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Acyclic Conformations

  • Page ID
    20932
  • An understanding of the factors that go into determining the lowest energy conformation of acyclic molecules is required to understand the stereochemical outcomes of many reactions. Here we will discuss how steric, electronic, and stereoelectronic effects influence the conformation of acyclic organic molecules.

    Ethane and Propane

    Eclipsed Interaction Energy (kcal/mol)

    \(\ce{H-H}\)

    1.0
    \(\ce{H-CH_3}\) 1.4

    Butane

    Using the eclipsing interactions extracted from ethane and propane we can estimate all but one of the eclipsed butane conformations

    The value of +3.8 kcal/mol agrees quite well with the value of +3.6 reported by Allinger (J. Comp. Chem., 1980, 1, 181-184.) His results are graphed below.

    From Allinger's results we can determine the energetic value of eclipsing methyl groups:

    Thus we can see that the interaction between eclipsing methyl groups is worth 3.1 kcal/mol.

    Pentane

    Allylic Strain

    The allylic stereocenter (marked with "*") and its substituents play an important part in determining the stereochemical outcome of reactions with the neighboring pi-bond. Allylic-1,3 and allylic-1,2 interactions (defined below) greatly affect the diastereoselectivity of reactions.

    More can be read about A(1,3) strain here: Chem. Rev., 1989, 89, 1841-1860.

    Propene and 1-Butene

    To understand the origins of A(1,2) and A(1,3) strain, we have to look at the consequences of introducing sp2 hybridized centers into our structures. The first major difference is the bond angle change from 109° to 120°.

    Changing from propane to propene has major affects on the preferred conformation of the molecule. Propene now prefers to sit in an eclipsed conformation.

    We can understand this change in preference by looking at the new orbital interactions that arise from the p-orbitals forming a pi-bond. New stabilizing and destabilizing interactions account for the stabilization of the eclipsed conformation and the destabilization of the staggered conformation, respectively.

    see: J. Am. Chem. Soc., 1985, 107, 5035-5041.; J. Am. Chem. Soc., 1987, 109, 6591-6600.

    These effects become more pronounced as we move to butane and 1-butene. The eclipsed methyl-methyl conformation is now lower in energy than the staggered conformation; however, it is not the lowest energy conformation.

    Analysis of the torsional energy profile shows that the lowest energy conformation for 1-butene arises when one of the hydrogen atoms is in the plane of the alkene. We often times will refer to drawing these lowest energy conformations as the "H-in plane conformation."

    2-pentene

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