In this organic chemistry tutorial, you will learn about alkynes, including their structure, their reactions, some quick facts, and some notable examples.
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Structure of Alkynes
Chemists use the term “alkyne” to refer to any organic compound with a carbon-carbon triple bond. In hydrocarbons, this triple bond serves as the main functional group. This triple bond has one unreactive sigma bond and two reactive pi bonds.
Alkynes have a general formula of RCΞCR’. Because they only have two electron domains, the triple bond and their respective variable group, the carbons of the alkyne have an sp hybridization. Ethyne has the simplest alkyne structure, with the formula HCΞCH:
Quick Facts on Alkenes
- Structure: A carbon triple bonded to another carbon.
- General formula: CnH2n-2, where n ≥ 2
- Acidity: More acidic than alkenes and alkanes
- Solubility: Insoluble in water but soluble in nonpolar organic solvents like diethyl ether
- CΞC bond length: 1.21 Å
- CΞC bond enthalpy: 812 kJ mol-1
- IR Spectroscopy: Alkyne triple bonds (CΞC) form a peak at 2140-2260 cm-1 depending on if monosubstituted or disubstituted. Csp–H bonds of terminal alkenes form a peak at 3267-3333 cm-1.
When hydrogen attaches to an alkyne functional group, chemists call the resulting group a “terminal alkyne”, because such groups locate at the ends of hydrocarbons. This variety of alkene is special because its bonded hydrogen is remarkably acidic.
Generally, hydrogens bonded to carbon tend to have very low acidity, due to the strength of C-H bonds. There are a few exceptions, such as hydrogens bonded to carbons adjacent to carbonyls, which are crucial for aldol condensations. Terminal alkyne hydrogens break this trend due to the high s characteristic of the sp C-H bond.
Due to this acidity, the hydrogen of the terminal alkyne dissociates when interacting with a strong enough base. The terminal alkyne then becomes a carbanion that can serve as a nucleophile in subsequent reactions.
Examples of Alkynes
Alkynes follow a familiar naming convention, equivalent to that of alkenes. For a hydrocarbon with an alkyne and no other functional group, then the compound’s name would be the lowest number carbon of the alkynes location, followed by the prefix denoting the number of carbons in the compound (eth-, prop-, but-, etc.), and finally the suffix -yne.
For instance, between hydrocarbons with a single terminal alkyne, the first 10 compound names include:
For compounds with other functional groups and more complicated nomenclature, alkynes follow the same rules as their alkene equivalents:
As you can see, many alkene molecules also have stable alkyne equivalents. One exception to this would be cycloalkenes, as “cycloalkynes” tend to break down as fast as they form due to the unbearable ring strain of the normally-linear CΞC bond.
Reactions with Alkynes
Alkynes behave similarly to alkenes, which involve carbon-carbon double bonds rather than triple bonds. Aside from the acidity of terminal alkynes, alkynes behave essentially like a double alkene, with two reactive pi bonds at the same pair of carbons.
Addition Reactions with Alkynes
Like with alkenes, certain nucleophile-electrophile compounds can perform addition reactions across each alkyne pi bond. For instance, let’s look at the addition reaction between an alkyne and HCl.
In this example, let’s use propyne. First, the HCl protonates the alkyne, forming a C-H bond using one of the pairs of pi electrons. The resulting molecule is a vinyl cation, which is a term chemists use to describe a cation with a positive charge on an alkene carbon. As with most carbocations, the vinyl carbocation favors placing its positive charge on a secondary carbon (C2) rather than a primary carbon (C1). “Secondary,” in this instance, describes a carbon bonded to two non-hydrogens, as opposed to a “primary” carbon bonded to only one non-hydrogen.
Second, the chloride anion attacks the carbocation, forming an alkene halide molecule.
Because the propyl molecule still has a pi bond, it will react further in an excess of HCl. Third, another HCl protonates the alkene, again forming a carbocation on C2. In this instance, the carbocation is further stabilized by the electron-withdrawing effect of the chloride. Finally, the second chloride attacks the carbocation, forming 2,2-dichloropropane.
Hydration of Alkynes
Though the above mechanism applies to most alkyne addition reactions, there exist some exceptions. The most important exception involves the addition of water to an alkyne. Let’s look at the mechanism, again using propyne.
First, like with HCl, the first pi bond is protonated and the resulting hydroxide attacks the secondary vinyl carbocation. The resulting compound is what chemists call an enol, with an alcohol bonded to an alkene carbon. Enol molecules tend to have little stability. Second, rather than performing a second addition, the enol undergoes a keto-enol tautomerization, forming a ketone.
Ozonolysis with Alkynes
Similar to alkenes, ozone can react with alkynes through ozonolysis, which forms carboxylic acids under aqueous conditions. Let’s look at the mechanism, this time with 2-pentyne, as terminal alkynes cannot perform ozonolysis.
First, the ozone performs a cycloaddition on the alkyne, involving a benzene-like cyclic movement of electrons. This forms a cyclic ozonide compound. Second, another cyclic movement of electrons occurs, which breaks the ring structure, forming an ionic carbonyl oxide group.
Third, the carbonyl oxide reacts with water from the aqueous environment, which eventually yields two carboxylic acid molecules. Interestingly, as you can see, ozonolysis completely cleaves the carbon-carbon triple bond.
In synthesis, ozonolysis tends to have little use, as alkynes have more synthetic versatility than carboxylic acids. However, chemists do use ozonolysis to analyze unknown alkynes by analyzing the resulting carboxylic acid fragments.