{"id":20764,"date":"2020-06-26T08:00:59","date_gmt":"2020-06-26T13:00:59","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=20764"},"modified":"2026-04-22T10:13:42","modified_gmt":"2026-04-22T15:13:42","slug":"electrocyclic-ring-opening-and-closure-2-six-or-eight-pi-electrons","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2020\/06\/26\/electrocyclic-ring-opening-and-closure-2-six-or-eight-pi-electrons\/","title":{"rendered":"Electrocyclic Ring Opening And Closure (2) – Six (or Eight) Pi Electrons"},"content":{"rendered":"

Electrocyclic Ring Opening And Closure (2) – Systems With 6 (and 8) Pi-Electrons.<\/strong><\/p>\n

In the last post we introduced electrocyclic ring opening and closure, with conjugated systems of two pi bonds (4 pi electrons). [See Post<\/em>: Electrocyclic Reactions<\/a>] . We saw that systems with 4pi electrons undergo “conrotatory<\/strong>” ring closure (and opening) under thermal conditions, and “disrotatory<\/strong>” ring closure (and opening) under photochemical conditions. [We’ll re-introduce those terms in the article below<\/em>]<\/p>\n

So what happens if we add an extra pi bond to the system, giving us 6 pi-electrons?\u00a0 How does this change things?<\/p>\n

In case you want to skip right to the answer, here’s the short summary.<\/p>\n

\"0-summary-thermal<\/a><\/p>\n

Electrocyclic ring-closing and ring opening in 6 pi systems occur in exactly the opposite sense of those in 4 pi systems.<\/p>\n

Why is that, you might ask? Great question!\u00a0 \u00a0In the article below, we’ll go through this subject from the beginning.<\/p>\n

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

    \n
  1. 6 pi Thermal Electrocyclic Ring Opening and Closing<\/a><\/li>\n
  2. Going “Under The Hood” of 6-\u03c0 Electrocyclic Ring Closure: Identifying The Highest-Occupied Molecular Orbital (HOMO)<\/a><\/li>\n
  3. Disrotatory and Conrotatory Ring Closure: The Importance of “Orbital Symmetry”<\/a><\/li>\n
  4. What About Electrocyclic Ring Opening?<\/a><\/li>\n
  5. Electrocyclic Reactions Are Stereospecific<\/a><\/li>\n
  6. Electrocyclic Ring Closure (And Opening) Under “Photochemical” Conditions<\/a><\/li>\n
  7. Summary: 6 pi Electrocyclic Ring Opening and Closure<\/a><\/li>\n
  8. [Bonus]\u00a0 What About 8 pi Systems?<\/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. 6 pi Thermal Electrocyclic Ring Opening and Closing<\/h2>\n

    So when we say, a “4 pi” , “6 pi”, or even “n-pi” system, what’s meant by that?<\/p>\n

    What we mean is a system of 4, 6 or even “n”\u00a0 atoms bearing p-orbitals able to overlap (“conjugate”) such that a set of 4, 6, or “n”\u00a0 pi molecular orbitals <\/strong>is formed. \u00a0[See: Conjugation and Resonance<\/a>].<\/em> Since each pi-bond is comprised of two adjacent p orbitals, a molecule with three conjugated pi bonds results in a “pi system” containing 6 pi molecular orbitals.<\/p>\n

    Let’s start with the simplest possible 6 pi example that can undergo electrocyclic ring closure: (Z)- hexatri-1,3,5-ene. When this triene is heated, a new species is formed that looks like this:<\/p>\n

    \"1-thermal<\/a><\/p>\n

    [Note: this wouldn’t this work for the\u00a0E isomer. Why not? [See Note 1]\u00a0<\/a><\/em><\/p>\n

    Note the bonds that form and break here. We’re breaking<\/strong> 3 C-C pi bonds and forming<\/strong> two C-C pi bonds along with a sigma bond.<\/p>\n

    The reverse of electrocyclic ring-closure is electrocyclic ring-opening.\u00a0. Heating 1,3-cyclohexadiene will result in at least some (Z)- <\/em>hexa-1,3,5-triene:<\/p>\n

    \"2-thermal<\/a><\/p>\n

    Note that the bonds that form and break here are the exact opposite<\/strong> than those found in the forward reaction.<\/p>\n

    Both electrocyclic ring-opening and ring-closure proceed in a concerted fashion (with no intermediate) and pass through a common transition state. These reactions belong to a class of concerted reactions with cyclic transition states known as pericyclic reactions<\/strong>, which includes the Diels-Alder<\/a>, the Cope and Claisen rearrangements<\/a>, and many others.<\/p>\n

    Since the reactions are reversible and pass through the same transition state, we can also draw it up as an equilibrium.<\/p>\n

    \"3-equilibrium<\/a><\/p>\n

    Note 2<\/a> about the equilibrium shown here.<\/p>\n

    <\/a>2. Going “Under The Hood” of 6-\u03c0 Electrocyclic Ring Closure: Identifying The Highest-Occupied Molecular Orbital (HOMO)<\/h2>\n

    If you look at the bonds formed \/ bonds broken table, you’ll see that there’s a net destruction<\/strong> of one pi bond and net formation<\/strong> of one sigma\u00a0 bond.<\/p>\n

    So what’s\u00a0actually<\/em>\u00a0going on when hexatriene is being converted to 1,3 cyclohexadiene?<\/p>\n

    Well, the new sigma bond is formed from the<\/strong> overlap of two p-orbitals<\/strong> as the ends of the hexatriene pi system (carbon 1 and carbon 6) rotate, and the hybridization of these carbons changes from sp2<\/sup> to sp3<\/sup> . (We’ll get to this in a second).<\/p>\n

    The pi molecular orbital involved in this transformation will be the one in which the electrons are the most loosely-held<\/strong>, or in other words, those of highest energy<\/strong>.<\/p>\n

    What are the highest-energy electrons in this case? The electrons in the highest-energy molecular orbital<\/strong> (HOMO for short).<\/p>\n

    So how do we know what the HOMO looks like? Glad you asked.<\/p>\n

    We covered how to build up pi molecular orbitals in this earlier post. [See: Pi molecular orbitals of butadiene<\/a><\/em>]<\/p>\n

    The pi molecular orbitals of 1,3,5 hexatriene look like this:<\/p>\n

    \"4-MOs<\/a><\/p>\n

    That’s a lot to absorb!\u00a0 Thankfully, for our purposes, we don’t need to use all<\/em> the molecular orbitals of 1,3,5 hexatriene in this instance.<\/p>\n

    What we really care about here is the highest-energy (most loosely-held) electrons; the electrons in the highest-occupied molecular orbital (HOMO)<\/strong>. In hexatriene, that’s \u03c03, which we’ll draw out below:<\/p>\n

    \"5-HOMO<\/a><\/p>\n

    We can simplify things even further. For the purposes of forming the new sigma bond, we’re only going to concern ourselves with the orbitals on C1 and C6<\/strong>, since these are the orbitals involved in forming the new sigma bond.<\/p>\n

    \"6-ends<\/a><\/p>\n

    Now we can map this on to 1,3,5 hexatriene drawn in the conformation where the reaction will occur.<\/p>\n

    \"7-superimpose<\/a><\/p>\n

    Now what?<\/p>\n

    <\/a>3. Disrotatory and Conrotatory Ring Closure: The Importance of “Orbital Symmetry”<\/h2>\n

    In order for the new sigma bond to form, C1 and C6 must rotate such that there is\u00a0constructive overlap<\/strong> orbitals. They must have the same phase.<\/p>\n

    In a 6 pi system like 1,3,5 hexatriene, the orbitals on each of the termini (C1 and C6) each have their phases aligned in the same<\/strong> direction<\/strong> (they are arbitrarily drawn with the “shaded” lobe pointing up, but it would work just as well if they were both drawn with the shaded lobes pointing down). [If you like, you can call this a “symmetric” orientation; the term “orbital symmetry” is generally used to refer to the relative orientation of these lobes.]\u00a0<\/em><\/span><\/p>\n

    Sigma bond formation can thus be achieved if one carbon rotates clockwise<\/strong> and the other counter-clockwise<\/strong>. This is referred to as a disrotatory<\/strong> mode of bond formation.<\/p>\n

    \"8-thermal<\/a><\/p>\n