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Alkynes: Reactions and Synthesis

Alkynes: Preparation

Reaction of calcium carbide with water

The reaction of calcium carbide with water is carried out at room temperature and, for a long time, was the most important method to generate acetylene.

Movie of the preparation of acetylene from calcium carbide

Today, the carbide method has been replaced by synthesis from methane. At 1500 °C, methane is pyrrolyzed using short reaction times. The reaction is endothermic, however, at very high temperatures it becomes exergonic (increase of entropy, four parts are formed from two parts).

Electric arc methodology

Acetylide anion is very nucleophilic and reacts with a multitude of electrophiles in SN2-type reactions. Using this method, terminal and internal alkynes can be synthesized.

Double elimination of 1,1- or 1,2-dihalogen alkanes with strong bases yields the corresponding compounds with triple bonds. Under mild conditions, elimination with sodium amide in ammonia, for example, takes place at ( -33 °C).

Dihalogen alkanes are easily obtained from alkenes by halogenation. As mentioned above, these compounds can be transferred into alkynes by double dehydrohalogenation.

Single-metallated ethyne is being used for the alkylation of ethinyl anions. However, at ca. 30 °C disproportionation to ethyne and a poorly soluble dianion (double Grignard reagent) takes place.

Alkynes by disproportionation of single-metalled ethyne
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