When (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid, (+)-1-chloro-2-methylbutane is formed. Which of the following correctly describes what happens to the configuration at the asymmetric carbon in this reaction?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It is retained, because no bond to the asymmetric carbon is broken.
Retention of configuration means the spatial arrangement of bonds around an asymmetric carbon is kept the same during a reaction. This happens when no bond to the stereocentre is broken.
\( \displaystyle \ce{CH3CH2CH(CH3)CH2OH + HCl} \xrightarrow{\text{heat}} \ce{CH3CH2CH(CH3)CH2Cl + H2O} \)
The asymmetric carbon is C-2, which carries H, CH3, C2H5 and CH2OH. The bond that breaks is the C–O bond of C-1, so the four bonds of C-2 are untouched and its configuration is retained.
Option (1) It is racemised, because a planar carbocation is formed at the asymmetric carbon. — Incorrect. Racemisation needs the bond to the asymmetric carbon to break, giving a planar carbocation there. Here the reaction takes place at C-1, so no carbocation forms at C-2.
Option (2) It is inverted, because the sign of optical rotation changes from (–) to (+). — Incorrect. The sign of rotation is not related to the configuration. Two different compounds with the same configuration may rotate light in opposite directions, as the alcohol and the chloride do here.
Option (4) It is inverted, because the chloride ion attacks from the side opposite to the –OH group. — Incorrect. Back-side attack inverts only the carbon that is attacked, here C-1, which is not asymmetric. The stereocentre C-2 is not attacked.
NCERT (2026-27 edition):
6.7.1 Reactions of Haloalkanes – “(iii) Retention: Retention of configuration is the preservation of the spatial arrangement of bonds to an asymmetric centre during a chemical reaction or transformation.”
“In general, if during a reaction, no bond to the stereocentre is broken, the product will have the same general configuration of groups around the stereocentre as that of reactant. Such a reaction is said to proceed with retention of the configuration. Consider as an example, the reaction that takes place when (–)-2-methylbutan-1-ol is heated with concentrated hydrochloric acid.”