{"id":10917,"date":"2017-08-25T05:00:51","date_gmt":"2017-08-25T09:00:51","guid":{"rendered":"https:\/\/www.masterorganicchemistry.com\/?p=10917"},"modified":"2025-10-21T10:52:11","modified_gmt":"2025-10-21T15:52:11","slug":"how-do-we-know-methane-is-tetrahedral","status":"publish","type":"post","link":"https:\/\/www.masterorganicchemistry.com\/2017\/08\/25\/how-do-we-know-methane-is-tetrahedral\/","title":{"rendered":"How Do We Know Methane (CH4) Is Tetrahedral?"},"content":{"rendered":"

What Do The Valence Electrons Of Carbon Tell Us About The Bonding In CH4<\/sub>?<\/strong><\/p>\n

(Hint: since the dipole moment of CH4<\/sub> is zero, the answer is, “not enough”)<\/em><\/strong><\/p>\n

If the orbital\u00a0 configuration\u00a0 of carbon is 2s2<\/sup>2p2\u00a0<\/sup>, then how can we use this information to figure out what the arrangement of the orbitals are in a simple organic molecule like methane (CH4<\/sub>)?<\/p>\n

It turns out that methane is tetrahedral, with 4 equal bond\u00a0 angles of 109.5\u00b0 and 4 equal bond lengths, and no dipole moment.<\/p>\n

This brings up two questions. First, how do we know<\/em> that CH4<\/sub> is tetrahedral? And secondly, how do\u00a0 we reconcile this\u00a0 electronic configuration (2s2<\/sup>2p2\u00a0<\/sup>) with the fact that we have\u00a0four<\/strong> equal C\u2013H bonds?<\/p>\n

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

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

    \n
  1. The Electronic Configuration Of The Valence Electrons Of Carbon Is 2s2<\/sup>2p2<\/sup><\/a><\/li>\n
  2. Can We Use This Information To Figure Out The Structure Of Methane (CH4<\/sub>)? (Spoiler: No)<\/a><\/li>\n
  3. Maybe Methane (CH4<\/sub>) Is Square Planar?<\/a><\/li>\n
  4. Disproving The Square Planar Structure Of CH4<\/sub> (1874) And Proposal Of A Tetrahedral Structure<\/a><\/li>\n
  5. Tetrahedral Carbons: Not A Popular Idea In 1874<\/a><\/li>\n
  6. So What Orbitals ARE Involved?<\/a><\/li>\n
  7. Notes<\/a><\/li>\n<\/ol>\n
    \n

    <\/a>1. The Electronic Configuration Of The Valence Electrons Of Carbon Is 2s2<\/sup>2p2<\/sup><\/h2>\n

    In our review of atomic orbitals<\/a>, we saw that the orbital configuration of the valence electrons<\/strong> of carbon is 2s2<\/sup>2p2<\/sup> as shown below:<\/p>\n

    \"electron-configuration-for-carbon-atom-shown-as-potential-energy-diagram-with-valence-electrons-4-and-filled-1s-electrons\"<\/p>\n

    Since the 2s orbital is lower in energy than 2p, it’s filled first. That means that there are two electrons in the 2s orbital, and a single electron in two of the three 2p orbitals. There’s also an empty 2p orbital.<\/p>\n

    [In addition, there are two electrons in the “inner shell” 1s orbital, which are not available for bonding].<\/em><\/span><\/p>\n

    <\/a>2. Can We Use This Information To Figure Out The Structure Of Methane (CH4<\/sub>)? (Spoiler: No)<\/h2>\n

    So far so good. This is fine if we’re just talking about isolated carbon atoms.<\/p>\n

    But in order to be truly useful, we need to be able to relate the orbitals of carbon to the structure and bonding of actual organic compounds.<\/p>\n

    The simplest organic compound is methane, CH4<\/sub>. So\u00a0let’s bring four hydrogen atoms into the picture and try to apply what we’ve learned to come up with some hypotheses about the bonding in this molecule.<\/p>\n

    \"valence-obrtials-of-carbon-look-like-this-with-three-p-orbitals-and-a-2s-orbital\"<\/p>\n

    The 3 p-orbitals in carbon are all at 90 degrees to each other, along the x, y, and z axes.<\/p>\n

    Shouldn’t we expect that the structure of methane would have three C-H bonds for each of the p orbitals (at 90 degrees to each other) and then the fourth C-H bond attached to the 2s orbital? Since electron pairs repel, maybe we should put that C-H bond the maximum distance away from the other C-H bonds; this would give an H\u2013C\u2013H bond angle of 135\u00b0.<\/p>\n

    Following this logic would give a structure like this:<\/p>\n

    \"reasonable-but-wrong-proposal-for-ch4-bonding-why-dont-c-h-bonds-line-up-along-x-y-and-z-axes-with-h-c-h-bond-angles-of-90-degrees\"As it turns out, it can be shown that this proposal is wrong.<\/p>\n

    Why?<\/strong><\/em><\/p>\n

    Dipole moment.<\/p>\n

    Recall that each C\u2013H bond has a small dipole<\/a> due to the difference in electronegativity between C (2.5) and H (2.2). We expect C to be partially negative and H to be partially positive. (See article: Dipoles and Dipole Moments<\/a>)<\/em><\/span><\/p>\n