Alkynes
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02:39
Nomenclature of Alkynes
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2025
Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of alkynes follows a set of rules similar to alkanes and alkenes; however, alkynes bear the suffix "-yne" instead of "-ane" or "-ene." There are two approaches to naming alkynes: • IUPAC names • Common names IUPAC nomenclature The IUPAC naming system follows a systematic approach to deduce names of alkynes based on their chemical structure.
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Acidity of 1-Alkynes
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2023
The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here. Ethane (pKa = 51) Ethene (pKa = 44)
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Preparation of Alkynes: Dehydrohalogenation
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2025
Introduction Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions. Reaction Mechanism – E2 pathway Vicinal dihalides In the first elimination step, the strong base abstracts the proton from the dihalide that is oriented anti to the leaving group. Since E2 reactions follow a...
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Mass Spectrometry: Alkyne Fragmentation
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2024
The fragmentation of alkynes preferentially occurs at the carbon–carbon bond between the α and β carbon of the alkyne bond to generate a 3-propynyl cation (or propargyl cation). In terminal alkynes, there is the only type of fragmentation that yields the 3-propynyl cation. The unsubstituted 3-propynyl cation exhibits a peak at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne fragments into methyl-substituted 3-propynyl cation, which...
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Structure and Physical Properties of Alkynes
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2023
Introduction: In nature, compounds containing both carbon and hydrogen are known as "hydrocarbons". Aliphatic hydrocarbons are compounds whose molecules contain saturated single bonds (i.e., alkanes) or unsaturated double or triple bonds. Alkenes contain carbon–carbon double bonds and have a structural formula CnH2n. Unsaturated hydrocarbons containing carbon–carbon triple bonds are called "alkynes" and are structurally represented by the formula CnH2n-2. The simplest alkyne is ethyne, or...
FAQs
In an alkyne, each triple-bonded carbon uses sp hybrid orbitals, producing a linear arrangement around the bond. The remaining p orbitals form two pi bonds, whose electron distribution contributes to the compound’s ability to participate in transformations such as hydrogenation, halogenation, and hydration. This combination of geometry and pi bonding helps explain the behavior of alkynes in organic synthesis.
These reactions are enabled by the two pi bonds within the carbon-carbon triple bond. The pi-bonding system provides the reactive feature that allows alkynes to undergo hydrogenation, halogenation, or hydration, while the sigma bond remains part of the carbon framework. Consequently, chemists can use these transformations to modify alkyne-containing molecules during synthesis.
A terminal alkyne contains a hydrogen attached to one of the triple-bonded carbon atoms. This hydrogen-bearing structure allows the compound to be converted into a useful carbon nucleophile, a carbon species that participates in bond formation. The resulting reactivity provides a route for creating new carbon-carbon bonds, making terminal alkynes valuable intermediates in organic synthesis.
The key distinction is whether a hydrogen is attached to a triple-bonded carbon. Terminal alkynes possess this hydrogen and can be converted into carbon nucleophiles for forming new carbon-carbon bonds. Alkynes without that stated structural feature do not provide the same terminal-hydrogen-based route described here, so the distinction affects which synthetic transformations are available.
Chemists can subject alkynes to hydrogenation, halogenation, or hydration, using the triple bond as the reactive site for molecular change. Terminal members also offer a carbon-nucleophile pathway that forms new carbon-carbon bonds. Together, these options allow alkyne-containing compounds to serve as adaptable building blocks rather than remaining limited to a single synthetic role.
Alkynes provide useful structural and reactive features for several areas of chemistry. Their reaction pathways support organic synthesis and pharmaceutical development, while their building-block role extends to polymer production and the preparation of advanced functional materials. These applications reflect both the reactivity of the triple bond and the carbon-carbon bond-forming potential of terminal alkynes.