Smog hanging in the air over Dhaka City, Bangladesh. Note the brown coloration of the haze, caused by NO₂

A photoinitiator is any chemical compound that decomposes into free radicals when exposed to light. Photoinitiators are found both in nature (in photochemical smog) and in industry (for example, in plastics production).

In nature, photoinitiators are present througout the atmosphere. For instance, nitrogen dioxide is produced in large quantities by gasoline-burning internal combustion engines. NO₂ in the troposphere gives smog its brown coloration and catalyzes production of toxic ground-level ozone. Molecular oxygen (O₂) also serves as a photoinitiator in the stratosphere, breaking down into atomic oxygen and in order to form the ozone in the ozone layer.

In industry, photoinitiators are primarily used to promote polymerization reactions, notably in the production of polyethylene plastic. There are a handful of medical applications as well; for instance, benzoyl peroxide creams are commonly prescribed as acne medication.

[edit] Reactions

All photoinitiators have bonds that cleave via photolysis. For example, peroxides like hydrogen peroxide are perfect examples, with the O-O bond cleaving to form two hydroxyl radicals.

H₂O₂ --> 2OH

Certain azo compounds (such as azobisisobutyronitrile), can also photolytically cleave, forming two alkyl radicals and nitrogen gas:

RCH₂-N=N-H₂CR --> 2RCH₂ + N₂

These free radicals can now promote other reactions.

[edit] Atmospheric Photoinitiators

[edit] Peroxides

Hydrogen peroxide, the simplest peroxide

Since molecular oxygen can abstract H atoms from certain radicals, the HOO radical is easily created. This particular radical can further abstract H atoms, creating H₂O₂, or hydrogen peroxide; peroxides can further cleave photolytically into 2 hydroxyl radicals. More commonly, HOO can react with free oxygen atoms to yield a hydroxy radical (OH) and oxygen gas. In both cases, the OH radicals formed can serve to oxidize organic compounds in the atmosphere.^[1]

H₂O₂ --> 2OH

HOO + O --> O₂ + OH

OH + CH₄ --> CH₃ + H₂O

[edit] Nitrogen Dioxide

Nitrogen dioxide, a large contributor to the production of smog

Nitrogen dioxide can also be photolytically cleaved by photons of wavelength less than 400 nm^[2] producing atomic oxygen and nitric oxide.

NO₂ --> NO + O

Atomic oxygen is a highly reactive species, and can abstract a H atom from anything, including water.

O + H₂O --> 2OH

Nitrogen dioxide can be regenerated through a reaction between certain peroxy-containing radicals and NO.

ROO + NO --> NO₂ + RO

[edit] Molecular Oxygen

In the stratosphere, molecular oxygen (O₂) is an important photoinitiator that begins the ozone-production process in the ozone layer. Oxygen can be photolyzed into atomic oxygen by light with wavelength less than 240 nm.^[3]

O₂ → 2O

Atomic oxygen can then combine with more molecular oxygen to form ozone.

O + O₂ → O₃

However, ozone can also be photolyzed back into O and O₂.

O₃ → O + O₂

Furthermore, atomic oxygen and ozone can combine into O and O₃.

O + O₃ → 2O₂

This set of reactions govern the production of ozone and can combine to calculate its equilibrium concentration.

[edit] Commercial Photoinitiators & Uses

[edit] AIBN

Azobisisobutyronitrile, a commonly used industrial photoinitiator, and its breakdown into two radicals and hydrogen gas

Azobisisobutyronitrile is a white powder often used as a photoinitiator for vinyl-based polymers such as polyvinyl chloride, also known as PVC. Because this particular photoinitiator produces nitrogen gas (N₂) upon decomposition, it is often used as a blowing agent to change the shape and/or texture of plastics.^[4]

[edit] Benzoyl Peroxide

Benzoyl peroxide, a common photoinitiator used in plastics production and in acne medication

Benzoyl peroxide, much like azobisisobutyronitrile, is a white powder used as a photoinitiator in various commercial and industrial processes, including plastics production. Unlike AIBN, however, benzoyl peroxide produces oxygen gas upon decomposing, giving this compound a host of medical uses as well.^[5]

Benzoyl peroxide-based Upon contact with the skin, benzoyl peroxide breaks down, producing oxygen gas, among other things. The oxygen gas is absorbed into the pores of the skin, where it kills off the acne-causing bacteria Propionibacterium acnes.

In addition, the free radicals produced can break down dead skin cells. Clearing out these dead cells prevents pore blockage and, by extension, acne breakouts.^[6]

[edit] See also

• Radical initiator

[edit] References

1. ^ vanLoon, Gary W.; Duffy, Stephen J. (2005). Environmental Chemistry: A Global Perspective. New York, NY: Oxford University Press. pp. 52–53. ISBN 0-19-927499-1. 
2. ^ vanLoon, Gary W.; Duffy, Stephen J. (2005). Environmental Chemistry: A Global Perspective. New York, NY: Oxford University Press. pp. 74–79. ISBN 0-19-927499-1. 
3. ^ vanLoon, Gary W.; Duffy, Stephen J. (2005). Environmental Chemistry: A Global Perspective. New York, NY: Oxford University Press. pp. 48–49. ISBN 0-19-927499-1. 
4. ^ "Azobisisobutyronitrile - Chemical Dictionary", lookchem.com, accessed October 22, 2009
5. ^ "Benzoyl Peroxide", chemicalland21.com, accessed October 29, 2009
6. ^ "Benzoyl Peroxide", http://www.about.com, accessed October 29, 2009
7. ^ Liao KC, Hogen-Esch T, Richmond FJ, Marcu L, Clifton W, Loeb GE., Percutaneous fiber-optic sensor for chronic glucose monitoring in vivo. Biosens Bioelectron. 2008 May 15;23(10):1458-65. Epub 2008 Jan 18.

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