Examining The Concept Of Photochemical Reaction Biology Essay


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The light excites atoms and molecules and then some of their electrons go to the higher energy level and thus make them more reactive (the first step in a photochemical process is photo excitation. In comparison to ordinary reactions using thermal energy alone, photochemical reactions can follow different routes and are more likely to produce free radicals, which can trigger and sustain chain reactions. One photochemical reaction is the action of sunlight on car exhaust fumes, which results in the production of ozone. (Suppan1996, Wiley2009). In experiment set up, usually, quartz is used for the reactors as well as to contain the lamp. Pyrex absorbs at wavelengths shorter than 275nm.

Solvents are very important parameter. Solvents containing instauration absorb at longer wavelengths and can usefully filter out short wavelengths. For example Cyclohexane (The solvent that we used in this experiment) absorbs strongly at wavelengths shorter than 215nm. (Wiley2009, Wayne2005)

Photochemical reactions involve electronic reorganization initiated by electromagnetic radiation. The reactions are several orders of magnitude faster than thermal reactions. Photo chemists typically work in only a few sections of the electromagnetic spectrum. Some of the most widely used sections, and their wavelengths, are the following: (Bulletin2005)

There are three main types of mercury lamp; low pressure, medium pressure, and high-pressure lamps. Each one has a different output spectrum. Low-pressure lamps (10-5 atm), operate at 40-50°C and emit two wavelengths radiation at 253.7 nm and 184.9 nm. The 185 nm emission is of no use to photochemists because it is absorbed by the quartz that surrounds the lamp; therefore the only interesting line is the weak 253.7 nm emission and for this reason these lamps are thought of as monochromatic. More useful for synthetic photochemistry are the medium-pressure (1-10 atm) lamps. Since these lamps have an operational temperature of 600-800°C, they are utilized in cold-water cooled immersion jackets (According to Figure 1) to prevent thermal excitation of the photochemical substrate. Medium-pressure lamps have a broad spectral output that targeting of many more chromophores and therefore a more diverse range of photochemical applications are possible compared to the low-pressure lamps.

According to above spectrum medium pressure lamps so various glassware filters such as Quartz, Pyrex, Uranium and Vycor Used to prevent high-energy short wavelength radiation from reaching the photochemical substrate and allow transmission of only specific regions of the mercury emission spectrum emit wavelengths as low as 200 nm. This can allow greater control over which chromophores of the substance are excited.

A cycloaddition reaction is the union of two small, independent Ï€-System. It is the most useful of all pericyclic reactions in organic synthesis. Cycloaditions are characterized by two components, they coming together to form two new δ-bonds, at the end of both mechanism, joining them together to form a ring, with a reducing of the length of the conjugated system of orbitals in each component. Cycloadditions are by far the most plentiful, featureful, and useful of all pericyclic reactions. Cycloaddition is type of X + Y  è X-Y complexation, and it follows the usual thermo chemistry rules. However, many cycloaddition reactions require moderate heating to overcome the activation go , but if it is heated too much the equilibrium will favour retrocycloaddition. The most important type of cycloaddition is the Diels-Alder reaction. (Movasaghi2007).

The compound cyclopentadiene slowly undergoes cycloaddition with itself: one molecule of cyclopentadiene acts as a 4 π-electron diene and the other as a 2 π-electron dieneophile. The product is a Diels-Alder "adduct", often called dicyclopentadiene. This dimeric material can be cracked back to cyclopentadiene by heating at 150°c for an hour and then distilling off the diene monomer. (Pericyclic Reactions-Ian Fleming-1999-page: 4,5,7)

A sigmatropic reaction in organic chemistry is a pericyclic reaction in which the net result is one δ-bond is changed to another δ-bond in an uncatalyzed intramolecular process. Sigmatropic rearrangements are unimolecular processes. Sigmatropic reactions involve the movement of a sigma-bond with the simultaneous rearrangement of the π-system. Two examples explain this (Chem 1999,Klarner1984).

The [1,5] shift of hydrogen in a 1,3-pentadiene system:

The [3,3] Cope rearrangement: Electrocyclic reactions:

Electronic reactions are unimolecular process, which the exchange of π-bonds for ring closing sigma-bonds. This is best illustrated by an example:

(Note that the 3-alkene must be cis the reaction to occur).

The reverse. Or retroelectrocyclic, reaction can also occur. This is seen with the ring opening of cyclobutene to 1,3-butadiene:

Group transfer reactions are a special class of complexation or fragmentation process. They are best exemplified by the reaction between a propene and ethane to give a 1-pentene. (Chem2009, Wiley1976) Cheletropic reactions:

Cheletropic reactions are a type of pericyclic reaction where the net result is the conversion of π-bond and a lone pair into a pair of sigma bonds; with both new sigma bonds adding into the same atom. [IPUAC Compendium of chemical terminology 2nd edition (1997)].

A pericyclic reaction is one that involves a transition state with a cyclic array of atoms and an associated cyclic array of interacting orbitals. (Thomson, Wiley 1976). reactions:

It is a type of organic reaction and more specifically a pericyclic valence isomerization in which two δ bonds simultaneously migrate intramolecularly. The reaction type is of some relevance to organic chemistry because it can explain how certain reactions occur and because it is a synthetic tool in the synthesis of organic molecules for example in total synthesis. It was first described by Manfred T.Reetz in 1971. (Chem2009)

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