Organic Photochemistry Part II

 See previous notes in below link

Photochemical Processes:

• 6.Certain chemicals change their color when exposed to suitable radiation and reverse when the radiation source is removed. These are known as photochromic materials. A well known example is spiropyrans.Such materials are used in photochromic sunglasses.
• 7. In photography, it is the photo reduction of silver ions into silver metal ,which is the main phenomenon.Colour photography is based on the same chemistry, where silver halide is in superimposed gelatin layers, each sensitive to only one of the primary colors.
• 8.Newborn babies, particularly those born prematurely, often suffer from jaundice or hyperbilirubinemia.This is because they have difficulty in excreting bilirubin formed through metabolism of heme.Bilirubin is toxic and high conc. can induce brain damage or even death. Only treatment of neonatal hyperbilirubinemia is the exposure to light to photo degrade the excess bilirubin.
9.Rickets is a disease due to vitamin D deficiency. One of the remedy is exposure to sunlight. The photochemical transformation(260-315nm) leads to conversion of 7-dehydro cholesterolin the epidermis to vitamin D
Photochemical Transformation

10.An instructive example for the technical exploitation of photochemistry is the development of optical brighteners. An optimum impression of whiteness is obtained when a body reflects all visible light. However, many materials cannot satisfy this condition, because they absorb some light in the blue range of the spectrum. This leads to what is called the blue defect in the light reflected and the materials assume a pale yellowish colouration. Nevertheless the impression of whiteness can be achieved by adding blue fluorescent compounds, which absorb invisible UV-light and transform it into visible blue fluorescent light.

Opical brightener, fluorescent dye

An important example of industrial application of photochemistry is the photo oximation.

Photo oximation of saturated hydrocarbon was discovered in 1919 by E V Lynn. By a coincidence, the American chemist let a solution of nitrosyl chloride in n-heptane,which he prepared for quite a different purpose, stand for a day in sunlight. The originally red brown solution turned blue and then completely discoloured .A,heavy yellow oil had collected at the bottom of the vessel and Lynn identified it as a mixture of the hydrochlorides of isomeric heptanone oximes.This first photo oximation reaction demonstrated the ease with which oximes could be produced in one step under such mild conditions from hydrocarbons ,considered to be inert.