Which of the following statement is not correct for Butan-2-ol?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It is optically inactive
A carbon atom bonded to four different groups is an asymmetric carbon (stereocentre). A molecule with one such carbon is chiral: it is not superimposable on its mirror image, and the two mirror-image forms (enantiomers) rotate plane polarised light equally in opposite directions.
Butan-2-ol is \( \ce{CH3-CH(OH)-CH2-CH3} \). Its C2 carries four different groups: H, OH, \( \ce{CH3} \) and \( \ce{C2H5} \). So butan-2-ol has one stereocentre, it is chiral, and each enantiomer is optically active. Only an equimolar (racemic) mixture of the two shows zero rotation, because the rotations cancel; that does not make the compound itself optically inactive.
Option (1) It exists as a pair of non-superimposable mirror images — True for butan-2-ol. Its two forms, (+)- and (−)-butan-2-ol, are non-superimposable mirror images, that is, enantiomers.
Option (2) It is optically inactive — Not true for butan-2-ol (it is optically active). Each enantiomer of butan-2-ol rotates plane polarised light; only the racemic mixture shows zero rotation.
Option (3) It is chiral — True for butan-2-ol. It has an asymmetric carbon and no plane of symmetry, so it is not superimposable on its mirror image.
Option (4) It has one stereocentre — True for butan-2-ol. Only C2 has four different groups; C1 and C4 are \( \ce{CH3} \) groups and C3 carries two H atoms.
NCERT (2026-27 edition):
Molecular Asymmetry, Chirality and Enantiomers – …
Molecular Asymmetry, Chirality and Enantiomers – “… Enantiomers possess identical physical properties namely, melting point, boiling point, refractive index, etc. They only differ with respect to the rotation of plane polarised light. If one of the enantiomer is dextro rotatory, the other will be laevo rotatory. … A mixture containing two enantiomers in equal proportions will have zero optical rotation, as the rotation due to one isomer will be cancelled by the rotation due to the other isomer. Such a mixture is known as racemic mixture or racemic modification. A racemic mixture is represented by prefixing dl or (±) before the name, for example (±) butan-2-ol.”