phenyl group is bulkier than a methyl group. Therefore, C H CH(C H ) Br is less reactive than C H CH(CH ) Br in S N reactions. William Nicol ( - ) developed the first prism that produced plane polarised light. two types of crystals.
Dutch scientist, J. Van’t Hoff and French scientist, C. Le Bel in the same year ( ), independently argued that the spatial arrangement of four groups (valencies) around a central carbon is tetrahedral and if all the substituents attached to that carbon are different, the mirror image of the molecule is not superimposed (overlapped) on the molecule; such a carbon is called asymmetric carbon or stereocentre . The resulting molecule would lack symmetry and is referred to as asymmetric molecule.
The asymmetry of the molecule along with non superimposability of mirror images is responsible for the optical activity in such organic compounds. The symmetry and asymmetry are also observed in many day to day objects: a sphere, a cube, a cone, are all identical to their mirror images and can be superimposed. However, many objects are non superimposable on their mirror images. For example, your left and right hand look similar but if you put your left hand on your right hand by moving them in the same plane, they do not coincide.
The objects which are non- superimposable on their mirror image (like a pair of hands) are said to be chiral and this property is known as chirality . Chiral molecules are optically active, while the objects, which are, superimposable on their mirror images are called achiral. These molecules are optically inactive. The above test of molecular chirality can be applied to organic molecules by constructing models and its mirror images or by drawing three dimensional structures and attempting to superimpose them in our minds.
There are other aids, however, that can assist us in recognising chiral molecules. One such aid is the presence of As you can see very clearly, propan- -ol (A) does not contain an asymmetric carbon, as all the four groups attached