{"id":8072,"date":"2014-03-24T11:43:37","date_gmt":"2014-03-24T16:43:37","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8072"},"modified":"2026-05-01T11:58:22","modified_gmt":"2026-05-01T16:58:22","slug":"cycloalkanes-how-to-calculate-ring-strain","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/03\/24\/cycloalkanes-how-to-calculate-ring-strain\/","title":{"rendered":"Calculation of Ring Strain In Cycloalkanes"},"content":{"rendered":"

Ring Strain In Cycloalkanes (1) – Calculation of Ring Strain<\/strong><\/p>\n

This post is all about how ring strain is calculated. If you want more specific details about ring strain in cyclopropane and cyclobutane, I suggest moving on to the next post.<\/p>\n

In the last post we learned about one consequence of the fact that carbon can form rings – that we can form stereoisomers (cis \/ trans).\u00a0 <\/span>This post attempts to explain another very interesting consequence of ring formation: ring strain<\/strong>.<\/p>\n

It all starts with this: it turns out you can learn a lot about molecules by burning them<\/strong>. In a controlled and quantified way, of course. : – )<\/p>\n

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

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

    \n
  1. Heat of Combustion of Alkanes<\/a><\/li>\n
  2. Heat of Combustion As A Function of Chain Length<\/a><\/li>\n
  3. The Heat Of Combustion For A Very Long Chain Is About 157 kcal\/mol Per CH2<\/sub><\/a><\/li>\n
  4. Cycloalkanes Have The General Formula (CH2<\/sub>)n<\/sub><\/a><\/li>\n
  5. Any Deviation From An Average Value of 157 kcal\/mol Per CH2 Represents Ring Strain<\/a><\/li>\n
  6. What Factors Might Lead To Ring Strain?<\/a><\/li>\n
  7. Notes<\/a><\/li>\n
  8. Quiz Yourself!<\/a><\/li>\n
  9. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Heat of Combustion Of Alkanes<\/strong><\/h2>\n

    The molar heat of combustion, as you might recall from gen chem, is the energy released upon burning one mole of a substance. It\u2019s a value of enthalpy [\u0394 H] – usually measured in kJ\/mol or kcal\/mol. We\u2019re going to use kcal\/mol here (See post: Why Do Organic Chemists Use Kilocalories<\/a><\/em><\/span>) but to convert to kJ\/mol, multiply by 4.184 and you get the same thing<\/strong>.<\/p>\n

    Let\u2019s start with something simple. Imagine we had a series of straight chain alkanes. Ethane, propane, butane, all the way up to dodecane (12).<\/p>\n

    As we increase the number of carbons, the increase energy released upon combustion will increase. I like to use the analogy of an infinite staircase: with every step we go higher, we\u2019re increasing the distance between us and the floor, and therefore increasing the energy released if we were to fall from the staircase down to the ground : – ) .\u00a0<\/strong><\/p>\n

    \"staircase-with-stair-number-edited-into-photo\"<\/p>\n

    <\/a>2. Heat of Combustion As A Function Of Chain Length<\/h2>\n

    Let’s ask a simple question: how does the heat of combustion change<\/strong> as we add extra carbons?<\/p>\n

    Do you think we would we expect to see an “equal spacing” of energies as we successively add a carbon (by analogy to a staircase with equally spaced steps)? Or would the spacing gradually change? \u00a0And, importantly, could we use this information to make predictions?<\/p>\n

    Let\u2019s burn<\/strong> various straight chain hydrocarbons and measure how much energy (in kcal\/mol) is released by each molecule.
    \n<\/span><\/p>\n

    We can then calculate the average energy released per carbon – this gives us the “spacing” between each step.\u00a0<\/strong>This will help us predict the energy released for the next step.\u00a0<\/span><\/p>\n

    [Here, use your imagination as we burn hydrocarbons of varying length and measure the heat of combustion for each case].<\/em><\/span><\/p>\n

    Let’s now look at the data. When we do this, we see the kcal\/mol per carbon starts high: 213 for methane (not shown) , \u00a0186 (for ethane) and then starts reaching a limit a little bit lower than 160 kcal\/mol. This is like a staircase where the steps start off large, but gradually decrease towards a\u00a0 uniform height.\u00a0<\/strong><\/p>\n

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

    <\/a>3. The Heat Of Combustion For A Very Long Straight Chain Alkane is About 157 kcal\/mol Per CH2<\/sub><\/h2>\n

    <\/a>What\u2019s up with the curve, by the way? Very subtle question. Answer is long, so it’s in Note 1<\/strong><\/a>.<\/p>\n