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Preparation of Alkynes: Dehydrohalogenation

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.

Preparation of Alkynes: Alkylation Reaction

Preparation of Alkynes: Alkylation Reaction

Introduction
Alkylation of terminal alkynes with primary alkyl halides in the presence of a strong base like sodium amide is one of the common methods for the synthesis of longer carbon-chain alkynes. For example, treatment of 1-propyne with sodium amide followed by reaction with ethyl bromide yields 2-pentyne.

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.

Electrophilic Addition to Alkynes: Halogenation

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.

Base-Promoted α-Halogenation of Aldehydes and Ketones

Base-Promoted α-Halogenation of Aldehydes and Ketones

α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base. The reaction begins with the abstraction of α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...

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Related Experiment Video

Updated: Aug 13, 2025

Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

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Published on: September 12, 2018

Recent progress in alkynylation with hypervalent iodine reagents.

Eliott Le Du¹, Jérôme Waser¹

¹Laboratory of Catalysis and Organic Synthesis, Institute of Chemical Sciences and Engineering École Polytechnique Fédérale de Lausanne EPFL, SB ISIC, LCSO, BCH 4306, 1015, Lausanne, Switzerland. jerome.waser@epfl.ch.

Chemical Communications (Cambridge, England)

|January 19, 2023

Summary

Ethynyl hypervalent iodine reagents enable electrophilic alkynylation, expanding organic synthesis. This review highlights recent advancements in their synthesis and diverse applications since 2018.

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Area of Science:

Organic Chemistry
Synthetic Chemistry

Background:

Alkynes are versatile building blocks in chemistry and material science.
Traditional alkynylation uses acetylenes as nucleophiles.
Ethynyl hypervalent iodine reagents offer electrophilic alkyne synthons.

Purpose of the Study:

To review progress in alkynylation reactions using ethynyl hypervalent iodine reagents since 2018.
To discuss the synthesis and applications of these reagents.

Main Methods:

Review of recent literature on ethynyl hypervalent iodine reagents.
Discussion of base-mediated, transition-metal catalyzed, and radical-based alkynylations.
Focus on atom-economical transformations.

Main Results:

Significant developments in the synthesis of ethynyl hypervalent iodine reagents.
Broadened scope of alkynylation reactions with electrophilic alkyne synthons.
Advancements in radical-based and atom-economical alkynylation strategies.

Conclusions:

Ethynyl hypervalent iodine reagents have revolutionized alkynylation chemistry.
Recent progress has expanded their utility in various synthetic transformations.
These reagents are key to developing efficient and sustainable synthetic methods.