Identify the stereochemically correct product from the following:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (CH3)3COH + HO–CHClBr with retention of configuration
An ester reacts with two equivalents of a Grignard reagent. The first \( \ce{CH3MgX} \) adds to the C=O group and the alkoxide group –O–CHClBr leaves, giving acetone; the second \( \ce{CH3MgX} \) adds to the acetone. After hydrolysis:
\( \ce{CH3COO-CHClBr} \) + \( \ce{CH3MgX} \) → \( \ce{(CH3)2C=O} \) + \( \ce{BrClCH-OMgX} \)
\( \ce{(CH3)2C=O} \) + \( \ce{CH3MgX} \) → \( \ce{(CH3)3C-OMgX} \)
On hydrolysis (\( \ce{H3O+} \)): \( \ce{(CH3)3C-OMgX} \) → \( \ce{(CH3)3COH} \) and \( \ce{BrClCH-OMgX} \) → HO–CHClBr
The acyl–oxygen bond (CO–O) breaks, but the bond between oxygen and the stereocentre (O–C) is not broken. So the stereocentre keeps its spatial arrangement: the product is formed with retention of configuration. In Option (1), Cl is up, H is on the wedge to the right and Br is down, just as in the ester, with OH in place of the ester oxygen.
Why the other options are incorrect
Option (2) — Incorrect. The positions of H and Br are exchanged, which is the other configuration; this would require a bond to the stereocentre to be broken.
Option (3) — Incorrect. 2-methylpropene would come from dehydration of tert-butyl alcohol, which does not occur under these conditions; the alcohol \( \ce{(CH3)3COH} \) is the product.
Option (4) — Incorrect. It shows the same wrong alkene, and its stereocentre is drawn with a different spatial arrangement from the starting ester.
Note: The question is from beyond the NCERT textbook.
NCERT (2026-27 edition):
Retention – “Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.”