to the tetrahedral carbon are not different. We rotate the mirror image (B) of the molecule by ° (structure C) and try to overlap the structure (C) with the structure (A), these structures completely overlap. Thus propan- -ol is an achiral molecule. Jacobus Hendricus Van’t Hoff ( - ) received the first Nobel Prize in Chemistry in for his work on solutions.
Fig . : Some common examples of chiral and achiral objects a single asymmetric carbon atom. Let us consider two simple molecules propan- -ol (Fig. .
) and butan- -ol (Fig. . ) and their mirror images. Fig .
: B is mirror image of A; B is rotated by o and C is obtained; C is superimposable on A. Haloalkanes and Haloarenes Butan- -ol has four different groups attached to the tetrahedral carbon and as expected is chiral . Some common examples of chiral molecules such as -chlorobutane, , -dihyroxypropanal, (OHC–CHOH–CH OH), bromochloro-iodomethane (BrClCHI), -bromopropanoic acid (H C–CHBr–COOH), etc. Fig.
. : A chiral molecule and its mirror image The stereoisomers related to each other as non- superimposable mirror images are called enantiomers (Fig. . ).
A and B in Fig. . and D and E in Fig. .
are 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- -ol. The process of conversion of enantiomer into a racemic mixture is known as racemisation.