342 18 · COMMON CLASSES OF ORGANIC COMPOUNDS
to be more basic than ammonia because alkyl groups push electrons on to the
nitrogen atom; this makes them better proton acceptors.The ‘electron push’ of alkyl groups is known as an inductive effect.
Optical isomerism
Some organic molecules can exist in two forms that are mirror images of one
another. Think of a pair of gloves – the left and right hand gloves cannot ‘fit over’
each other or be superimposed. The right glove has the shape of the image of the left
glove as seen in a mirror (Fig. 18.1).
Similarly, where a carbon atom is bonded to four differentatoms or groups, then
two such non-superimposable isomers are possible, as shown in Fig. 18.2 (make two
models and try to superimpose one upon the other). The carbon atom is known as a
chiral centre.
The mirror image isomers are called enantiomers. They generally have very simi-
lar chemical and physical properties, but differ in the way that they rotate plane
polarized light. The isomers are optically active– they will rotate the light by equal
amounts, but in opposite directions.18.6
BOX 18.4
Plane polarized light
Ordinary light travels in waves and the wave vibrations are at right angles to the direction of
travel. Plane polarized light is light that vibrates in only one plane and is obtained by passing
ordinary light through a ‘Polaroid’ lens or polarizer. If the plane polarised light is passed
through a solution of one enantiomer of a compound, the light will be rotated, say to the
right, by a certain angle. This is shown in Fig. 18.3. The light is rotated by an equal angle, but
to the left, by a solution of the other enantiomer of the compound with the same
concentration.Fig. 18.3Rotation of plane polarized light by an optically active compound.Fig. 18.1A glove and its
mirror image.
The basicity of
aminesArrange the following
compounds in order of
increasingbasicity:
CH 3 NH 2 , CH 3 NHCH 3 , NH 3.Exercise 18L
Fig. 18.2Two non-
superimposable isomers (a, b,
c and d are different atoms or
groups).