Conformations

Fused Rings

August 5, 2014

At the beginning of this series I said that the fact that carbon can form rings leads to all kinds of interesting consequences. We’re going to see many examples of that in our post today! So far, we’ve only talked about cyclic molecules containing one ring. But, of course, molecules with multiple rings are very common [...]

Read the full article →

Which Cyclohexane Chair Is Of Lower Energy?

July 23, 2014

In the last post, we introduced A values and said they were a useful tool for determining which groups are “bulkiest” on a cyclohexane ring. The greater the A-value (bulk), the more favoured the equatorial conformer will be (versus axial). We saw that hydroxyl groups (OH) have a relatively low A-value (0.87), methyl groups are [...]

Read the full article →

Substituted Cyclohexanes: “A Values”

July 1, 2014

In the last post we saw that adding a methyl group to cyclohexane results in two chair conformers that are unequal in energy. We saw that the conformer where the methyl group was equatorial is the most stable, since it avoids destabilizing diaxial interactions (technically, gauche interactions) that are present in the conformer when the methyl group is axial. We also said [...]

Read the full article →

Substituted Cyclohexanes – Equatorial vs Axial

June 27, 2014

Just to bring you up to speed, here’s what we mentioned in the last post. At the bottom, I’ll also correct a little fib I made. 1. Cyclohexane, at room temperature, undergoes a conformational interconversion known as a “chair flip”. In this chair flip, all axial groups become equatorial, and all equatorial groups become axial.[but [...]

Read the full article →

The Cyclohexane Chair Flip – Energy Diagram

June 6, 2014

In the last post,  we showed a video of  a cyclohexane ring flip – turning a cyclohexane chair conformation into a boat and then into the opposite chair. The key observation we made here was that a chair flip converts all axial groups into equatorial groups and all equatorial groups into axial groups. However all [...]

Read the full article →

The Cyclohexane Chair Flip

May 30, 2014

In our last post, an aerial tour of the cyclohexane chair, we showed that there are two different positions a substituting can occupy on a cyclohexane chair – axial (straight up and down, relative to the ring) and equatorial (off to the side of the ring). This brings up an interesting thought experiment. Let’s take [...]

Read the full article →

How To Draw A Cyclohexane Chair

May 20, 2014

Now that we’ve had an aerial tour of the cyclohexane chair, we’re going to move on to a very important skill: how to properly draw one of these beasties. This takes practice. Your first few attempts in drawing a cyclohexane chair from memory might see you end up with one of these “variations”, which in their [...]

Read the full article →

An Aerial Tour Of The Cyclohexane Chair

May 14, 2014

  When I first learned about the cyclohexane chair (as we did in the last post) I was in denial about its importance in organic chemistry. From an aesthetic standpoint, it bothered me that cyclohexane wasn’t a beautifully flat hexagon. The cyclohexane chair was so UGLY to me, and I felt disappointment that I actually [...]

Read the full article →
Ring Strain in Cyclopentane and Cyclohexane

Ring Strain in Cyclopentane and Cyclohexane

April 18, 2014

In the last post, we saw that ring strain of cyclopropane and cyclobutane were 27 and 26 kcal/mol respectively.  They are the unhappiest of rings – constrained into uncomfortable angles, with hydrogens forced by geometry to grumpily line up side-by-side with their repulsive neighbours. The situation for cyclopentane (ring strain: 6 kcal/mol) and cyclohexane (ring strain: [...]

Read the full article →

Cycloalkanes – Ring Strain In Cyclopropane And Cyclobutane

April 3, 2014

In the last post we saw that cyclopropane and cyclobutane have an unusually high “ring strain” of 27 kcal/mol and 26 kcal/mol respectively.  We determined this by comparing heats of combustion from rings of various sizes, and saw that the ΔHcombustion per CH2 was essentially constant as ring sizes went above 12. Based on these [...]

Read the full article →