Optical activity

Question 1

What is the device that light passes through which cuts out all vibrations apart from those in one plane?

Answer 1

Polaroid

Question 2

What can optical rotation be measured by?

Answer 2

Polarimeter

Question 3

How can you distinguish between two enantiomers?

3 stages

Answer 3

1) Polarised light is passed through two solutions of the same concentration, each containing a different optical isomer of the same substance.

2) One solution will rotate the plane of polarisation through a particular angle, clockwise.
- This is the (+)-isomer

3) The other will rotate the plane of polarisation by the same angle, anticlockwise.
- This is the (-)-isomer

Question 4

Outline the synthesis of 2-hydroxypropanoic acid (lactic acid)

Answer 4

1) Hydrogen cyanide is added across the C=O bond to form 2-hydroxypropanenitrile.
- This is a nucleophilic reaction

2) The nitrile group is converted into a carboxylic acid group
- The 2-hydroxypropanenitrile is reacted with water acidified with a dilute solution of hydrochloric acid - this is a hydrolysis reaction