The dehydrohalogenation of alkyl halides gives alkenes. For example, we can react alkyl halides with strong bulky and non-bulky bases to prepare the Zaitsev or Hofmann alkene:

Preparation of Alkynes

As the name suggests, in these dehydration reactions, the leaving group halogen is removed with one hydrogen that is the β hydrogen. So, if one dehydrohalogenation forms one π bond, then two dehydrohalogenations should give us two π bonds if the halogens are in close proximity. More specifically, they must be either vicinal or geminal.

To distinguish these, you can go with geminal as being “jammed” on one carbon. More scientifically, it comes from the Latin geminus – “twin.”

The double dehydrohalogenation of vicinal or geminal dihalides indeed results in the formation of a triple bond. This is the standard strategy for preparing alkynes:

Although the elimination can be performed with regular bases such as hydroxides and alkoxides, a stronger base is needed to achieve an elimination of the intermediate vinyl halide. Most often Sodium amide (NaNH₂), dissolved in liquid ammonia (NH₃), is used.

Converting Alkyl Halides to Alkynes

We now know how to prepare alkynes from vicinal and geminal dihalides, therefore, to convert alkyl halides to alkynes, we need to first convert them to alkenes, halogenate the latter to obtain the dihalides which can then be transformed to alkynes:

An aqueous workup is needed to protonate the alkynide ion that is formed because of the relatively acidic proton of the terminal alkyne (pKa of 25).

The Mechanism of Dihalide Elimination

The reaction is based on the E2 mechanism, similar to the preparation of alkenes. The difference here is that two halogen atoms are needed because the triple bond is formed by two consecutive elimination reactions:

Even when an internal alkyne is prepared, an excess of sodium amide is often used to make sure the reaction goes to completion. Therefore, the water work-up is used in any case the protonate the alkynide ion and also quench the possible excess of NaNH₂.

Check Also

• Alkyl Halides to Alkenes
• General Features of Elimination

• The E2 Mechanism
• Zaitsev’s Rule – Regioselectivity of E2 Elimination Reactions
• The Hofmann Elimination of Amines and Alkyl Fluorides
• Stereoselectivity of E2 Elimination Reactions
• Stereospecificity of E2 Elimination Reactions
• Elimination Reactions of Cyclohexanes with Practice Problems
• POCl₃ for Dehydration of Alcohols
• The E1 Mechanism with Practice Problems
• Regioselectivity of E1 Reactions
• Stereoselectivity of E1 Reactions
• How to tell if it is E2 or E1 Mechanism
• Dehydration of Alcohols by E1 and E2 Elimination
• Mesylates and Tosylates as Good Leaving Groups
• Mitsunobu Reaction
• S_N1 S_N2 E1 E2 – How to Choose the Mechanism
• The Role of the Solvent in S_N1, S_N2, E1, and E2 Reactions
• S_N1 S_N2 E1 or E2 – the Largest Collection of Practice Problems
• The Hammond Postulate
• The E1cB Elimination Mechanism
• Nucleophilic Substitution and Elimination Practice Quiz