A Butane Analogue, 3-Hexyne, Is Eclipsed

Abstract

In ethane the facts of a staggered equilibrium structure and a 2.9 kcal/mol torsional barrier are established. 1 However, the origin of the barrier to internal rotation in ethane remains controversial. This debate centers around the relative importance of hyperconjugation vs steric/exchange interactions in determining the staggered structure and torsional barrier. A distinction between the competing models is that steric interactions primarily depend on the distance between interacting atoms, whereas hyperconjugation depends also on the symmetry (orientation) of the interacting orbitals. The orbital orientation distinction suggests another perspective on the barrier. The staggered gauche and trans conformers of butane are a corollary of the staggered ethane structure. 3-Hexyne (CH 3 CH 2 CtCCH 2 CH 3 ) is butane elongated by a cylindrically symmetrical CtC spacer, which separates the ethyl groups. If hyperconjugation were the dominant interaction in this butane analogue, the equilibrium structure would be staggered. But if steric interactions were dominant, the structure would be eclipsed. The equilibrium conformation of 3-hexyne is experimentally determined and its structure is syneclipsed, C 2V , not staggered.

Acute leukemia during pregnancy

“…A microwave study of the molecular structure of 3‐hexyne has recently been reported 1. The observed rotational structure can unambiguously be assigned to a syn ‐eclipsed C 2 v conformation.…”

Assessment of hydrophobic interactions and their contributions through the analysis of the methane dimer

“…[19][20][21][22][23] These last correlations are involved in many important chemical and biological observations like the degree of antimalarial activity of quinine, quinidine, and their diasteromers. [26][27][28] n-C 4 H 10 [29] and C 2 H 5 CBCC 2 H 5 [30] have also been relevant models in the analysis of conformational isomerism for which it has been argued that steric effects are the most important in the determination of the most stable conformers. There is an active discussion about the causes that determine the conformational equilibrium of this molecule.…”

“…[24,25] The reasons underlying the preponderance of certain conformers over others have been studied by using different models like ethane. [30,31] Other particularly fundamental examples of conformational analyses are those of substituted cyclohexane rings wherein the conformational preferences of molecules such as CH 3 AC 6 H 11 , (CH 3 ) 3 CAC 6 H 11 , and C 6 H 5 AC 6 H 11 are directly affected by intramolecular noncovalent interactions (INCIs) of the CAHÁÁÁHAC type. Exchange interactions (Pauli repulsion, steric contacts), electrostatics, and hyperconjugation are amongst the many effects that have been used to explain the preference of the staggered over the eclipsed conformation of C 2 H 6 .…”

Microwave spectroscopy and structures of two conformers of 6-methyl-3-heptyne and three of 2-methylpentane

“…Any polarization of the C"C triple bond from cylindrical symmetry by asymmetric alkyl substituents appears to be negligible. This has been demonstrated in 3-hexyne [1] (CH 3 CH 2 C"CCH 2 CH 3 ), which is heavy atom planar with syn-eclipsed C 2v symmetry, and 3-heptyne [2] (CH 3 CH 2 C"CCH 2 CH 2 CH 3 ). In the latter case, the substituted propyl group exists in anti and gauche configurations.…”