Other Reactions of Alkynes
Other Reactions of Alkynes
Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. They are unsaturated compounds with the general formula $C_nH_{2n-2}$. Alkynes are known for their reactivity due to the presence of the high-energy triple bond. In this discussion, we will explore various reactions that alkynes undergo, beyond the typical addition reactions.
Hydrogenation
Alkynes can be hydrogenated to form alkenes and ultimately alkanes. The process involves the addition of hydrogen (H₂) across the triple bond. This reaction requires a metal catalyst such as palladium, platinum, or nickel.
Partial Hydrogenation
$$ \text{RC} \equiv \text{CR'} + \text{H}_2 \xrightarrow{\text{Pd/CaCO}_3} \text{RC} = \text{CR'} $$
Complete Hydrogenation
$$ \text{RC} \equiv \text{CR'} + 2\text{H}_2 \xrightarrow{\text{Pt}} \text{RCH}_2\text{-CH}_2\text{R'} $$
Halogenation
Alkynes react with halogens (X₂) to form dihalides and tetrahalides. The reaction proceeds through the addition of halogen across the triple bond.
Addition of One Equivalent of Halogen
$$ \text{RC} \equiv \text{CR'} + \text{X}_2 \rightarrow \text{RCX} = \text{CXR'} $$
Addition of Two Equivalents of Halogen
$$ \text{RC} \equiv \text{CR'} + 2\text{X}_2 \rightarrow \text{RCX}_2\text{-CX}_2\text{R'} $$
Hydrohalogenation
The addition of hydrogen halides (HX) to alkynes results in vinyl halides or geminal dihalides, depending on the amount of HX added.
Addition of One Equivalent of HX
$$ \text{RC} \equiv \text{CR'} + \text{HX} \rightarrow \text{RC} = \text{CHR'}\text{X} $$
Addition of Two Equivalents of HX
$$ \text{RC} \equiv \text{CR'} + 2\text{HX} \rightarrow \text{RCHX}_2\text{-CHR'}\text{X} $$
Hydration
Alkynes can be hydrated to form enols, which tautomerize to ketones or aldehydes. The reaction typically requires an acid catalyst and mercury(II) sulfate.
$$ \text{RC} \equiv \text{CR'} + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4, \text{HgSO}_4} \text{RC(OH)} = \text{CHR'} \xrightarrow{\text{tautomerization}} \text{RCOR'} $$
Ozonolysis
Ozonolysis of alkynes leads to the cleavage of the triple bond and formation of carboxylic acids or their derivatives.
$$ \text{RC} \equiv \text{CR'} + \text{O}_3 \xrightarrow{\text{workup}} \text{RCOOH} + \text{R'COOH} $$
Alkylation of Terminal Alkynes
Terminal alkynes can undergo alkylation via the reaction with a strong base to form an acetylide anion, which can then be alkylated with an alkyl halide.
$$ \text{RC} \equiv \text{CH} + \text{NaNH}_2 \rightarrow \text{RC} \equiv \text{C}^- \text{Na}^+ + \text{NH}_3 $$ $$ \text{RC} \equiv \text{C}^- \text{Na}^+ + \text{R'X} \rightarrow \text{RC} \equiv \text{CR'} + \text{NaX} $$
Cycloaddition Reactions
Alkynes can participate in [2+2] and [4+2] cycloaddition reactions, such as the Diels-Alder reaction, to form cyclic compounds.
Diels-Alder Reaction
$$ \text{RC} \equiv \text{CR'} + \text{dienophile} \xrightarrow{\text{heat}} \text{cyclic adduct} $$
Comparison Table
| Reaction Type | Reactant | Catalyst/Reagent | Product | Notes |
|---|---|---|---|---|
| Hydrogenation | H₂ | Pd, Pt, Ni | Alkane/Alkene | Can be partial or complete |
| Halogenation | X₂ | None | Dihalide/Tetrahalide | X = Cl, Br, I |
| Hydrohalogenation | HX | None | Vinyl halide/Geminal dihalide | X = Cl, Br, I |
| Hydration | H₂O | H₂SO₄, HgSO₄ | Ketone/Aldehyde | Enol intermediate |
| Ozonolysis | O₃ | Workup (e.g., Zn) | Carboxylic acids | Cleavage of triple bond |
| Alkylation | NaNH₂, R'X | None | Alkylated alkyne | Terminal alkynes only |
| Cycloaddition | Dienophile | Heat | Cyclic compound | Diels-Alder reaction |
Examples
Hydrogenation Example
The partial hydrogenation of 1-butyne using a Lindlar catalyst yields cis-2-butene:
$$ \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_3 + \text{H}_2 \xrightarrow{\text{Lindlar catalyst}} \text{CH}_3\text{CH} = \text{CHCH}_2\text{CH}_3 $$
Halogenation Example
The addition of bromine to 2-butyne forms 2,2-dibromo-2-butene:
$$ \text{CH}_3\text{C} \equiv \text{CCH}_3 + \text{Br}_2 \rightarrow \text{CH}_3\text{CBr} = \text{CBrCH}_3 $$
Hydration Example
Hydration of 1-hexyne yields 2-hexanone after tautomerization:
$$ \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_2\text{CH}_2\text{CH}_3 + \text{H}_2\text{O} \xrightarrow{\text{H}_2\text{SO}_4, \text{HgSO}_4} \text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_2\text{CH}_3 $$
Ozonolysis Example
Ozonolysis of 3-hexyne leads to the formation of two molecules of acetic acid:
$$ \text{CH}_3\text{C} \equiv \text{CCH}_2\text{CH}_2\text{CH}_3 + \text{O}_3 \xrightarrow{\text{workup}} 2 \text{CH}_3\text{COOH} $$
Understanding these reactions is crucial for students preparing for exams, as they provide a comprehensive view of the chemistry of alkynes and their versatility in organic synthesis.