Organic Photochemistry Part I

Organic Photochemistry Part I


Learning Objectives: 

In this unit we shall learn about the 
i) Basic Principles of Photochemistry.
ii). Several photophysical processes .
iii). Photochemical reactions of a few classes of organic compounds.
INTRODUCTION

Classical Chemistry: It involves the reactions of molecules in their ground electronic states.

Photochemistry: It involves the reactions of molecules in excited electronic states. 

Organic photochemistry is the study of reactions that occur through excited states of organic molecules produced by the absorption of electromagnetic radiation in the UV-Vis region.

Classical Chemistry & Photochemistry

There are a few advantages of photochemistry over classical chemistry.
1.In photochemistry selective activation of individual reactant in a mixture of number of reactant is possible.
2.There is an activation energy requirement for every chemical reaction. However if the activation energy is very high, the desired reaction cannot be performed and instead decomposition and unwanted products are formed. This is because thermal energy is supplied in terms of small quanta of energy. As the temperature is increased the vibrational energy is increased and it may so happen that the molecule may fragment before the activation energy for the reaction is achieved by the molecule.
But unlike vibrational  and rotational energy levels which are closely spaced the electronic energy levels  have large energy difference. Photochemistry is the art of taking advantage of the large energy difference which exists between electronic states to produce directly highly energetic molecules without having forced them to climb step by step through various vibrational energy levels, thereby avoiding thermally induced fragmentation reactions.

Photochemical Process

A few examples of photochemical processes :
1.The most important photochemical process taking place in the nature is the photosynthesis.
2.In our daily experience, photochemistry is responsible for many noticeable changes.Coloured fabrics and materials fade much more when they have been exposed to sunlight. Similarly colored photographs undergo a change with the passage of time due to action of sunlight. 3.The formation of ozone layer in the stratosphere is a photochemical phenomenon.
4.Our vision process is also photochemical phenomenon, involving the pigment rhodopsin.
5.Photochemical degradation of environmental pollutants like polycyclic aromatic hydrocarbons, halogenated hydrocarbons etc.
To be continued...........