{"id":8402,"date":"2014-08-05T09:04:26","date_gmt":"2014-08-05T13:04:26","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8402"},"modified":"2026-05-07T09:18:00","modified_gmt":"2026-05-07T14:18:00","slug":"fused-rings","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/08\/05\/fused-rings\/","title":{"rendered":"Fused Rings – Cis-Decalin and Trans-Decalin"},"content":{"rendered":"

Why Is trans-Decalin More Stable Than cis-Decalin?<\/strong><\/p>\n

At the beginning of this series I said that the fact that carbon can form rings leads to all kinds of interesting consequences.\u00a0We’re going to see many examples of that in our post today!<\/p>\n

So far, we’ve only talked about cyclic molecules containing one ring. But, of course, molecules with multiple rings are very common in nature. A prime example is the steroid structure, exemplified by the common oral contraceptive ethinyl estradiol.<\/p>\n

\"drawing-of-the-oral-contraceptive-ethinyl-estradiol-showing-multiple-rings\"<\/p>\n

Here, we’re just going to talk about two very simple cases of molecules containing two rings. We’re going to focus our efforts on cyclohexane, and examine in close detail the situation where two cyclohexane rings are bonded to each other in the most common way – with the two ring junctions (“bridgeheads<\/strong>“) on adjacent carbons, a situation we refer to as\u00a0fused<\/em> rings.\u00a0<\/strong><\/p>\n

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

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

    \n
  1. cis<\/em>-Decalin and trans-<\/em>Decalin<\/a><\/li>\n
  2. A Model of\u00a0cis-<\/em>Decalin Shows It Has A Tent-Like Shape<\/a><\/li>\n
  3. A Model of\u00a0trans-<\/em>Decalin Shows It To Be Flat<\/a><\/li>\n
  4. Why Is\u00a0trans<\/em>-Decalin More Stable Than\u00a0cis-<\/em>Decalin? All Carbons Are Equatorial<\/a><\/li>\n
  5. cis-<\/em>Decalin Has Two\u00a0Gauche\u00a0<\/em>Interactions<\/a><\/li>\n
  6. Doing A “Ring-Flip” On\u00a0cis-<\/em>Decalin<\/a><\/li>\n
  7. trans<\/em>-Decalin Cannot Undergo Ring Flips. It Is Conformationally “Locked”<\/a><\/li>\n
  8. Summary:\u00a0cis<\/em>-Decalin versus\u00a0trans<\/em>-Decalin<\/a><\/li>\n
  9. Notes<\/a><\/li>\n
  10. Quiz Yourself!<\/a><\/li>\n
  11. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. cis-Decalin and trans<\/em>-Decalin<\/strong><\/h2>\n

    The parent molecule is called “decalin” since there are ten carbons in total. However, remember that any time we have two substituents on a cyclohexane ring (as we do here), it is essential to draw in the stereochemistry in order to avoid ambiguity! Two stereoisomers are possible here: one where the hydrogens at both ring junctions are “cis”, and the other where they are “trans”.<\/p>\n

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

    These molecules look so simple when we draw them on paper! \u00a0What could be simpler than two hexagons joined together?<\/p>\n

    Well, just like with cyclohexane itself, the complexity comes when we examine their most stable three-dimensional structures. Each six membered ring will adopt a chair conformation. Believe it or not, the cis<\/strong> and\u00a0trans<\/strong> stereoisomers of decalin\u00a0have\u00a0remarkably different shapes.<\/p>\n

    There’s no better way to see this than by making a model.<\/p>\n

    <\/a>2. A Model Of cis<\/em>-Decalin Shows It Has A Tent-Like Shape<\/h2>\n

    Here’s\u00a0cis-decalin<\/strong>. The hydrogens on the “bridgeheads” are highlighted in red and white. Notice how both cyclohexanes are in chair conformations, but the molecule adopts almost a tent-like shape.
    \n