Example 6.8

Example . Identify chiral and achiral molecules in each of the following pair of compounds. (Wedge and Dash representations according to Class XI. Fig . : E is mirror image of D; E is rotated by o to get F and F is non superimposable on its mirror image D. However, the sign of optical rotation is not necessarily related to the absolute (actual) configuration of the molecule. (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 (–)- -methylbutan- -ol is heated with concentrated hydrochloric acid. It is important to note that configuration at a symmetric centre in the reactant and product is same but the sign of optical rotation has changed in the product. This is so because two different compounds with same configuration at asymmetric centre may have different optical rotation. One may be dextrorotatory (plus sign of optical rotation) while other may be laevorotatory (negative sign of optical rotation). (iv) Inversion, retention and racemisation: There are three outcomes for a reaction at an asymmetric carbon atom, when a bond directly linked to an asymmetric carbon atom is broken. Consider the replacement of a group X by Y in the following reaction; If (A) is the only compound obtained, the process is called retention of configuration. Note that configuration has been rotated in A. If (B) is the only compound obtained, the process is called inversion of configuration. Configuration has been inverted in B. Haloalkanes and Haloarenes If a mixture of A and B is obtained then the process is called racemisation and the product is optically inactive, as one isomer will rotate the plane polarised light in the direction opposite to another. Now let us have a fresh look