Dehydrohalogenation and Related Reactions

What is a dehydrohalogenation reaction:

The Dehydrohalogenation of alkyl halides is normally carried out either in methanol with sodium methoxide, or in ethanol with sodium ethoxide, or in tert-butyl alcohol with potassium tert-butoxide, or in dimethyl sulfoxide, (CH₃)₂SO. A reaction in which a hydrogen atom and a halogen atom in a molecule or a compound are separated from neighboring atoms, and usually produce an alkene or an alkyne.

The Alkyl halides undergo a 1,2-elimination reaction to produce alkenes when heated with solid bases. usually Bases are KOH or NaOH or NaOR or KOR (alkoxide) in alcohol as a solvent, in particular NaOEt or KOtBu. It goes in Relative reactivity order of Alkyl halide: I > Br > Cl > F

Phenylacetylene is an alkyne hydrocarbon that contain a phenyl group in it. It is a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle as compared to acetylene gas.

Sodium amide, which is also commonly known as sodamide (systematic name sodium azanide), is a inorganic compound with the formula NaNH₂. Sodium amide is a salt composed of sodium cation and azanide anion. This solid is dangerously reactive toward water, it is white in its pure form, but commercial samples are typically gray due to the presence of small quantities of metallic iron from its manufacturing process. Such impurities do not affect the utility of the reagent NaNH₂ which conducts electricity in the fused state, its conductance being similar to that of NaOH in similar state, NaNH₂ has been widely employed as a strong base in organic synthesis.

Dehydrohalogenation

The Sodium amide induces the loss of two equivalents of hydrogen bromide from a vicinal dibromoalkane to produce a carbon-carbon triple bond, as in a preparation of phenylacetylene. Generally two equivalents of sodium amide yields the desired alkyne whereas three equivalents are necessary in the preparation of a terminal alkynes because the terminal CH of the resulting alkyne protonates an equivalent amount of base.

Hydrogen chloride (HCl) and ethanol (C₂H₅OH) can also be eliminated in this way, like in the preparation of 1-ethoxy-1-butyne. 

The Sodium amide molecule decomposes violently on contact with water, producing ammonia and sodium hydroxide:

NaNH₂ + H₂O → NH₃ + NaOH

When burned in the presence of oxygen, it will give oxides of sodium (which react with the water produced to produce sodium hydroxide) along with nitrogen oxides:

4 NaNH₂ + 5 O₂ → 4 NaOH + 4 NO + 2 H₂O

4 NaNH₂ + 7 O₂ → 4 NaOH + 4 NO₂ + 2 H₂O

In the presence of limited quantities of air and moisture, for example in a improperly closed container, explosive mixtures of peroxides may form after the combustion, This is accompanied by a yellowing or browning of the solid. As such, sodium amide is to be stored in a tightly closed container, under an atmosphere of inert gas. The Sodium amide samples that are yellow or brown in color represent explosion risks. 

Dehydrohalogenation of alkyl halides

When alkyl halide is heated with the base then it undergoes 1,2 elimination reaction. Nucleophilic substitution is a potential side reaction which results in alcohols and ethers. Typical bases will include NaOH or KOH or NaOR or KOR. Stability of product alkenes is done by regioselectivity. Hence, we have discussed the dehydrohalogenation of alkyl halides. 

Conclusion

In this article, we have gone through the definition of dehydrohalogenation reaction. The Dehydrohalogenation of alkyl halides is done either in methanol with sodium methoxide, or in ethanol with sodium ethoxide. We have also seen the mechanism of dehydrohalogenation reaction. We have seen the usage of phenylacetylene. We have also discussed sodium amide. Sodium amide is a salt composed of sodium cation and azanide anion. It is highly reactive toward water, but commercial samples are typically grey due to the presence of small quantities of metallic iron from its manufacturing process.