Posts Tagged ‘IRC’

Dynamic effects in nucleophilic substitution at trigonal carbon (with Na+) revisited.

Monday, August 13th, 2012
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This reaction looks simple but is deceptively complex. To recapitulate: tolyl thiolate (X=Na) reacts with the dichlorobutenone to give two substitution products in a 81:19 ratio, a result that Singleton and Bogle argue arises from a statistical distribution of dynamic trajectories bifurcating out of a single transition state favouring 2 over 3. On the grounds (presumably) that the presence of both the cation X (=Na+) and H-bonded solvent (ethanol) are uninfluential, neither species was explicitly included in the transition state model used to derive the dynamics. I speculated whether in fact the spatial distribution of counterions and solvent (set up by explicit hydrogen bonds and O…Na+ interactions) might in fact be perturbed from un-influential randomness by co-ordination to the carbonyl group present in the system. I also raised the issue of what the origin of the electronic effects leading to the major product might be. 

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The SN1 Mechanism for a third time. Exploration of the intrinsic reaction coordinate.

Tuesday, October 25th, 2011
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As the title hints, I have been here before. The SN1 solvolysis mechanism of t-butyl chloride was central to the flourishing of physical organic chemistry from the 1920s onwards, and it appears early on in most introductory lecture courses and text books. There we teach that it is a two-stage mechanism. Firstly the C-Cl bond heterolyses to form a stable tertiary carbocation intermediate, which in a second stage reacts with nucleophile (water) to form e.g. t-butanol. This is contrasted with the SN2 mechanism, where these two stages are conflated into a single concerted process, involving no intermediates. Here I explore an intrinsic reaction coordinate for the hydrolysis of t-butyl chloride which attempts to tease out whether this simple picture is realistic.

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Anatomy of a simple reaction: the hydration of an alkene.

Sunday, September 4th, 2011
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The hydration of an alkene by an acid is one of those fundamental reactions, taught early on in most chemistry courses. What can quantum mechanics teach us about the mechanism of the reaction?

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