Alkanes

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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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 [...]

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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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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 [...]

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Synthesis (2) – Reactions of Alkanes

Synthesis (2) – Reactions of Alkanes

December 19, 2013

In this post we’re going to begin building our reaction map, starting with the simplest organic compounds of all: alkanes.*We’ve only learned one synthetically important class of alkane reaction: free-radical halogenation. [Combustion is also technically a reaction of alkanes, but producing CO2 and H2O from organic starting materials is not very useful from a synthetic [...]

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Halogenation At Tiffany’s

November 13, 2013

As we discussed in the last post on radicals, bromine radicals are considerably more selective than chlorine radicals in the halogenation of alkanes. For example, a hydrocarbon exposed to Br2 in the presence of heat or light (for initiation) can selectively be brominated at the tertiary C–H bond, resulting in one major product:In contrast, free-radical [...]

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Selectivity In Free Radical Reactions

September 23, 2013

In the last post we showed some examples of how different isomers might be formed in free-radical halogenation of alkanes. And I left off with a question.  If we performed a free-radical chlorination on propane with 1 equiv Cl2 under normal conditions (25°C, initiated with light), what  be the expected ratio of 1-chloropropane and 2-chloropropane? Notice [...]

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Isomers From Free Radical Reactions

September 17, 2013

Last time we covered a comparatively simple reaction: free-radical chlorination of methane to (CH4) to give chloromethane (CH3Cl) and saw that the reaction proceeds through three stages – initiation (where free radicals are created), propagation (the main “product-forming” step of the chain reaction, where a chloroalkane is created without net formation of new free radicals) [...]

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Initiation, Propagation, Termination

September 6, 2013

Waaay back when we started to go through free radical substitution reactions, you might recall that we looked a reaction like this one:   Now that we know a little more about what free radicals are and their key properties, the question for today is, “how does this reaction work?“. We’re going to go through the [...]

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