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I know that in ethane, the extra energy present in the eclipsed conformer is caused by torsional strain.

In butane, the gauche conformation experiences steric strain. But the eclipsed conformation at 0 degrees has substantial amounts of both steric strain and torsional strain.

What is the difference?

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TL;DR Torsional strain can be thought as the repulsion due to electrostatice forces between electrons in adjacent MOs. Meanwhile steric strain (also known as van der Waals strain) can be thought as the repulsion when two bulky groups which are not directly bonded to each other become too close to each other and hence there isn't enough space for them.

Here is the more detailed version.

Torsional Strain

Lets consider an ethane molecule. The C-C sigma bond is free to rotate and in principle there are an infinite number of possible conformations. However only 2 are significant, these are staggered and eclipse conformations. Different conformers are usually drawn as Newman projections as they can easily be compared with each other. Below are the Newman projections for the eclipsed and staggered conformer:

enter image description here

The staggered conformer is the most stable conformer while the eclipsed conformer is the least stable conformer. The staggered conformer is approximately $\mathrm{12~kJ~mol^{-1}}$ more stable than the eclipsed conformer. This energy difference between this maxima and minima is known as the torsional barrier.

So what is torsional strain? The reason for the eclipse conformer being higher energy than any other conformers is due to the destabilising electrostatic repulsion between the electron pairs of the C-H sigma bonds on the two carbons. Furthermore, there also exists a stabilising feature which is greater in the staggered conformer. In the staggered conformer there is a constructive orbital interaction involving the bonding and anti-bonding MOs of the adjacent H atoms. This results in hyper conjugation which stabilises the compound.

enter image description here

The two effects that I have mentioned above is what torsional strain refer to. So you can think of torsional strain as the strain that is the result of electrostatic forces.

Steric Strain

Now lets consider butane. Rotation of the $\mathrm{C_2-C_3}$ sigma bond also lead to infinite possible conformers. However there are 4 main conformers shown below:

enter image description here

Here there are 2 types of eclipsed conformers which are the least stable of the conformers. The most unstable one is known as the eclipsed syn form. The syn form is approximately $\mathrm{20~kJ~mol^{-1}}$ higher in energy than the staggered conformer. The reason for this can partly be attributed to torsional strain as there is repulsion between the electrons in the sigma bonding orbitals. However also it can largely be attributed to the repulsion of the two relatively bulky methyl groups as they become too close to each other and there isn't enough space for them.

This repulsion is known as steric strain. Therefore steric strain can be define as the repulsion which occurs when non-bonded groups which are not directly bonded approach each other too closely. This repulsion only exists for bulky substituents, such as methyl or ethyl groups. So in ethane there is no steric hindrance as the hydrogen atoms are not that bulky.

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  • $\begingroup$ Nice answer and nice dashed line in the bottom right of the Newman projection image ;) $\endgroup$ – Jan Oct 3 '15 at 13:31
  • $\begingroup$ It's probably helpful to point out a few more things too. Torsional strain is defined as not existing when the conformations are staggered, so there is technically no torsional strain at 60, 120, and 180. Torsional strain also only ever exists between atoms separated by three bonds, such as the hydrogens in CH3-CH3, more precisely those three bonds are H-CH2-CH2-H. Steric strain exists only in atoms separated by four or more bonds, such as in propane (H-CH2-CH2-CH2-H). It's probably more helpful to consider bonds of more than four, however, such as butane. $\endgroup$ – Ryan Ward Jun 6 '16 at 15:50
  • $\begingroup$ Excluding the effect of MO constructive interference, do both torsional strain and steric strain arise from electrostatic repulsion? When you say that steric strain comes from the repulsion of bulky groups because there isn’t enough space for them, it’s also a repulsion between the electrons of those groups, correct? If you ignored the MO interference part, would it be suitable to consider steric strain as, in a way, additional torsional strain, as it were? $\endgroup$ – lightweaver Jun 25 '16 at 13:47
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    $\begingroup$ Thanks for the fantastic answer. To answer lightweaver's question, yes, there will be electron repulsion as well because the two bulky groups' filled sigma bond orbitals will approach each other and feel a "filled-filled" 4 e- interaction which is similar to the MO illustration above. $\endgroup$ – TOLA3HPPA Oct 4 '16 at 2:07
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For simplicity's sake, torsional strain is defined as as the strain experienced by the bonds when conformations are not staggered. So, at any angle other than 60, 120, or 180, there is torsional strain. Additionally, torsional strain can only ever exist in atoms separated by only three bonds.

Therefore, the hydrogens in ethane only ever experiences torsional strain, and it is zero (technically minimized) when it's staggered.

Ethane

Steric strain exists only in molecules who have four or more bonds, since steric strain is defined as the repulsion felt between atoms at four or more bonds separated from each other forced closer than their van der Waals radius would typically allow.

In the butane below, there is torsional strain between the two central carbons and hydrogens from each central methylene group. However, atoms separated by four or more bonds experience steric strain. Steric hindrance will never be zero, but it can be minimized when the atoms are separated by as much space as possible.

enter image description here

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  • $\begingroup$ How energy is increased if nothing adds energy to the molecule ? It violates the law of conservation of energy. $\endgroup$ – ado sar Feb 14 '19 at 21:51

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