9.4 Reduction of Alkynes | Organic Chemistry

The video begins by introducing the topic of alkyne reduction, which is the focus of the lesson. It is compared to the addition reactions of alkenes, indicating that the reactions for alkynes will be similar. The presenter outlines the lesson structure, which includes presenting known alkene reactions followed by analogous alkyne reactions. There are three different alkyne reduction reactions to be covered, contrasting with the single relevant reduction for alkenes. The video is part of an organic chemistry playlist that will be released weekly during the 2020-21 school year. The presenter encourages viewers to subscribe and enable notifications to stay updated with new lessons.

The first alkene reduction reaction discussed involves the addition of H2 using a metal catalyst like palladium, platinum, or nickel, with palladium on carbon being a common choice. This syn addition reaction reduces the alkene to an alkane, potentially forming chiral centers if present. The analogous alkyne reduction occurs in two steps, adding two equivalents of H2 across the two pi bonds. The first step converts the alkyne to an alkene, and the second step reduces the alkene to an alkane. However, by using a 'poisoned catalyst' like Lindlar's catalyst, which includes barium sulfate, calcium carbonate, and quinoline with palladium, the reaction can be halted at the alkene stage. This modification of the catalyst decreases its reactivity, preventing the second reduction step and resulting in a cis alkene. The video also mentions other less common poisoned catalysts but emphasizes that Lindlar's catalyst is the primary one students are likely to encounter.

In addition to the hydrogenation reactions, the video introduces an alternative reduction method called dissolving metal reduction. This process involves using sodium, lithium, or potassium, typically sodium, dissolved in liquid ammonia to achieve a different outcome—producing a trans alkene instead of a cis alkene. The mechanism behind this involves the donation of an electron from sodium to the alkyne carbon, resulting in a radical anion intermediate. The geometry of this intermediate favors a trans arrangement of the methyl groups, leading to the formation of a trans alkene. The role of ammonia in the process is to deprotonate, picking up a hydrogen and forming an amide ion. The video cautions against confusing sodium in liquid ammonia (Na + NH3) with sodium amide (NaNH2), as they are distinct reagents used for different purposes in organic chemistry. The presenter also provides information on how to support the channel and where to find additional study materials and practice problems.