In this article, we will discuss about the synthesis and reactions of alkynes. Alkynes are unsaturated hydrocarbons, containing triple bond, with general formula CnH2n-2. These reactions are very crucial and provide several pharmaceutical products, and other organic compounds. We will also discuss the mechanisms of these reactions along with references to support our concept.
INTRODUCTION OF SYNTHESIS AND REACTIONS OF ALKYNES:
Alkynes, a class of hydrocarbons, are unsaturated compounds that contain at least one carbon-carbon triple bond. They have attracted significant interest due to their remarkable reactivity and diverse applications in organic synthesis. Understanding the preparations and reactions of alkynes, along with their underlying mechanisms, is crucial for the development of new methodologies and the synthesis of complex organic molecules.
SYNTHESIS AND REACTIONS OF ALKYNES:
1. PREPARATIONS OF ALKYNES:
One common method for preparing alkynes is through the dehydrohalogenation of geminal dihalides, which involves the elimination of two halogen atoms (usually bromine or iodine) from adjacent carbon atoms. This reaction is typically carried out using a strong base such as sodium or potassium hydroxide in an alcohol solvent like ethanol.
b) DEHALOGENATION OF VICINAL DIHALIDES:
Another approach is based on the dehalogenation of vicinal dihalides, where two halogen atoms are eliminated to form a triple bond between the two adjacent carbon atoms. This reaction can be catalyzed by various reagents such as copper(I) iodide or cuprous oxide in the presence of a base like potassium hydroxide.
2. REACTIONS OF ALKYNES:
a) HYDROGENATION OF ALKYNES:
Alkynes can be hydrogenated to form alkanes using a variety of catalysts, such as palladium or platinum. The reaction proceeds through the addition of two equivalents of hydrogen across the triple bond, sequentially resulting in an alkene and then an alkane.
b) HALOGENATION OF ALKYNES:
Under appropriate conditions, alkynes can react with halogens (e.g., chlorine or bromine) to yield vicinal dihalides. The addition of one equivalent of halogen across the triple bond forms a vinyl halide, which subsequently reacts with the second equivalent of halogen to generate the final product.
c) ACID-CATALYSED HYDRATION:
In the presence of an acid catalyst, alkynes can be hydrated to form ketones or aldehydes. The addition of water across the triple bond leads to the formation of an enol, which tautomerizes to a carbonyl compound upon protonation.
d) METAL-AMMONIA REDUCTION:
Alkynes, when subjected to metal-ammonia reduction, undergo a process known as Lindlar’s reduction. This reaction selectively converts alkynes into cis-alkenes by employing a palladium catalyst poisoned with lead or quinoline.
The preparations and reactions of alkynes offer a broad array of synthetic opportunities in organic chemistry. From their dehydrohalogenation and dehalogenation for preparation to hydrogenation, halogenation, acid-catalyzed hydration, and metal-ammonia reduction for reactions, alkynes exhibit versatile reactivity. A comprehensive understanding of the mechanisms involved, as well as the application of these reactions, is vital for researchers and synthetic chemists alike.
Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry. Oxford University Press. https://global.oup.com/academic/product/organic-chemistry-9780199270293?cc=pk&lang=en&
Carey, F. A., & Sundberg, R. J. (2007). Advanced Organic Chemistry: Part A: Structure and Mechanisms. Springer. https://link.springer.com/book/10.1007/978-0-387-44899-2
Smith, M. B., & March, J. (2007). March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley. https://rushim.ru/books/mechanizms/march6ed.pdf