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9.5 : Preparation of Alkynes: Dehydrohalogenation

Introduction

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.

Chemical reaction diagram, alkyne synthesis, 2NaNH2/NH3, hydrogen elimination, organic chemistry.

Reaction Mechanism – E2 pathway

Vicinal dihalides

In the first elimination step, the strong base abstracts the proton from the dihalide that is oriented anti to the leaving group. Since E2 reactions follow a concerted pathway, the abstraction of a proton and departure of the halide leaving group occur simultaneously to form a haloalkene.

Birch reduction mechanism diagram, showing electron flow in organic compound reaction.

In the second elimination reaction, another equivalent of the strong base reacts with the haloalkene in a similar fashion to give the desired alkyne.

Elimination reaction mechanism diagram, E2 mechanism, showing alkene formation from alkyl halide.

Geminal dihalides

Likewise, geminal dihalides, when treated with two equivalents of a sodium amide, undergo double dehydrohalogenation to give alkynes.

Alkyne formation reaction; sodium amide abstracting proton; chemical equation diagram; synthesis method.

Chemical reaction mechanism, arrow-pushing diagram, alkyne formation using sodium amide.

Terminal dihalides

Dehydrohalogenation of terminal dihalides yields terminal alkynes as the final product. In the presence of a strong base like sodium amide, terminal alkynes get converted to acetylide ions. In such cases, a third equivalent of the base is required to complete the dehydrohalogenation of the remaining haloalkene.

Hofmann elimination reaction; chemical equations diagram; synthesis process using NaNH2; alkyne formation.

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

Organic reaction mechanism with sodium acetylide, water; diagram illustrating nucleophilic attack.

Application in Organic Synthesis

Dehydrohalogenation of vicinal dihalides is a useful intermediate step in the conversion of alkenes to alkynes. For example, chlorination of 1-propene gives 1,2-dichloropropane – a vicinal dihalide, which upon double dehydrohalogenation yields 1-propyne.

Alkene to alkyne conversion diagram; chemical reaction mechanism, dichlorination, dehydrohalogenation.

Similarly, alkynes can also be synthesized from ketones via dehydrohalogenation of geminal dihalides. For example, treatment of acetone with phosphorous pentachloride yields 2,2-dichloropropane – a geminal dihalide, which undergoes double dehydrohalogenation to give 1-propyne.

Organic synthesis mechanism; ketone to alkyne via substitution and elimination; chemical reaction diagram.

Tagi

AlkynesDehydrohalogenationVicinal DihalidesGeminal DihalidesStrong BaseSodium AmideLiquid AmmoniaReaction MechanismE2 PathwayHaloalkeneTerminal DihalidesAcetylide IonsOrganic Synthesis

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