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4.2: Conformations of open-chain organic molecules

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    170431

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    Before we begin our exploration of stereochemistry and chirality, we first need to consider the subject of conformational isomerism, which has to do with rotation about single bonds.

    We learned in section 2.1 that single bonds in organic molecules are free to rotate, due to the 'end-to-end' (sigma) nature of their orbital overlap. Consider the carbon-oxygen bond in ethanol, for example: with a 180o rotation about this bond, the shape of the molecule would look quite different:

    Ethanol molecule with hydroxy hydrogen to the right of oxygen. The carbon oxygen bond is rotated 180 degrees and is in equilibrium with ethanol that has the hydrogen bonded to the left of oxygen.

    Or ethane: rotation about the carbon-carbon sigma bond results in many different possible three-dimensional arrangements of the atoms.

    Rotations of ethane. Red hydrogen on solid lines, blue hydrogens on dashes, and black hydrogens on wedges. Rotated 120 degrees. The right carbon now has blue on a solid line, black on a dash, and red on a wedge. Left carbon stays the same. Rotated again by 60 degrees. Left carbon stays the same. Right carbon has black on dash, blue on wedge and red on solid line.

    These different arrangements, resulting from sigma bond rotation, are referred to in organic chemistry as conformations. Any one specific conformation is called a conformational isomer, or conformer.

    In order to better visualize different conformations of a molecule, it is convenient to use a drawing convention called the Newman projection. In a Newman projection, we look lengthwise down a specific bond of interest – in this case, the carbon-carbon bond in ethane. We depict the ‘front’ atom as a dot, and the ‘back’ atom as a larger circle.

    Ethane molecule with one C H 3 blue (right) and one C H 3 red (left). Visualization of looking down the carbon-carbon bond from the side to form a staggered Newman projection. Blue hydrogens are in the front connected by a dot. Red hydrogens are in the back connected by a larger circle around the blue dot.

    Interactive mode of ethane conformationsl

    The six carbon-hydrogen bonds are shown as solid lines protruding from the two carbons. Note that we do not draw bonds as solid or dashed wedges in a Newman projection.

    Looking down the C-C bond in this way, the angle formed between a C-H bond on the front carbon and a C-H bond on the back carbon is referred to as a dihedral angle. (The dihedral angle between the hour hand and the minute hand on a clock is 0o at noon, 90o at 3:00, and so forth).

    The lowest energy conformation of ethane, shown in the figure above, is called the ‘staggered’ conformation: all of the dihedral angles are 60o, and the distance between the front and back C-H bonds is maximized.

    If we now rotate the front CH3 group 60° clockwise, the molecule is in the highest energy ‘eclipsed' conformation, where the dihedral angles are all 0o (we stagger the bonds slightly in our Newman projection drawing so that we can see them all).

    Eclipsed conformation of Newman projection of ethane. Blue and red hydrogens are only slightly staggered to show that dihedral angles are zero degrees.

    The energy of the eclipsed conformation, where the electrons in the front and back C-H bonds are closer together, is approximately 12 kJ/mol higher than that of the staggered conformation.

    Another 60° rotation returns the molecule to a second staggered conformation. This process can be continued all around the 360° circle, with three possible eclipsed conformations and three staggered conformations, in addition to an infinite number of conformations in between these two extremes.

    Now let's consider butane, with its four-carbon chain. There are now three rotating carbon-carbon bonds to consider, but we will focus on the middle bond between C2 and C3. Below are two representations of butane in a conformation which puts the two CH3 groups (C1 and C4) in the eclipsed position, with the two C-C bonds at a 0o dihedral angle.

    Eclipsed (A) conformation of a butane molecule (two carbons each bonded to one methyl group). Both C H 3 groups are on a solid line in bond-line structure and face upwards on the Newman projection. Blue (front) and red (back) substituents are only slightly staggered. Methyl groups are next to each other.

    Interactive model of butane conformations

    If we rotate the front, (blue) carbon by 60° clockwise, the butane molecule is now in a staggered conformation.

    Staggered conformation of butane in gauche. Front (blue) C H 3 faces up to the right and back (red) C H 3 points straight up and is next to the front C H 3.

    This is more specifically referred to as the gauche conformation of butane. Notice that although they are staggered, the two methyl groups are not as far apart as they could possibly be.

    A further rotation of 60° gives us a second eclipsed conformation (B) in which both methyl groups are lined up with hydrogen atoms.

    Eclipsed (B) conformer of butane. Substituents only slightly staggered. Back (red) C H 3 points up to the left and front (blue) C H 3 points down to the right. Methyl groups opposite of each other.

    One more 60 rotation produces another staggered conformation called the anti conformation, where the two methyl groups are positioned opposite each other (a dihedral angle of 180o).

    Anti conformation of butane. Back (red) C H 3 points up and is 180 degrees from front (blue) C H 3 which points down.

    As with ethane, the staggered conformations of butane are energy 'valleys', and the eclipsed conformations are energy 'peaks'. However, in the case of butane there are two different valleys, and two different peaks. The gauche conformation is a higher energy valley than the anti conformation due to steric strain, which is the repulsive interaction caused by the two bulky methyl groups being forced too close together. Clearly, steric strain is lower in the anti conformation. In the same way, steric strain causes the eclipsed A conformation - where the two methyl groups are as close together as they can possibly be - to be higher in energy than the two eclipsed B conformations.

    The diagram below summarizes the relative energies for the various eclipsed, staggered, and gauche conformations.

    Graph of relative energy (k cal per mol) versus degrees of rotation about C 2 C 3 bond. Eclipsed (A) with zero degrees at 4.5 k cals. Gauche with 60 degrees at 0.9 k cals. Eclipsed (B) with 120 degrees at 3.8 kcals. Anti  with 180 degrees at 0 k cals.

    Because the anti conformation is lowest in energy (and also simply for ease of drawing), it is conventional to draw open-chain alkanes in a 'zigzag' form, which implies anti conformation at all carbon-carbon bonds. The figure below shows, as an example, a Newman projection looking down the C2-C3 bond of octane.

    Linear structure and zigzag structure of octane molecule with C 2 and its substituents in blue, C 3 and its substituents in red and the remaining carbons in green. Goes to the Newman projection in anti with carbon 4 through 8 becoming "R" in the back facing up.

    Exercise 3.1

    Using free rotation around C-C single bonds, show that (R,S) and (S,R)-tartaric acid are identical molecules.

    soderberg 3tartartic.svg

    Exercise 3.2

    Draw a Newman projection, looking down the C2-C3 bond, of 1-butene in the conformation shown below (C2 should be your front carbon).

    image026.png

    Solutions to exercises

    Online lectures from Khan Academy
    Newman projections part I
    Newman projections part II

    Contributors and Attributions


    This page titled 4.2: Conformations of open-chain organic molecules is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.


    This page titled 4.2: Conformations of open-chain organic molecules is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Tim Soderberg via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.