Alkynes are organic molecules with a carbon-carbon triple bond. They have a linear geometry, and the bond angle is 180 degrees. Alkynes have similar physical properties to alkenes and alkanes. Alkynes use the suffix -yne to indicate the triple bond. Alkynes are not as common in nature as alkenes, but they are used in welding and in synthesizing pharmaceuticals. Terminal alkynes are weak acids, because of the ease of deprotonation, and they act as nucleophiles. They may be prepared by double E2 elimination of vic-dihalides or gem-dihalides. Alkynes undergo addition reactions similar to alkenes. Alkynes undergo hydration reactions, and hydration of a terminal triple bond results in an aldehyde or ketone. Hydrogenation occurs through heavy-metal catalytic reactions, while alkylation creates longer carbon chains. A chain of any length and potentially with multiple functionality is created by combining alkylation of terminal alkynides with various hydrogenation reagents.

At A Glance

  • Alkynes have linear geometry with a bond angle of 180 degrees. Nomenclature of alkynes is similar to alkene nomenclature.
  • Alkynes can be prepared by the double E2 elimination of vic-dihalides (two halogens on neighboring carbons) or by the double E2 elimination of gem-dihalides (two halogens on the same carbon).
  • Reactions of alkynes include addition of HX, addition of X2, hydration, and catalytic hydrogenation. The stoichiometry of HX and X2 addition determines whether the reagent adds once or twice.
  • Terminal alkynes are acidic and can be deprotonated to form alkynides, which are excellent nucleophiles in SN2 reactions. Deprotonation and alkylation of terminal alkynes is a useful synthetic method to extend the carbon chain.