Beginner Topic 736 of 1102 Organic Chemistry
Alkynes
Hydrocarbons containing a carbon-carbon triple bond with formula CnH2n-2.
In-depth explanation of Alkynes
Alkynes are hydrocarbons containing a carbon-carbon triple bond, with the general formula CnH2n-2 for acyclic members. The triple bond consists of one sigma bond and two pi bonds, and the carbons are sp hybridized, so the geometry around each triple-bonded carbon is linear with a bond angle of 180 degrees. Because the sp carbon has 50 percent s character, its electrons are held closer to the nucleus, making terminal alkynes weakly acidic with a C-H pKa near 25. Alkynes can add up to two equivalents of a reagent across the triple bond, first giving an alkene and then an alkane or a carbonyl product.
Going deeper: Alkynes
Key ideas: A C-C triple bond (one sigma and two pi) is linear and sp hybridised. Additions occur in two stages (alkyne to alkene to alkane). Hydration with Hg2+ and dilute H2SO4 gives a ketone via enol (ethyne gives acetaldehyde). Lindlar catalyst gives a cis alkene, sodium in liquid ammonia gives a trans alkene. Terminal alkynes have pKa about 25 and give acetylides with NaNH2.
Common mistakes and connections: Alkynes are less reactive than alkenes toward electrophiles despite having more pi bonds, because the vinyl cation intermediate is less stable and sp carbon holds its electrons tightly. Students also forget that hydration always gives a ketone except for ethyne. Links to acidity of terminal alkynes, tautomerism and the Baeyer test.
Visualization of Alkynes
Ethyne is linear with 180 degree H-C-C angles around sp carbons; the triple bond is one sigma and two pi bonds. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ethyne, also called acetylene, burns at about 3000 degrees Celsius in an oxyacetylene welding torch.
Frequently asked questions about Alkynes
What is Alkynes?
Hydrocarbons containing a carbon-carbon triple bond with formula CnH2n-2.
Is Alkynes a beginner, intermediate or advanced topic?
Alkynes is a beginner topic in Organic Chemistry. It needs no earlier chemistry lessons and is a good starting point.
What should I learn before Alkynes?
Study these earlier topics in Organic Chemistry first: Homologous Series, Alkanes, Alkenes.
What comes after Alkynes?
The next topics in Organic Chemistry are Cycloalkanes and Aromatic Hydrocarbons.
Can I practise Alkynes with quizzes?
Yes. The interactive lesson includes a self-check question on Alkynes, and the Organic Chemistry quiz draws on every topic in this area.
Where Alkynes fits in Organic Chemistry
Alkynes is topic 8 of 148 in Organic Chemistry.