{"id":18096,"date":"2020-03-27T09:29:34","date_gmt":"2020-03-27T14:29:34","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=18096"},"modified":"2026-04-22T12:53:56","modified_gmt":"2026-04-22T17:53:56","slug":"conformational-isomers-of-propane","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2020\/03\/27\/conformational-isomers-of-propane\/","title":{"rendered":"Conformational Isomers of Propane"},"content":{"rendered":"

The Conformational Isomers of Propane (Are Awesome)<\/strong><\/p>\n

In this post we’ll explore the different conformations of propane and see that it has a slightly higher barrier to rotation than ethane due to a new steric interaction. We’ll draw out the Newman projection of propane from multiple perspectives, show the different Newman projections as we rotate through 360\u00b0 , and map out the energy diagram vs dihedral angle!<\/p>\n

\"-summary<\/a><\/p>\n

Table of Contents<\/strong><\/p>\n

    \n
  1. Recap: Staggered, Eclipsed, Newman Projections, Dihedral Angle.<\/a> <\/span><\/li>\n
  2. What About Propane?<\/a><\/li>\n
  3. Converting Propane to Newman Projections<\/span><\/a><\/li>\n
  4. The Role of Perspective: Analyze One Bond At A Time<\/span><\/a><\/li>\n
  5. Eclipsing Interactions in Propane<\/span><\/a><\/li>\n
  6. Visualizing The Rotational Barrier In Propane<\/span><\/a><\/li>\n
  7. Summary<\/span><\/a><\/li>\n
  8. Notes<\/a><\/li>\n
  9. Quiz Yourself!<\/a><\/li>\n
  10. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Recap: Staggered, Eclipsed, Newman Projections, Dihedral Angle<\/h2>\n

    In the previous post [see: Staggered vs Eclipsed Conformations of Ethane<\/a><\/em>] we saw that ethane has two important<\/strong> conformational isomers, depending on the orientation of the two methyl groups along the C\u2013C bond.<\/p>\n

      \n
    • In the eclipsed<\/strong> conformation,\u00a0 the hydrogens on the front carbon block (or “eclipse”) the hydrogens on the back carbon when one looks directly along the C-C bond, like two Mercedes-Benz symbols superimposed on top of each other.<\/li>\n
    • In the staggered<\/strong> conformation, the hydrogens on the front carbon bisect<\/strong> the hydrogens on the back carbon, so that if one looks along the C\u2013C bond, the hydrogens are all spaced out by 60 degrees.<\/li>\n
    • We saw that the Newman projection<\/strong> is a useful visual aid for visualizing the eclipsed and staggered conformations, as well as bond rotations.<\/li>\n
    • We also defined the dihedral angle<\/strong> as the angle between an arbitrarily chosen individual substituent<\/strong> on the front carbon and an individual<\/strong> substituent<\/strong> on the back carbon when looking along a carbon-carbon bond.\u00a0 A useful analogy for dihedral angle is the angle between the hour hand and minute hand on a clock face, where the dihedral angle is 0\u00b0 at 12:00,\u00a0 60\u00b0 at 2:00, 120\u00b0 at 4:00, and so on.<\/li>\n
    • When two substituents have a dihedral angle of\u00a0 \u201390\u00b0 \u2264 0\u00b0 \u2264+90 they are syn<\/em>. When they have a dihedral angle of +90\u00b0 \u2264 180\u00b0 \u2264 \u201390\u00b0 they are anti<\/em><\/li>\n
    • Conformational isomers can differ in energy<\/strong>; in ethane the staggered conformation is more stable than the eclipsed conformation<\/strong> by 3.0 kcal\/mol which works out to about 1.0 kcal\/mol per eclipsed H-H interaction. One way to think about this is repulsions between the hydrogens (or their electron clouds, to be more specific). There are other ways to think about it too.<\/li>\n<\/ul>\n

      <\/a>2. The Conformational Isomers Of Propane<\/h2>\n

      Now that we’ve looked at ethane, let’s go to the next higher alkane:\u00a0 propane<\/strong>.<\/p>\n

      Propane is simple to draw as a line diagram. Just like stick figures of people omit granular details like fingers and toes, line diagrams of molecules omit the placement of hydrogens.<\/p>\n

      Most of the time that’s OK, but if we’re going to discuss conformations, we need to expand out our line diagram of propane to show where the hydrogens are.<\/p>\n

      Obviously, the first order of business is to make sure that the hydrogens are arranged in a properly tetrahedral fashion around each carbon. (Yes, it’s possible to mess this up, as Khan Academy sometimes does [see post: Common Mistakes: Drawing Tetrahedral Carbons<\/a>]<\/em> )<\/span><\/p>\n

      \"proper-way-to-draw-tetrahedral-carbons-of-propane\"<\/a><\/p>\n

      Keep two substituents in the plane of the page, with one wedge and one dash, and an\u00a0acute<\/em><\/span>\u00a0<\/em>angle (<90\u00b0) between the wedge and dash.\u00a0<\/strong><\/p>\n

      Here’s a drawing of propane showing all the hydrogens. I’ve arbitrarily numbered the carbon on the left as C-1.<\/p>\n

      \"line-drawing-of-propane-with-expanded-drawing-showing-all-hydrogens-eclipsed\"<\/a><\/p>\n

      In expanding out the hydrogens on each carbon we now encounter the pesky details of conformational isomerism!<\/p>\n

      In the figure above, when I expanded out propane, I chose to draw an eclipsed<\/strong> conformation along both the C1-C2 and C2-C3 bonds. That’s one way to do it, and it’s not wrong<\/strong> (even if it is higher in energy).<\/p>\n

      However, there are other ways to draw out propane as well. I just as well could have drawn the C1-C2 carbon as staggered<\/strong> and the C2-C3 as eclipsed<\/strong> (below) or C1-C2 and C2-C3 as both staggered<\/strong>.<\/p>\n

      \"conformations-of-propane-staggered-eclipsed-c1-c2-c3\"<\/a><\/p>\n

      (There are of course other conformations that could be drawn out – an infinite number, but since they aren’t energy minima or maxima they generally aren’t as interesting to us<\/strong>. There are 3600 seconds in an hour, but when was the last time someone asked to meet you at 7:46:23 instead of just “8 o’clock” ?)<\/em><\/span><\/p>\n

      It doesn’t really matter\u00a0too\u00a0<\/em>much what conformation we depict propane in. Since there is free rotation about the C1-C2 and C2-C3 bonds<\/strong>,\u00a0 all the conformations are interconvertible<\/strong>.<\/p>\n