Reactions of Alkynes: Addition, Substitution, Elimination

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. The triple bond provides them with unique reactivity, making them important in organic synthesis. In this lesson, we will explore the reactions of alkynes, including addition, substitution, and elimination.

Addition reactions:

Addition reactions of alkynes involve the addition of atoms or groups to the carbon-carbon triple bond, resulting in the formation of a double or single bond. Addition reactions involve the addition of an electrophile (a positively charged or electron-deficient species) to the triple bond of an alkyne. The most common electrophiles used in the reaction are hydrogen halides (HBr, HCl, HI) and halogens (Cl2, Br2). The reaction is initiated by the attack of the electrophile on the π-electrons of the alkyne triple bond, forming a cyclic intermediate known as a vinyl cation. Typically, the reaction continues and reacts with the resulting alkene product and ungergoes a second addition reaction, resulting in the formation of a saturated compound (alkane).

An example is show below using our Mechanism Solver to draw any molecule, select your reagent, and get an answer!

Elimination reactions:

In elimination reactions, a molecule is eliminated from the alkene, resulting in the formation of a double bond. One example of this is the dehydrohalogenation reaction, in which halides are removed from an alkyl halide forming an alkyne. For example, when 1,1,2,2-tetrabromobutane (C4H6Br4) is reacted with sodium amide (NaNH2)and water (H2O), the product is 2-but-yne (C4H6), as shown below using OrgoSolver's Reaction Solver:

For a complete list of alkyne reactions used in Organic Chemistry 1, check out our Reaction Library!

Summary

Alkynes are unsaturated hydrocarbons that contain a triple bond between two carbon atoms. They have unique physical and chemical properties that differentiate them from alkanes and alkenes, including their high acidity and reactivity towards addition, substitution, and elimination reactions.

Test Your Knowledge:

What is the product of the reaction between chloride gas and but-1-yne?

What type of reaction involves the removal of molecules, resulting in the formation of a triple bond?