Steric Repulsion - Simple Explanation?

[rwooduk]

From here:

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

Steric hindrance or steric resistance occurs when the size of groups within a molecule prevents chemical reactions that are observed in related smaller molecules. Although steric hindrance is sometimes a problem, it can also be a very useful tool, and is often exploited by chemists to change the reactivity pattern of a molecule by stopping unwanted side-reactions (steric protection). Steric hindrance between adjacent groups can also restrict torsional bond angles. However, hyperconjugation has been suggested as an explanation for the preference of the staggered conformation of ethane because the steric hindrance of the small hydrogen atom is far too small

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.

 

[DrClaude]

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).

 

[rwooduk]

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?

 

[Ygggdrasil]

Here's a fairly general picture of the interaction between two molecules:

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.

 

[rwooduk]

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!

 

[Ygggdrasil]

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.

 

[rwooduk]

I see, okay, thanks again!