9.3.4. Preparation

Alkynes on partial reduction with calculated amount of dihydrogen in the presence of palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline give alkenes. Partially deactivated palladised charcoal is known as Lindlar\u2019s catalyst. Alkenes thus obtained are having cis geometry. However, alkynes on reduction with sodium in liquid ammonia form trans alkenes.

Alkynes can be converted into alkenes through a process known as partial reduction. This involves using a specific type of catalyst called Lindlar's catalyst, which is palladised charcoal that has been treated to reduce its activity. When alkynes undergo this process, they gain a double bond while losing one of their triple bonds, resulting in alkenes. The addition of dihydrogen gas occurs, and the structure of the resulting alkene has a cis configuration, meaning the hydrogen atoms attached to the double bond are on the same side. If the alkyne is treated with sodium in liquid ammonia, a different product is formed, leading to a trans alkene structure, where the hydrogen atoms are on opposite sides of the double bond.

Think of the alkyne compound as a rope stretched tightly between two points (the carbon atoms in the triple bond). By applying a little pressure (the dihydrogen), we can loosen the rope to form a loop (the alkene), but how we apply that pressure changes the shape of the loop. Using Lindlar's catalyst creates a loop that is more closed (cis), whereas using sodium in liquid ammonia creates a loop that is stretched out (trans).

Alkyl halides (R-X) on heating with alcoholic potash (potassium hydroxide dissolved in alcohol, say, ethanol) eliminate one molecule of halogen acid to form alkenes. This reaction is known as dehydrohalogenation i.e., removal of halogen acid. Nature of halogen atom and the alkyl group determine rate of the reaction.

Alkyl halides can be transformed into alkenes through a process called dehydrohalogenation. This occurs when the alkyl halide is heated with alcoholic potash, leading to the elimination of a hydrogen halide molecule (like HCl or HBr). The effectiveness and speed of this reaction depend on the nature of the halogen and the type of alkyl group (primary, secondary, or tertiary). Generally, tertiary alkyl halides react faster than secondary or primary ones because of their structure, which stabilizes the formation of the alkene.

Imagine trying to unscrew a tight lid (the alkyl halide) from a jar (the solvent) while heating the jar. The resistance you face when applying force depends on how tightly the lid is stuck, similar to how the structure of the alkyl halide affects the reaction speed. When the lid finally pops loose, that\u2019s like the alkene being formed, showcasing how effective the heating can be when paired with the right tools (alcoholic potash).