Alkyne Reactions

This paragraph discusses the hydrogenation reactions of alkynes using hydrogen gas and palladium on carbon as a catalyst. Initially, one molecule of hydrogen adds to the alkyne to form an alkene through syndication, with the catalyst placing both hydrogen atoms on the same side. Subsequently, an additional hydrogen molecule adds to the alkene to yield an alkane, resulting in a total of four hydrogen atoms across the triple bond. The paragraph also explains the use of Lindler's catalyst, which is a poisoned version of palladium on carbon, to stop the reaction at the alkene stage, specifically yielding a cis alkene.

The second paragraph delves into the metal-ammonia reduction reaction, where alkynes react with sodium metal and liquid ammonia to form trans alkenes. The reaction mechanism involves the formation of a radical anion, followed by the reaction with ammonia to form a vanillic radical. The final step involves the conversion of this radical into a vanillic anion, which upon reaction with another sodium atom and ammonia yields the trans alkene. The paragraph also contrasts this with the oxymercuration-demercuration reaction of alkynes, which leads to the formation of enols that tautomerize into ketones.

This section explains the hydroboration-oxidation reaction applicable to alkynes, highlighting the use of R2BH (such as 9-BBN) instead of BH3 to prevent overreaction with terminal alkynes. The reaction proceeds through the formation of an enol, which then tautomerizes into an aldehyde or ketone depending on the substitution pattern. The paragraph emphasizes the distinction between the Markovnikov and anti-Markovnikov addition, and the preference for R2BH in terminal alkynes to avoid side products.

The focus of this paragraph is on the conversion of alkynes to ketones using mercury sulfate in water and sulfuric acid, and to geminal dihalides through reactions with hydrobromic acid. The proposed mechanisms involve the initial formation of a pi complex, followed by the addition of halide ions to form carbocations, which are then stabilized by resonance. The paragraph also discusses the formation of various isomers, including E and Z isomers, and the preference for the addition of halogens to secondary carbons over primary ones.

This paragraph explores the detailed mechanism of alkynes reacting with hydrobromic acid, leading to the formation of geminal dihalides. It describes the nucleophilic attack of the alkyne on the hydrogen of HBr, followed by the attack of the bromide ion on the carbocation, resulting in the addition of bromine atoms across the double bond. The paragraph also addresses alternative mechanisms and the stabilization of carbocations through resonance with electron-withdrawing groups.

The paragraph discusses the synthesis of vicinal dihalides from alkynes using various reagents and the subsequent conversion of these dihalides back to alkynes using sodium amide or potassium hydroxide. It explains the preference for the formation of terminal alkynes with sodium amide and internal alkynes with potassium hydroxide at high temperatures, emphasizing the stability and equilibrium between internal and terminal alkynes.

This section provides a mechanism for the conversion of vicinal dihalides to internal alkynes using potassium hydroxide at elevated temperatures. The E2 elimination reaction is described, where the hydroxide ion abstracts a proton to form a double bond and expel a bromine atom, followed by the formation of a triple bond and the expulsion of another bromine atom to yield the internal alkyne.

The paragraph outlines the conversion of internal alkynes to terminal alkynes using sodium amide and ammonia. It describes the reversible steps involving the deprotonation of the alkyne by the amide ion and the stabilization of the resulting intermediates through resonance. The final, non-reversible step involves the formation of the terminal alkyne, which is stabilized by the addition of water to regenerate the hydroxide ion.

The final paragraph examines the reaction of acetylide ions, generated by the deprotonation of terminal alkynes with sodium amide, with alkyl halides like ethyl bromide. The reaction mechanism involves a nucleophilic attack of the acetylide ion on the electrophilic carbon of the alkyl halide, resulting in the formation of a new carbon-carbon bond and the addition of an ethyl group to the alkyne.