Dehalogenation

Dehalogenation is a chemical reaction that involves the cleavage of C-Halogen bond to form product. Dehalogenation can be divided into two subclasses: reductive dehalogenation and hydro dehalogenation.

Organic halides belong to a class of organic compounds that contain carbon-halogen bond. In 1832, scientist named Justus von Liebig synthesized the first organic halide (charcoal) via chlorination of ethanol. Since then, organohalides have gained a lot of attention. Organohalides are commonly used as pesticides, biodegradables, soil fumigants, refrigerants, chemical reagents – solvents, and polymers. It has been classified as pollutant despite of their wide use in various applications. Due to which, dehalogenation is a key reaction to convert toxic organohalides to less hazardous products.

Among halogens, fluorine is the most electronegative atom and will have the highest tendency to make the strongest bond with carbon. The rate of dehalogenation depends on the bond strength between carbon and halogen atom. The bond dissociation energies of carbon-halogen bonds are described as: H3C-I (234 KJmol-1), H3C-Br (293 KJmol-1), H3C-Cl (351 KJmol-1), and H3C-F (452 KJmol-1). Thus, for the same structures the bond dissociation rate for dehalogenation will be: F<< Cl < Br < I. Additionally, the rate of dehalogenation for alkyl halide also varies with steric environment and follows this trend: primary > secondary > tertiary halides.

The rate of dehalogenation differs with the type of substrate, metal’s oxidation state, and reducing agents used during the reaction.

Alkali and alkaline earth metals such as Lithium, sodium, potassium, magnesium, and calcium has proven to be great dehalogenation catalysts. During dehalogenation reaction, the metals act as a reducing agent to break down carbon-halogen bond. Halogen can then leave as a leaving group. The generic way to synthesize alkanes using alkali and alkaline-earth metal is shown in scheme 2:

Yus and his co-workers synthesized various lithium naphthalenide compounds that acts as a catalyst for lithiation of different functionalized halogenated arenes. The Li-arene reacted with water or deuterium to produce the dehalogenated product. The major drawback of using lithium naphthalene catalysts is that it is hard to separate from the reaction mixture because naphthalene adsorbs on surface of arene substrates. In polymer chemistry, sodium metal has been used for dehalogenation process. Removal of halogen atom from arene-halides in the presence of Grignard agent and water for the formation of new compound is known as Grignard degradation. Dehalogenation using Grignard reagents is a two steps hydrodehalogenation process. The reaction begins with the formation of alkyl/arene-magnesium-halogen compound, followed by addition of proton source to form dehalogenated product. Egorov and his co-workers have reported dehalogenation of benzyl halides using atomic magnesium in 3P state at 600oC. Toluene and bi-benzyls were produced as the product of the reaction. Morisson and his co-workers also reported dehalogenation of organic halides by flash vacuum pyrolysis using magnesium.

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