Different Strains

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elizabethrojas1G
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Joined: Fri Sep 25, 2015 3:00 am

Different Strains

Postby elizabethrojas1G » Mon Mar 07, 2016 8:15 pm

What's the difference between torsional and steric strain? Would it be possible to have both at the same time?

004636950
Posts: 29
Joined: Fri Sep 25, 2015 3:00 am

Re: Different Strains

Postby 004636950 » Mon Mar 07, 2016 8:24 pm

Torsional Strain is the force that opposes the rotation of a part of a molecule about a bond. For example, an eclipsed conformation has lots of torsional strain. Steric strain is when atoms are bumping into each other. It is possible for a molecule to have both. For example, butane has both torsional and steric strain when the C1 and C4 atoms are eclipsed at 0 degrees.

004636950
Posts: 29
Joined: Fri Sep 25, 2015 3:00 am

Re: Different Strains

Postby 004636950 » Mon Mar 07, 2016 8:32 pm

There is also bond angle strain when the spa hybridized carbon doesn't have its 109.5 bond angle.

AuraCruzHeredia_1L
Posts: 34
Joined: Fri Sep 25, 2015 3:00 am

Re: Different Strains

Postby AuraCruzHeredia_1L » Mon Mar 07, 2016 8:35 pm

Torsional Strain: When rotating a molecule around a bond, the torsional strain is the repulsion caused by the electrons in between different groups when they pass by each other. (rotational)

--> Basically, C-C bonds can rotate so the placements of the substituents relative to each other influence the stability of the molecule. If you look at the chair diagram, for example, you can see that cyclohexane prefers a staggered conformation vs. an eclipsed conformation due to the electron repulsions.

Steric Strain: Strain caused by the electrons in between different groups. Steric strain cannot be lessened by rotating the molecule around a bond. (structural)

--> Basically, this is caused by the proximity of two substituents, but it can't be "fixed" by rotating the molecule into a different configuration but by breaking/creating new bonds. So physically moving a substituent to some other, further part of the molecule.

taken from http://www.chem.ucla.edu/harding/notes/strain_01.pdf (which has more info)

--Yeah, you can definitely have both kinds of strains. This obviously leads to a more unstable molecule.
So Total Strain = Steric + Torsional. The more of each, the less stable.

does that make sense? I can try to further simplify it if it doesn't make sense! let me know.


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