{"id":19754,"date":"2020-04-30T10:46:10","date_gmt":"2020-04-30T15:46:10","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=19754"},"modified":"2026-05-06T16:18:02","modified_gmt":"2026-05-06T21:18:02","slug":"alkene-stability","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2020\/04\/30\/alkene-stability\/","title":{"rendered":"Alkene Stability"},"content":{"rendered":"

Alkene Stability (And Instability)<\/strong>
\nWhat factors affect alkene stability? If you’ve studied elimination reactions, no doubt you’ve learned about Zaitsev’s Rule<\/a> – about how elimination reactions generally favor the “more substituted”<\/strong> alkene.<\/p>\n

In this post we explore how increasing substitution at carbon increases the stability of alkenes, as well as the effects of conjugation and strain.<\/p>\n

\"alkene-stability-summary-image-trends-monosubstituted-less-stable-than-tetrasubstituted\"<\/a><\/p>\n

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

    \n
  1. Heat Of Hydrogenation As A Measure Of Alkene Stability<\/a><\/li>\n
  2. Stability of Alkenes Increases With Increasing Substitution<\/a><\/li>\n
  3. Heats Of Hydrogenation For Some Monosubstituted Alkenes<\/a><\/li>\n
  4. The Relative Stability of cis-<\/em> and trans-\u00a0<\/em>Alkenes<\/a><\/li>\n
  5. Alkenes Stabilized By Conjugation: Resonance Energy<\/a><\/li>\n
  6. Alkene Stability: Summary<\/a><\/li>\n
  7. Notes<\/a><\/li>\n
  8. Bonus Topic #1: Why Is Alkyl Substitution Stabilizing?<\/a><\/li>\n
  9. Bonus Topic #2:\u00a0trans<\/em>-Cycloalkenes<\/a><\/li>\n
  10. Quiz Yourself!<\/a><\/li>\n
  11. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Heat Of Hydrogenation As A Measure Of Alkene Stability<\/h2>\n

    We might not spend as much discussing thermodynamics in here organic chemistry as you did in general chemistry, but that doesn’t mean the concepts have just gone away!<\/p>\n

    One area where we’ve previously seen the usefulness of thermodynamic data is the use of heat of combustion data to quantify ring strain. [See: Cycloalkanes – How To Calculate Ring Strain<\/em><\/a>]. The heat of combustion for cyclopropane works out to about\u00a0 166 kcal\/mol per CH2 <\/sub>compared to the heat of combustion for unstrained cyclohexane [157 kcal\/mol per CH2<\/sub>]. That “extra” heat of combustion seen in cyclopropane is attributed to the instability arising from the strain of bent C-C bonds far away from their ideal angle of 109.5\u00b0. That’s\u00a0angle strain<\/strong>.<\/p>\n

    Another area of organic chemistry where thermodynamic studies are useful in the stability of alkenes<\/strong>.<\/p>\n

    Back in 1935, Prof. Kiasatakowsky\u00a0 and co-workers at Harvard published a method for measuring the heat of hydrogenation of ethylene (aka “ethene”) as it was passed over a finely divided metal catalyst containing adsorbed hydrogen. [Note 1<\/a>] Because hydrogenating a molecule is considerably more gentle than, say, BURNING<\/em> it, the method tends to be more sensitive for determining subtle differences in enthalpies.<\/p>\n

    In a hydrogenation reaction, a C-C bond is broken, and two new C-H bonds are formed.<\/p>\n

    \"-heat-of-hydrogenation-of-ethene\"<\/a><\/p>\n

    It was found that hydrogenation of ethylene released 32.5 kcal\/mol (136 kJ\/mol) of heat. [Note 2<\/a>]<\/p>\n

    Once the heat of hydrogenation of ethene was obtained, the next logical step was to measure the heat of formation for a huge variety of other alkenes, and to see what patterns emerged from the data.<\/p>\n

    So what happens to the heat of hydrogenation when alkyl groups are added to the alkene?<\/p>\n

    <\/a>2. Stability of Alkenes Increases With Increasing Substitution<\/h2>\n

    Well, as you might imagine from someone who had invented a new technique, Kiastakowsky went to town on this, investigating the heat of hydrogenation of a huge variety of alkenes. [Note 3<\/a>] In the following decades, even more data has been accumulated, which is easily obtainable (with references) from the NIST Chemistry Web Book.<\/p>\n

    For our purposes, there are\u00a0six\u00a0<\/strong>substitution patterns on an alkene (seven if you count ethene).<\/p>\n

    \"six-types-of-alkene-substitution-monosubstituted-disubstituted-trisubstituted-tetrasubstituted\"<\/a><\/p>\n

    The most notable trend that was found is that the heat of hydrogenation<\/strong>\u00a0decreases<\/strong> as C-H bonds are replaced with C-C bonds.\u00a0<\/strong><\/p>\n

    So what does that\u00a0mean?\u00a0<\/em><\/p>\n

    Since the same bonds are formed and broken in every hydrogenation reaction, the heat of hydrogenation is measuring the stability<\/strong> of each type of alkene.<\/p>\n

    This means that the lower the heat of hydrogenation, the greater the stability of the alkene.<\/strong><\/p>\n

    The way to visualize “stability” here is to compare it to potential energy, much like a ball becomes more “unstable” with increasing height.<\/p>\n