Dehydrohalogenation of Alkyl Dihalides

Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.

Dehalogenation can occasionally result in the formation of complex chemical molecules such as medicinal medications. The dehalogenation of organohalides is one method of detoxification because many of them are dangerous to health.

Dehydrohalogenation from alkyl halides

Alkyl halides have traditionally been used as substrates for dehydrohalogenation reactions. Due to the requirement that the alkyl halide be capable of forming an alkene, substrates such as methyl and benzyl halides are not appropriate. Aryl halides are likewise inappropriate for this use. When treated with a strong base, chlorobenzene dehydrohalogenation, resulting in the formation of phenol through a benzyne intermediate.

• Alkenes are subjected to base-promoted reactions.

Many alkyl chlorides undergo transformation into their corresponding alkenes when exposed to a strong base.

It is sometimes referred to as a -elimination reaction, and it is a form of elimination reaction in general. The following are some examples of prototypes:

In this reaction, ethyl chloride interacts with potassium hydroxide, which is commonly carried out in a solvent such as ethanol, to produce ethylene. Propene is produced through the reactions of 1-chloropropane and 2-chloropropane.

The rule of Zaitsev can be used to predict the regioselectivity of this reaction type.

Generalizations aside, the reaction of a haloalkane with potassium hydroxide can compete with an SN2 nucleophilic substitution reaction by OH, which is a powerful and unimpeded nucleophile in most cases. Alcohols, on the other hand, are often considered minor products. Strong bases, such as potassium tert-butoxide (K+ [CH3]3CO), are frequently used in dehydrohalogenation reactions.

Reaction Mechanism – E2 pathway

Vicinal dihalides

As part of the first elimination phase, a strong base is used to extract proton from a dihalide that is orientated antithetical to the remaining group. Because E2 reactions follow a coordinated route, the abstraction of a proton and the departure of the halide leaving group take place at the same time, resulting in the formation of a haloalkene compound.

Another equivalent of the strong base interacts with the haloalkene in the same manner as the first, yielding the required alkyne as a result of the second elimination reaction.

Geminal dihalides

Similarly, when treated with two equivalents of a sodium amide, geminal dihalides undergo twofold dehydrohalogenation, resulting in the formation of alkynes.

Terminal dihalides

The ultimate result of dehydrohalogenation of terminal dihalides is terminal alkynes, which are formed during the process. A strong base such as sodium amide causes terminal alkynes to be transformed to acetylide ions, which are toxic to living things. As a result, a third equivalent of the base is required to complete the dehydrohalogenation of the residual haloalkene in these situations.

Protonation of the acetylide ions with water or an aqueous acid completes the reaction.

Application in Organic Synthesis

It is possible to employ dehydrohalogenation of vicinal dihalides as an intermediary step in the conversion of alkenes and alkynes. For example, chlorination of 1-propene results in the formation of 1,2-dichloropropane, a vicinal dihalide that, after being dehydrohalogenation twice, provides 1-propyne.

Similarly, alkynes may be produced from ketones by dehydrohalogenation of geminal dihalides, which is a reaction that occurs in the presence of halogen. The treatment of acetone (and other solvents) with phosphorus pentachloride (and other solvents) results in 2,2-dichloropropane – a geminal dihalide that undergoes double dehydrohalogenation to create 1-propyne.

Conclusion 

Alkynes can be made by dehydrohalogenation vicinal or geminal dihalides in the presence of a strong base, such as sodium amide in liquid ammonia, and a strong base, such as sodium amide in liquid ammonia. In order for the reaction to continue, two equivalents of hydrogen halide (HX) must be lost in two subsequent E2 elimination processes, one after the other.

Alkyl halides have traditionally been used as substrates for dehydrohalogenation reactions. Due to the requirement that the alkyl halide be capable of forming an alkene, substrates such as methyl and benzyl halides are not appropriate. Aryl halides are likewise inappropriate for this use. When treated with a strong base, chlorobenzene dehydrohalogenation, resulting in the formation of phenol through a benzyne intermediate.