Q9 · CSIR-NET Chemistry, December 2013
Paper: CSIR-NET December 2013 · Subject: Organic Chemistry · Chapter: General Organic Chemistry · Topic: Stereochemistry R S E Z · Marks: 2 · Difficulty: Medium
The absolute configuration of the two stereogenic (chiral) centres in the following molecule is 
(a)5R, 6R
(b)5R, 6S
(c)5S 6R
(d)5S, 6S
Answer
Answer (as printed): C
Explanation
The ketone carbon is C-1, and C-2 carries the ring methyl with the C2=C3 double bond. The stereocentres are C-5 (isopropenyl) and C-6 (methyl + ethyl).
C-5: priorities are C-6 (C,C,C; it leads on to C-1 with O,O,C) > isopropenyl (C,C,C) > C-4 (C,H,H) > H. The isopropenyl is hashed, so H points toward the viewer. C-6 → isopropenyl → C-4 runs clockwise, which reverses to 5S.
C-6: priorities are C-1 (O,O,C) > C-5 (C,C,H) > CH₂CH₃ > CH₃. CH₃ is on the wedge (toward the viewer). C-1 → C-5 → ethyl runs counterclockwise, which reverses to 6R.
Answer: 5S, 6R (c).
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