{"id":8393,"date":"2014-07-23T13:32:15","date_gmt":"2014-07-23T18:32:15","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8393"},"modified":"2025-12-12T04:19:54","modified_gmt":"2025-12-12T10:19:54","slug":"which-cyclohexane-chair-is-of-lower-energy","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/07\/23\/which-cyclohexane-chair-is-of-lower-energy\/","title":{"rendered":"Cyclohexane Chair Conformation Stability: Which One Is Lower Energy?"},"content":{"rendered":"

Finding The Most Stable Conformation Of A Cyclohexane Chair<\/strong><\/p>\n

You’re given a structure with two or more substituents on a cyclohexane ring, and you’re asked to draw the most stable conformation. How do you do that? That’s what this post is about.<\/p>\n

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

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

    \n
  1. A-Values Are A Useful Measure of Bulkiness<\/a><\/li>\n
  2. A-Values Are Additive<\/a><\/li>\n
  3. Example: Determining The Most Stable Conformation Of\u00a0cis<\/em>– And\u00a0trans-\u00a0<\/em>1,2-Dimethylcyclohexane<\/a><\/li>\n
  4. To Determine Chair Conformation Stability, Add Up The A-Values For Each Axial Substituent. The Lower The Number, The More Stable It is.\u00a0<\/a><\/li>\n
  5. Summary: Stability of Cyclohexane Conformations<\/a><\/li>\n
  6. Notes<\/a><\/li>\n
  7. Quiz Yourself!\u00a0<\/a><\/li>\n
  8. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. A-Values Are A Useful Measure Of Bulkiness<\/strong><\/h2>\n

    In the last post<\/strong>, we introduced A values<\/strong> and said they were a useful tool for determining which groups are “bulkiest” on a cyclohexane ring. (See post: Ranking the Bulkiness Of Substituents On Cyclohexane Rings With A-Values<\/a><\/em>)<\/p>\n

    The greater the A-value (bulk), the more favoured the equatorial conformer will be (versus axial). We saw that hydroxyl groups (OH) have a relatively low A-value (0.87), methyl groups are higher (1.7) and the t-butyl group is one of the highest of all (>4.5) .<\/p>\n

    We also saw that by knowing the A value (which is essentially the energy difference in kcal\/mol) we could figure out the % of axial and equatorial conformers in solution using the formula\u00a0\u0394G = \u2013RT ln K<\/strong><\/p>\n

    In this post we’re going to extend this concept and see what happens when we have MORE than one group on a cyclohexane ring.<\/p>\n

    <\/a>2. A-Values Are Additive<\/h2>\n

    The nice thing about A values is that they are\u00a0additive.\u00a0<\/strong>We can make the (safe) assumption that groups on adjacent carbons don’t bump into one another [Note 1<\/a>] so figuring out the torsional strain of a cyclohexane chair is simply a matter of\u00a0adding up the A values of the axial groups in any chair conformation.<\/p>\n

    We can apply this to cyclohexanes with two, three, or even more substituents.<\/p>\n

    Here are some examples:<\/p>\n

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

    <\/a>3. Example: Determining The Most Stable Conformation Of cis-<\/em> And\u00a0trans<\/em>– 1,2-Dimethylcyclohexane<\/h2>\n

    That’s nice, you might say, but when might we ever want to do that? The key example is when we are examining two chair conformers of the same molecule<\/strong>. A-values are essential in helping us figure out which one is most stable.<\/p>\n

    Here’s an example of the type of question we might be asked: draw the two chair conformations of cis-<\/em>1,2-dimethylcyclohexane and\u00a0trans<\/em>-1,2-dimethylcyclohexane, and determine which is most stable.<\/p>\n

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