{"id":8418,"date":"2014-08-14T12:18:54","date_gmt":"2014-08-14T16:18:54","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=8418"},"modified":"2026-01-06T10:19:47","modified_gmt":"2026-01-06T16:19:47","slug":"bridged-bicyclic-compounds-and-how-to-name-them","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2014\/08\/14\/bridged-bicyclic-compounds-and-how-to-name-them\/","title":{"rendered":"Naming Bicyclic Compounds – Fused, Bridged, and Spiro"},"content":{"rendered":"

Bicyclic Compounds – Fused Rings, Bridged Bicyclic Rings, and Spiro Bicyclic Rings<\/strong><\/p>\n

In the previous post<\/a> we started our discussion of\u00a0structures with more than one ring, using decalin<\/strong> as our key example of a fused ring<\/strong>. We saw how much the stereochemistry at the ring junction can affect the overall shape of the molecule, as well as its stability.<\/p>\n

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

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

    \n
  1. Ring Junctions: Fused Rings, Bridged Bicyclic Rings, and Spiro Rings<\/a><\/li>\n
  2. In Or Out? Either Both In, Or Both Out!<\/a><\/li>\n
  3. Putting Bridged Bicyclic Compounds\u00a0 In Perspective<\/a><\/li>\n
  4. Naming Bridged Bicyclic Compounds – A Five-Step Guide<\/a><\/li>\n
  5. A Few More Examples Of Bridged Bicyclic Molecules<\/a><\/li>\n
  6. Naming Spiro Compounds<\/a><\/li>\n
  7. Summary: Naming Bicyclic Compounds<\/a><\/li>\n
  8. Notes (and a complex example)<\/a><\/li>\n
  9. Quiz Yourself!<\/a><\/li>\n
  10. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Fused Rings, Bridged Bicyclic Rings, And Spiro Rings<\/strong><\/h2>\n

    What we didn\u2019t talk about is that the ring junction of decalin represents\u00a0only\u00a0one<\/b> way to arrange the ten carbons of decalin into a pair of adjacent rings: the so-called \u201cfused\u201d ring structure, where the two “bridgehead” carbons are directly connected.<\/p>\n

    There are actually two other modes of ring junction.<\/em> In \u201cbridged bicyclic<\/strong>\u201d molecules, the two bridgeheads are separated by \u201cbridges\u201d containing at least one carbon. In \u201cspiro<\/strong>\u201d fused molecules, the two rings are both joined at the same carbon.<\/p>\n

    In this post we\u2019ll focus on \u201cbridged bicyclic\u201d molecules (and how to name them), and also briefly touch on \u201cspiro\u201d fused bicyclic molecules.<\/strong><\/p>\n

    By the way, in the diagram below, only one “bridged bicyclic” isomer of decalin with a \u00a0six-membered ring is drawn. Can you find the other bridged bicyclic isomer that also contains a six-membered ring? (Answer in Note 1<\/a>)<\/p>\n

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

    <\/a>2. In Or Out? Either Both In, Or Both Out.\u00a0<\/strong><\/h2>\n

    First, stereochemistry. One important note about bridged bicyclic molecules is that the two carbons forming each bridge will always be \u201ccis\u201d to each other, never trans, relative to the other ring.* [exceptions? yes, but you probably won’t see ’em – check<\/span> Note 2<\/a>\u00a0if you’re curious<\/em>]<\/p>\n

    Why? Like we said in the last post when discussing trans-decalin, for much the same reason that you can\u2019t kiss yourself on the back\u00a0– there just isn\u2019t enough flexibility for this to happen without breaking something<\/strong>.\u00a0 Given the strict bond angle (109\u00b0) and length (1.50 \u00c5) requirements of alkanes, five, six, and seven membered rings don\u2019t have enough slack to tolerate anything other than cis<\/i> ring junctions.\u00a0 A trans<\/i> ring junction (much like a trans<\/i> double bond in rings of these sizes) would simply lead to too much ring strain<\/strong>.<\/p>\n

    Note that in the diagram below, the \u201ccis\u201d ring junctions is implied by the fact that both hydrogens are on the same side (both dashes in this case). The \u201cimpossible\u201d trans ring junction is shown in grayscale for comparison. The picture shows a model of this molecule highlighting the angle strain<\/strong> and Van Der Waals strain<\/strong> resulting from this arrangement.<\/p>\n

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

    <\/a>3. Putting Bridged Compounds In Perspective<\/h2>\n

    Next – and this is where a lot of students get confused<\/strong> – we come to the topic of how to depict these things<\/strong>. Merely using dashes and wedges doesn\u2019t truly capture their three-dimensional beauty.<\/p>\n

    So when drawing bridged bicyclic molecules, it\u2019s very common to show them in perspective from the side<\/strong>. The first reaction my students have upon seeing these drawings is utter disgust and confusion. \u201cWhat is THAT<\/em>?\u201d, I recall Mike from Sault Ste. Marie asking me one night during a tutoring session.<\/p>\n

    It\u2019s the same thing as the \u201ctop down\u201d view, just drawn from a different perspective<\/strong>. Using the \u201ctop down\u201d view is a perfectly acceptable way to draw these molecules – however, it\u2019s vital to be able to interpret <\/i><\/b>these \u201cperspective\u201d drawings as many bicyclic molecules in your textbook will be shown in this way.<\/p>\n

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

    Here\u2019s a \u201cflyover\u201d view of the same molecule, from top-down to the side-view. [BTW: note how the six membered ring on the left is in the chair conformation]<\/em><\/span><\/p>\n