WHY DO CHIRAL COMPOUNDS ROTATE PLANE POLARISED LIGHT?

INTRODUCTION:

Any material that rotates the plane of polarised light is optically active. Optically active compounds are non-superimposable on their mirror image. This property of non-superimposability of an object on its mirror image is called chirality. 

In each case of optical activity of a compound, there are two isomers called enantiomers, which differ in structure only in left and right-handedness of their structure. They rotate the plane of polarised light in opposite directions, although in equal amounts.

The isomer that rotates the plane to the left is called the levo isomer and is designated (-), while the one that rotates the plane to the right is called dextro and is designated as (+).

THE CAUSE OF OPTICAL ACTIVITY/ ROTATION OF THE PLANE

• When light interacts with any molecule, it is slowed down. The extent of interaction depends on the polarizability of the material.

• Plane polarised light can be considered as made up of two circularly polarised light.

• Circularly polarised light can be imagined as helix propagating around the axis of light motion, and one kind is left- and the other is a right-handed helix.

• When the plane polarised light passes through a symmetrical region, the two circularly polarized components travel at the same speed.

• A chiral molecule has different polarizability depending on whether it is approached from the right or left.

• If one circularly polarized component approaches the molecule to say left, it sees different polarizability( hence on a net scale, a different refractive index) than the other and is slowed down to a different extent.

• This would seem to mean that the two circularly polarised light travel at different however it is impossible for two components of the same light to travel at different velocities.

• The “faster” component seems to pull the other component towards it, resulting in rotation of the plane.

SOURCE- JERRY MARCH ADVANCED ORGANIC CHEMISTRY(SIXTH EDITION)

DOES AN ELECTRON ACTUALLY SPIN?

In this blog, I will cover what is a quantum spin, do electrons spin similar to planets and the Stern-Gerlach experiment; direct observation of the spin.

WHAT IS SPIN?

Spin is an inherent property possessed by the electron. However, it does not rotate. In quantum mechanics, we speak of an en electron as having an intrinsic angular momentum called spin. The reason we use this term is that electrons possess an angular momentum & a magnetic moment just like a rotating charged body.

 DO ELECTRONS SPIN SIMILAR TO PLANETS?

IT IS MISLEADING TO IMAGINE AN ELECTRON AS A SMALL SPINNING OBJECT DUE TO THE FOLLOWING:

• An electron’s spin is quantified. It has only two possible orientations, spin up and down, unlike a  tossed ball.
• To regard an electron as spinning, it must rotate with a speed greater than light to have the correct angular momentum[Griffiths, 2005, problem 4.25].
• Similarly, the electron’s charge would have to rotate faster than the speed of light to generate the correct magnetic moment[Rohrlich, 2007, Pg 127].
• Unlike a tossed ball, the spin of an electron never changes. It has only two possible orientations: spin up and down.
  

  STERN-GERLACH EXPERIMENT[]

  

• In 1922, Otto Stern and Walther Gerlach performed an experiment sending a beam of silver atoms through an inhomogeneous magnetic field and observing their deflection.
• The result illustrated that the electron possesses a quantized intrinsic angular momentum i.e. spin. If the particles were classical spinning objects, one would expect random and continuous spin angular momentum vectors. Each particle would be deflected by an amount proportional to its magnetic moment, producing density distribution on the screen.
• Instead, the particles passing through the Stern-Gerlach apparatus are either deflected up or down by a specific amount.

To conclude, the quantum spin of fundamental particles like electrons is very different from the concept of spin in classical physics.