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Steric Repulsion - Simple Explanation?

  1. Aug 26, 2016 #1
    From here:

    https://in.answers.yahoo.com/question/index?qid=20100421234931AAnRi1H

    Please could someone tell me exactly what is causing the repulsion? When it says size of groups, what does it mean? What groups? What's a group? Shouldn't larger things attract, not repel? Is it a structural issue?

    Thanks for any help with a more simple explanation, I've checked Wiki and I'm not finding it much use tbh.
     
  2. jcsd
  3. Aug 26, 2016 #2

    DrClaude

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    Staff: Mentor

    I think that the basics are correctly explained in this Wikipedia page https://en.wikipedia.org/wiki/Steric_effects

    Repulsion means electron-electron repulsion. The definition of a group is
    [quote="IUPAC Gold Book"}
    A defined linked collection of atoms or a single atom within a molecular entity. This use of the term in physical organic and general chemistry is less restrictive than the definition adopted for the purpose of nomenclature of organic compounds.
    [/quote]
    The size of the group goes as the number of atoms. In the case of steric hindrance, it relates to the physical size occupied by the group, as this physically blocs reagents from attacking the molecule (or which side will be attacked).
     
  4. Aug 26, 2016 #3
    Many thanks for the reply!

    So basically in the physical size scenario its a reduction in the surface area, or reduction in the accessibility of reactive areas.

    Could I perhaps take this further? A paper I'm reading talks about adsorbed molecules on a bubbles surface. It proposes that if two of the bubbles were to approach then the adsorbed molecules on their respective surfaces could cause the bubbles to repel (via steric repulsion). I understand the bubbles may not get as close to one another due to, as you say, a physical blocking, but why in this instance would they repel? Where would the repelling force come from? Is it talking about a rebound? Or an actual repulsive force?
     
  5. Aug 26, 2016 #4

    Ygggdrasil

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    Science Advisor

    Here's a fairly general picture of the interaction between two molecules:
    tumblr_nsiaqbXsbQ1uwnewko2_1280.jpg
    This graph shows a Lennard-Jones potential. As molecules approach each other, they experience an attraction (due to van der Waals interactions). However, if they get too close, the overlap of their orbitals begins to create a repulsive force (called Pauli repulsion or Pauli exclusion). The distance at which these attractive and repulsive forces balance is the equilibrium bond length between the two molecules.
     
  6. Aug 26, 2016 #5
    hmm so why is it referred to specifically as steric repulsion and not Pauli repulsion?

    But excellent, that explains the repulsion aspect. Many thanks for the help!
     
  7. Aug 26, 2016 #6

    Ygggdrasil

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    Different fields have different names for the same phenomena. In general, in chemistry, when one talks about sterics it relates to the shape of the molecule. Steric complementarity means that two molecules have shapes that interlock well (allowing the molecules to get close so that van der Waals interactions across the entire interface can cooperate to hold the molecules together). Steric hinderance talks about how the shape of the molecule can interfere with the ability of other reactants to approach the molecule in the correct orientation for a reaction to occur. If you say steric repulsion to an undergraduate chemistry student they'll likely be able to figure out what you mean if they have not heard the term before. The same student wouldn't be likely to figure out what a term like Pauli exclusion or Pauli repulsion means unless they are already familiar with the term.
     
  8. Aug 26, 2016 #7
    I see, okay, thanks again!
     
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