Integrated SequencePhysics Chemistry Organic Biology

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Virtual Textbook of Organic Chemistry - Organic Radicals
Excellent discussion of the structure and reactivity of organic free radicals. Synthesizes a lot of material you learn in bits and pieces in other topics. Highly recommended.

Purdue University - Free Radical Reactions
Good general discussion covering some of the important free radical reaction mechanisms.



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Special points of emphasis

The Chemical Bond

Reactions with Radical Intermediates

Like carbocations, alkyl radicals appear as intermediates in various organic reaction mechanisms. Because the nature of the alkyl radical can often play a role in determining the structure of the product, it is important to be familiar with their nature. Orbital hybridization of alkyl radicals conforms to the sp2 model with the six bonding electrons in three σ bonds and the lone electron in the unhybridized 2p orbital. The more highly substituted a radical, the more stable.



Reactions with Radical Intermediates

Reactions of Alkanes

The quintessential reaction involving carbon radical intermediates is free radical halogenation, a chain reaction mechanism. The first step of the reaction, initiation, begins with homolysis with ultraviolet radiation of a halogen molecule, chlorine, in this example, to form two chlorine atoms. A chlorine atom has an unpaired electron and acts as a free radical in the next phase, chain propagation. In the chain propagation phase, a hydrogen atom is removed by chlorine from the alkane and then the alkane radical pulls a chlorine atom from a new chlorine molecule. This step not only produces the alkyl halide product, but it also regenerates another chlorine radical, which can then take part in another propagation reaction. It is a chain reaction. The final step, chain termination occurs with recombination of two free radicals.

The MCAT has been somewhat willing over the years to present this reaction in the context of free radical halogenation of alkenes. In that case remember that a position adjacent to C=C double bonds will be the site of hydrogen extraction, an allylic position, due to the the proximity of the adjacent π system. Extraction is favored at that position because it forms a resonance stabilized allylic radical. Through resonance, the allylic radical will have two reactive positions for chain propagation.




Reactions with Radical Intermediates

Reactions of Alkenes

Reactions of Conjugated Species

Free radicals also appear in alkene chemistry. Free radical mechanisms involving alkenes include free radical addition of HX to alkenes and free radical polymerization. The free radical addition of HX to alkenes is notable in that, unlike electrophilic addition of HX to alkenes, the product is anti-Markovnikov. The halogen is added to the least substituted carbon of the original double bond.



Reactions with Radical Intermediates

Coordination Chemistry

Proteins

Bioenergetics and Cellular Respiration

Integration of Metabolism

The Digestive System and Nutrition

Here is an example of free radical chemistry in biochemistry to read for comprehension.

The coenzyme, vitamin B12, contains a coordinated cobalt atom. An unusual aspect is that one of the coordination positions is occupied by carbon, which is the only known carbon-metal bond in a biomolecule. This carbon metal bond is weak. In the activity of the enzyme, it is homeolytically cleaved to generate a methyl radical, which then abstracts a hydrogen atom as a prelude to intramolecular rearrangement, methylation, and reduction in certain biochemical transformations such as the addition of a carbon to the carbon skeleton of L-methylmalonyl CoA to form succinyl CoA, a portion of the pathway of the degradation of the amino acids methionine, isoleucine, and valine. In addition to enzyme processes involving vitamin B12, important enzymatic process involving free radicals include the actions of superoxide dismutase and ribonucleotide reductase.








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