Unit 9: Alkynes
The straight, sp-hybridized cousin of the alkene: a proton acidic enough to remove, and a triple bond that can absorb two full additions.
6 topics · 22 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Alkyne Structure and Nomenclature
- How it worksHow does sp hybridization on each alkyne carbon leave two unhybridized p orbitals free to form two separate π bonds?
- Worked exampleFind the correct IUPAC name for the five-carbon chain with a triple bond between carbons 2 and 3.
- Going furtherWhat happens to a molecule's local shape when a triple bond sits in the middle of a long carbon chain?
Your turn1 of 2
What is the correct IUPAC name for the five-carbon chain with a triple bond between carbons 2 and 3?Acidity of the Terminal Alkyne
- How it worksHow does the high s-character of an sp carbon hold its electrons close enough to make that C-H bond acidic?
- Worked exampleFind the product when 1-hexyne is treated with sodium amide, NaNH2, a very strong base.
- Common mistakeWhy isn't a terminal alkyne's proton anywhere near as acidic as the proton on a carboxylic acid or an alcohol?
- Going furtherWhat happens to the acetylide ion once it forms: what kind of electrophile can it go on to attack?
Your turn1 of 2
A terminal alkyne's C-H proton has a pKa of 26. It reacts with a strong base whose own conjugate acid has a pKa of 38. Using Keq = 10 to the power (pka2 minus pka1), what is the equilibrium constant for this acid-base reaction?Addition of HX and X2 to Alkynes
- How it worksHow does the first equivalent of HBr add through a carbocation to form a vinyl halide before a second equivalent adds?
- Worked exampleFind the product when 2-butyne reacts with two equivalents of HBr, following Markovnikov selectivity both times.
- Going furtherWhat happens to a product's geometry when only one equivalent of bromine adds to an internal alkyne?
Your turn1 of 2
2-butyne reacts with two equivalents of HBr, following Markovnikov selectivity both times. What is the final product?Hydration of Alkynes and Tautomerization
- How it worksHow does an unstable enol flip into its more stable keto form right after water adds across the triple bond?
- Worked exampleFind the ketone product formed when 2-pentyne undergoes acid-catalyzed hydration with mercuric sulfate.
- Common mistakeWhy doesn't the enol intermediate in alkyne hydration ever survive long enough to be isolated on its own?
- Ties togetherHow does this keto-enol tautomerization preview the same equilibrium that runs alpha-carbon chemistry later on?
Your turn1 of 2
2-pentyne, CH3-C triple bond C-CH2CH3, undergoes acid-catalyzed hydration with mercuric sulfate. Since the triple bond's two carbons are attached to alkyl groups of only modestly different size, what ketone product(s) actually form?Hydroboration of Alkynes
- How it worksHow does boron add to the terminal, less hindered carbon and then oxidize into an enol that tautomerizes to an aldehyde?
- Worked exampleFind the aldehyde product formed when 1-hexyne undergoes hydroboration-oxidation.
- Going furtherWhat happens to this reaction's regiochemistry when the starting alkyne is internal instead of terminal?
Your turn1 of 2
1-hexyne undergoes hydroboration-oxidation. What is the aldehyde product?Reduction of Alkynes to Alkenes and Alkanes
- How it worksHow does hydrogen add to both faces of the alkyne on a poisoned Lindlar catalyst to deliver a cis alkene?
- How it worksHow does dissolving-metal reduction with sodium in ammonia form a radical anion that relaxes into a trans alkene?
- Worked exampleFind the product when 3-hexyne is hydrogenated over Lindlar's catalyst.
- Worked exampleFind the product when 3-hexyne is treated with sodium metal dissolved in liquid ammonia.
- Common mistakeWhy doesn't full hydrogenation with excess hydrogen over plain palladium ever stop cleanly at the alkene stage?
Your turn1 of 2
3-hexyne is hydrogenated over Lindlar's catalyst. What is the product?