Conformations

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: [...]

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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 [...]

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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 [...]

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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 [...]

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A Modest Proposal

A Modest Proposal

October 2, 2012

Right around this time of year, tens of thousands of organic chemistry students are learning about the cyclohexane chair. It looks like this: It’s more of a deck chair than a wingback. See why we call it a chair? It’s got a “headrest” and a “footrest”? The cyclohexane chair form is more stable than flat [...]

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Khan Academy for Organic Chemistry – Part 2

September 21, 2012

Khan Academy for Organic Chemistry, Part 2: Bonding, Conformations, and Stereochemistry This second  installment of the guide to Khan Academy organic chemistry videos covers videos 13 to 26, covering the topics of chemical bonding, conformational analysis, and stereochemistry. It also goes into the naming conventions for alkenes. Video #13 Title: “sp3 Hybridized Orbitals And Sigma [...]

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How To Do Bond Rotations

How To Do Bond Rotations

July 6, 2012

Bond rotation can be a tricky skill for first timers, and it’s a subject that doesn’t really lend itself to a text-laden blog post. You just have to “see it in action”. This video walks through it using the “steering wheel” analogy. Hope it’s useful to students out there who are having difficulty with this [...]

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The Ups and Downs of Cyclohexanes

March 16, 2011

Drawing cyclohexane chair forms can be a tricky business. While it’s often intuitive to pick out which groups are up and down when they’re axial (i.e. pointing straight up or down), it’s tougher for beginners to see that equatorial groups (the ones pointing “somewhat up” or “somewhat down”) can be considered up or down too. [...]

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The World’s Cheapest Molecular Model Kit

February 11, 2011

I see a lot of students who have trouble visualizing how molecules appear from different directions.  Probably the best way to solve this problem is to buy and practice with a model kit, which really helps to make structures more concrete. However, as much as people like me nag their students to buy (and use) [...]

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The Meso Trap

January 12, 2011

Question: Are these molecules enantiomers, diastereomers, or the same? If you immediately recognized this as a molecule with an internal plane of symmetry (and thus an achiral molecule, incapable of having an enantiomer), congratulations. If not, you just fell into The Meso Trap. The Meso Trap is a common feature on exams and tests to [...]

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