{"id":8093,"date":"2014-04-03T09:00:35","date_gmt":"2014-04-03T13:00:35","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8093"},"modified":"2026-05-07T09:18:56","modified_gmt":"2026-05-07T14:18:56","slug":"cycloalkanes-ring-strain-in-cyclopropane-and-cyclobutane","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/04\/03\/cycloalkanes-ring-strain-in-cyclopropane-and-cyclobutane\/","title":{"rendered":"Cycloalkanes – Ring Strain In Cyclopropane And Cyclobutane"},"content":{"rendered":"

Ring Strain In Cyclopropane and Cyclobutane<\/strong><\/p>\n

In the last post<\/a> we saw that cyclopropane and cyclobutane have an unusually high \u201cring strain\u201d of 27.6 kcal\/mol and 26.3 kcal\/mol respectively. \u00a0We determined this by comparing heats of combustion from rings of various sizes, and saw that the \u0394Hcombustion<\/sub> per CH2<\/sub> was essentially constant as ring sizes went above 12.<\/p>\n

Based on these calcuations, we saw that cyclopropane and cyclobutane are much<\/span> more unstable \u00a0than we might naively expect for an \u201cunstrained\u201d ring of that size<\/strong>. In addition, the C-C bond dissociation energy in cyclopropane is considerably weaker (65 kcal\/mol) than we observe for a typical C-C bond (80-85 kcal\/mol).<\/p>\n

Therefore, \u00a0there must be some structural feature of cyclopropane and cyclobutane which leads to this additional strain.<\/p>\n

What might that be?\u00a0<\/strong>\u00a0Let\u2019s take a look.<\/p>\n

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

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

    \n
  1. Angle Strain In Cyclopropane and Cyclobutane<\/a><\/li>\n
  2. Torsional Strain In Cyclopropane\u00a0<\/a><\/li>\n
  3. Torsional Strain In Cyclobutane – Some Puckering Is Possible<\/a><\/li>\n
  4. Summary: Ring Strain In Cyclopropane and Cyclobutane<\/a><\/li>\n
  5. Notes<\/a><\/li>\n
  6. Quiz Yourself!<\/a><\/li>\n
  7. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Angle Strain In Cyclopropane And Cyclobutane<\/strong><\/h2>\n

    \"c<\/a><\/p>\n

    The first thing to notice about cyclopropane and cyclobutane is the non-ideal bond angles<\/strong>. The ideal bond angle in tetrahedral carbon is 109 degrees (See post: How Do We Know Methane Is Tetrahedral<\/a><\/em><\/span>)<\/p>\n

    However, constrained to a triangle and a square, the interior angles of cyclopropane (60\u00b0) and cyclobutane (90\u00b0) are considerably less than the ideal angle.<\/p>\n

    This means that the electron clouds surrounding each atom will be considerably closer together than ideal, and since like charges repel<\/b>, this will be energetically less<\/strong> favorable<\/strong> than for a straight-chain alkane.<\/p>\n

    The additional instability caused by this constraint is called angle strain<\/b> or Von Baeyer strain.<\/p>\n

    [\u00a0Note – although the atoms of cyclopropane do form a triangle, the electron clouds between each atom do not necessarily follow the \u201clines\u201d of it. \u00a0The nature of the bonding in cyclopropane is a deep topic – Mike Evans put together a video on it here<\/a>.]<\/em><\/span><\/p>\n

    <\/a>So is it that simple? Is angle strain all there is to talk about? Not quite – there\u2019s one more factor to consider.<\/p>\n

    <\/a>2. Torsional Strain In Cyclopropane<\/strong><\/h2>\n

    From earlier chapters on conformations, you may recall the concept of \u201cTorsional Strain\u201d (aka \u201ctwisting strain) (See Article: Staggered vs. Eclipsed Conformations of Ethan<\/a>e<\/em><\/span>).<\/p>\n

    Although those two words often generate confusion in students\u2019 minds, the concept is very simple: strain generated through simple rotation<\/strong>.<\/p>\n

    \u00a0Have you ever flown a toy plane with a wind-up propeller? As you twist the propeller, you will progressively encounter more and more resistance from the elastic bands.<\/p>\n

    When wind-up is complete, you can really feel that propeller digging into your finger! The elastic bands in this case are under a lot<\/b> of torsional strain. Releasing the propeller results in untwisting of the elastic bands, which is the driving force for propelling the plane.<\/p>\n