{"id":10752,"date":"2017-05-10T13:18:42","date_gmt":"2017-05-10T17:18:42","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10752"},"modified":"2026-05-07T10:43:36","modified_gmt":"2026-05-07T15:43:36","slug":"the-pi-molecular-orbitals-of-cyclobutadiene","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/05\/10\/the-pi-molecular-orbitals-of-cyclobutadiene\/","title":{"rendered":"The Pi Molecular Orbitals of Cyclobutadiene"},"content":{"rendered":"

Cyclobutadiene: Molecular Orbital Diagram, Antiaromaticity, and Structure<\/strong><\/p>\n

Previously, we’ve seen what the molecular orbitals\u00a0of benzene look like<\/a>, and that the fact that they are partially duplexed (or to use the proper nomenclature, “degenerate<\/a>“) helps to explain benzene’s unusual stability.<\/p>\n

Let’s flip the coin. What about cyclobutadiene<\/strong>, a molecule we usually class as antiaromatic<\/a>. Why is it so unusually unstable<\/strong>?<\/p>\n

\"cyclobutadiene<\/p>\n

Again, examining the\u00a0pi molecular orbitals will give us some useful clues.\u00a0<\/strong><\/p>\n

Notably, in cyclobutadiene, the highest-occupied molecular orbitals are unpaired electrons<\/strong> of equal energy.<\/p>\n

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

    \n
  1. Building Up The Molecular Orbital Diagram Of Cyclobutadiene: The Lowest-Energy Molecular Orbital Has Zero Nodal Planes<\/a><\/li>\n
  2. The Highest-Energy Molecular Orbital Has Two Nodal Planes<\/a><\/li>\n
  3. The Two Intermediate pi Molecular Orbitals Each Have One Nodal Plane (two different ways)<\/a><\/li>\n
  4. The Molecular Orbital Diagram Of Cyclobutadiene Reveals Why Cyclobutadiene Is Extremely Unstable: It Has Unpaired Electrons Of Equal Energy<\/a><\/li>\n
  5. Summary: The Molecular Orbital Diagram of Cyclobutadiene<\/a><\/li>\n
  6. Notes<\/a><\/li>\n
  7. Quiz Yourself!<\/a><\/li>\n
  8. (Advanced) References and Further Reading<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. Building Up The Molecular Orbital Diagram Of Cyclobutadiene: The Lowest-Energy Molecular Orbital Has Zero Nodal Planes<\/h2>\n

    Today, let’s build up the orbitals of cyclobutadiene\u00a0using the principles we’ve discussed in previous posts [e.g. see this post on butadiene<\/em><\/a>] and see if we can gain some useful insights.<\/p>\n

    Cyclobutadiene has a pi system comprised of 4 individual atomic p orbitals and thus should have a total of 4 pi molecular orbitals.<\/strong><\/p>\n

    The lowest-energy molecular orbital: zero nodal planes<\/strong><\/p>\n

    Following our “apartment building” analogy from last time, \u00a0the lowest-energy molecular orbital (the “ground floor” of cyclobutadiene, if you will) \u00a0should have all phases of the p-orbitals aligned and zero nodal planes, like this:<\/p>\n

    \"lowest<\/p>\n

    <\/a>2. The Highest-Energy Molecular Orbital Has Two Nodal Planes<\/strong><\/h2>\n

    Conversely, the highest-energy pi orbitals (the “penthouse”) \u00a0will have all phases alternating, and thus have two<\/strong> nodal planes. (As we said last time, the “penthouse” is not exactly desirable real estate for electrons)<\/em><\/p>\n

    \"highest<\/p>\n

    <\/a>3. The Two Intermediate pi Molecular Orbitals Each Have One Nodal Plane (two different ways)<\/strong><\/h2>\n

    That leaves us with the intermediate pi orbitals, which each have a single<\/strong> nodal plane. As with benzene, there are two ways to place a single nodal plane on cyclobutadiene, either through the bonds, or through the atoms:<\/p>\n

    \"wo<\/p>\n

    That gives us our four molecular orbitals. Now lets populate them with the “tenants”: the pi electrons.<\/p>\n

    <\/a>4. The Molecular Orbital Diagram Of Cyclobutadiene Reveals Why Cyclobutadiene Is Extremely Unstable: It Has Unpaired Electrons Of Equal Energy<\/strong><\/h2>\n

    Cyclobutadiene has a total of 4 pi electrons. So ranking all the pi molecular orbitals by energy, and populating the orbitals according to Hunds rule, we get the following picture:<\/p>\n

    \"full<\/p>\n

    Can you see why\u00a0cyclobutadiene might be\u00a0unstable?<\/p>\n