Alkanes

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 →

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 →

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 →

Cycloalkanes – How To Calculate Ring Strain

March 24, 2014

In the last post we learned about one consequence of the fact that carbon can form rings – that we can form stereoisomers (cis / trans).  This post attempts to explain another very interesting consequence of ring formation. This whole post is about strain.  It all starts with this: it turns out you can learn [...]

Read the full article →
Cycloalkanes – Dashes And Wedges

Cycloalkanes – Dashes And Wedges

March 20, 2014

In the last post, we mentioned that one of the consequences of the fact that carbon can form rings is that small rings (less than 8 carbons) are so rigid that they can’t be turned inside out [video]. One of the important consequences of this, as we’ll see today, is that it leads to the [...]

Read the full article →
Introduction to Cycloalkanes (1)

Introduction to Cycloalkanes (1)

February 18, 2014

In the first few weeks of an organic chemistry class, we learn that: Carbon can form up to four single bonds  Carbon with four single bonds adopts a tetrahedral geometry (ideal bond angle: 109.5°) Compared to other atoms on the periodic table, [O, N, S, Si for example] carbon forms very strong bonds with itself [...]

Read the full article →
Synthesis (5) – Reactions of Alkynes

Synthesis (5) – Reactions of Alkynes

January 29, 2014

Today, we’re going to add the reactions of alkynes to our reaction map, which will bring to a close all the major reactions we’ve discussed so far in a typical first semester course. Like alkenes, the main pathway found in the reactions of alkynes is “addition” – that is, breaking the C-C π bond and forming [...]

Read the full article →