Advices For Studying Organic Chemistry

Figure 5.14 (a) A beam of plane-polarized light encounters a molecule of
(R)-2-butanol (a chiral molecule) in a particular orientation. This
encounter produces a slight rotation of the plane of polarization. (b)
exact cancellation of this rotation requires that a second molecule be
oriented as an exact mirror image. This cancellation does not occur
because the only molecule that could ever be oriented as an exact
mirror image at the first encounter is a molecule of (S)-2-butanol,
which is not present. As a result, a net rotation of the plane of
polarization occurs.

5.8A RACEMIC FORMS

1. A 50:50 mixture of the two chiral enantiomers.

5.8B RACEMIC FORMS AND ENANTIOMERIC EXCESS (e.e.)

% Enantiomeric excess = MMMM+– x 100

+ –

–

+

Where M+ is the mole fraction of the dextrorotatory enantiomer, and M–

the mole fraction of the levorotatory one.

% optical purity = [α][α]mixture

pure enantiomer

x 100

1. [α]pure enantiomer value has to be available


2. detection limit is relatively high (required large amount of sample for small
   rotation compounds)