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 (CH3CH2C⋮CCH2CH3) is butane elongated by a cylindrically symmetrical C⋮C spacer, which separates the ethyl groups. If hyperconjugation were the dominant interaction in this butane analogue, the equilibrium structure would be staggere...