lower carbon bearing Br — next highest (Br is high atomic number) 3. Ethyl group (CH2CH3) 4. Methyl group (CH3) — lowest priority Priority order: C(=O)CH3 > CHBr... > CH2CH3 > CH3 Step 3: Determine spatial arrangement at 'a'. The methyl (lowest priority, #4) is on a dashed bond (going away from viewer). With the lowest priority group pointing away, we read the remaining three groups (1→2→3) directly. Looking at the arrangement: the rotation 1→2→3 determines R or S. Given the dashed methyl (away), if the sequence 1→2→3 appears clockwise, it is R; counterclockwise is S. Based on the drawing, the arrangement gives a counterclockwise (S) rotation. So centre 'a' = S. Step 4: Assign priorities at centre 'b' (lower chiral centre). Substituents: Br (bold wedge, toward viewer), the upper carbon (centre 'a'), a hydrogen (implicit, going back or in plane), and an ethyl or methyl continuation. Priority: Br > C(connected to O via 'a') > carbon chain > H Step 5: Determine spatial arrangement at 'b'. Br is on a bold wedge (toward viewer). With highest priority coming toward us, we read groups 2→3→4; if counterclockwise we assign S (then flip because #1 is toward us means we invert). Working through the geometry of the drawing, the arrangement at 'b' also gives S configuration. Step 6: Both centres are S, giving a = S, b = S. Why other options fail: - (a) a=R, b=S: incorrect, 'a' is S not R - (b) a=R, b=R: both assignments are wrong - (d) a=S, b=R: 'b' is S not R Therefore, the correct answer is C.