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Related Concept Videos

Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.
Stability of Substituted Cyclohexanes02:30

Stability of Substituted Cyclohexanes

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in...
Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
Nomenclature of Alkynes02:39

Nomenclature of Alkynes

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:
Relative Stabilities of Alkenes01:59

Relative Stabilities of Alkenes

The relative stability of alkenes can be determined by comparing their heats of hydrogenation. The lower heat of hydrogenation indicates the more stable alkene.  The three main factors determining the relative stability of alkenes are i) the number of substituents attached to the double-bond carbon atoms, ii) hyperconjugation, and iii) the stereochemistry of the double bond.
Nucleophilic Aromatic Substitution: Elimination–Addition01:11

Nucleophilic Aromatic Substitution: Elimination–Addition

Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is confirmed through isotopic...

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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

Published on: September 18, 2016

The substituted alkyne 3-heptyne is eclipsed.

Geoffrey B Churchill1, Robert K Bohn

  • 1Department of Chemistry, University of Connecticut, Storrs, CT 06269-3060, USA.

The Journal of Physical Chemistry. A
|April 19, 2007
PubMed
Summary

The stable conformations of 3-heptyne, an analogue of pentane, are eclipsed, unlike the staggered forms of pentane. This shift is driven by dispersion attractions in low-polarity acetylenes.

Area of Science:

  • Physical Chemistry
  • Molecular Spectroscopy
  • Organic Chemistry

Background:

  • Butane and pentane exhibit stable staggered conformations (anti and gauche).
  • 3-hexyne, with a central triple bond, shows a stable eclipsed conformation, differing from butane.
  • Understanding conformational preferences in similar molecules provides insight into molecular interactions.

Purpose of the Study:

  • To investigate the conformational behavior of 3-heptyne, an elongated analogue of pentane.
  • To determine if the anti-anti (AA) and gauche-gauche (GA) conformations of pentane translate to staggered forms in 3-heptyne.
  • To explore the influence of the carbon-carbon triple bond on molecular conformation.

Main Methods:

  • Rotational microwave spectroscopy was employed to study 3-heptyne.

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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes
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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes
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Chemoselective Preparation of 1-Iodoalkynes, 1,2-Diiodoalkenes, and 1,1,2-Triiodoalkenes Based on the Oxidative Iodination of Terminal Alkynes

Published on: September 12, 2018

  • Analysis of spectral data allowed for the determination of molecular structure and conformations.
  • Comparison with known conformational data for pentane and 3-hexyne.

    • Main Results:

    • The most stable conformer of 3-heptyne is eclipsed, analogous to the extended anti-anti (AA) form of pentane.
    • The elongated analogue of the gauche-gauche (GA) conformer of pentane in 3-heptyne is also nearly eclipsed.
    • Conformations of low-polarity substituted acetylenes are dictated by dispersion attractions between terminal groups.

    Conclusions:

    • The presence of a C[triple bond]C triple bond significantly alters stable conformations from staggered to eclipsed in 3-heptyne.
    • Dispersion attractions between end groups are the primary drivers for conformational preferences in these molecules.
    • This study provides crucial data on the conformational landscape of alkynes and their relationship to alkanes.