Posts Tagged ‘energy profile’
Thursday, April 4th, 2019
Previously, I explored the Graham reaction to form a diazirine. The second phase of the reaction involved an Sn2′ displacement of N-Cl forming C-Cl. Here I ask how facile the simpler displacement of C-Cl by another chlorine might be and whether the mechanism is Sn2 or the alternative Sn1.
The reason for posing this question is that as an Sn1 reaction, simply ionizing off the chlorine to form a diazacyclopropenium cation might be a very easy process. Why? Because the resulting cation is analogous to the cyclopropenium cation, famously proposed by Breslow as the first example of a 4n+2 aromatic ring for which the value of n is zero and not 1 as for benzene.[1] Another example of a famous “Sn1” reaction is the solvolysis of t-butyl chloride to form the very stable tertiary carbocation and chloride anion (except in fact that it is not an Sn1 reaction but an Sn2 one!)
(more…)
References
-
R. Breslow, "SYNTHESIS OF THE s-TRIPHENYLCYCLOPROPENYL CATION", Journal of the American Chemical Society, vol. 79, pp. 5318-5318, 1957. http://dx.doi.org/10.1021/ja01576a067
Tags:animation, Carbenium ion, Cations, Chemical elements, chemical reaction, Chemistry, Chlorine, computational chemistry, Cyclopropenium ion, Diazirine, energy, energy profile, free energy, Halogens, Natural sciences, Nucleophilic aromatic substitution, Oxidizing agents, Physical sciences, potential energy surface, SN1 reaction, Substitution reactions
Posted in reaction mechanism | No Comments »
Monday, February 18th, 2019
Students learning organic chemistry are often asked in examinations and tutorials to devise the mechanisms (as represented by curly arrows) for the core corpus of important reactions, with the purpose of learning skills that allow them to go on to improvise mechanisms for new reactions. A common question asked by students is how should such mechanisms be presented in an exam in order to gain full credit? Alternatively, is there a single correct mechanism for any given reaction? To which the lecturer or tutor will often respond that any reasonable mechanism will receive such credit. The implication is that a mechanism is “reasonable” if it “follows the rules”. The rules are rarely declared fully, but seem to be part of the absorbed but often mysterious skill acquired in learning the subject. These rules also include those governing how the curly arrows should be drawn.† Here I explore this topic using the Graham reaction.[1]‡
(more…)
References
-
W.H. Graham, "The Halogenation of Amidines. I. Synthesis of 3-Halo- and Other Negatively Substituted Diazirines1", Journal of the American Chemical Society, vol. 87, pp. 4396-4397, 1965. http://dx.doi.org/10.1021/ja00947a040
Tags:/RT, activation energy, activation free energy, animation, arrow pushing, arrow-head, cellular telephone, Chemical kinetics, chemical reaction, Chemistry, computed energy, Ed Smith, energy, energy maximum, energy minima, energy plot, energy profile, energy surface, free energy, lecturer, mechanism, Natural sciences, Organic chemistry, overall reaction energy, Physical sciences, Reaction rate constant, Resonance, Transition state, Transition state theory, tutor, Tutorial
Posted in Curly arrows, Interesting chemistry | No Comments »
Sunday, October 1st, 2017
I noted in my WATOC conference report a presentation describing the use of calculated reaction barriers (and derived rate constants) as mechanistic reality checks. Computations, it was claimed, have now reached a level of accuracy whereby a barrier calculated as being 6 kcal/mol too high can start ringing mechanistic alarm bells. So when I came across this article[1] in which calculated barriers for a dyotropic ring expansion observed under mild conditions in dichloromethane as solvent were used to make mechanistic inferences, I decided to explore the mechanism a bit further.
(more…)
References
-
H. Santalla, O.N. Faza, G. Gómez, Y. Fall, and C. Silva López, "From Hydrindane to Decalin: A Mild Transformation through a Dyotropic Ring Expansion", Organic Letters, vol. 19, pp. 3648-3651, 2017. http://dx.doi.org/10.1021/acs.orglett.7b01621
Tags:animation, bicyclic ring product, energy derivative gradient norm, energy profile, final non-ionic product, Organic chemistry, possible products, potential energy surface, realistic model for the reaction
Posted in pericyclic, reaction mechanism | 3 Comments »
Wednesday, May 25th, 2016
The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre occurs with inversion of configuration at the carbon, the mechanism being known by the term SN2 (a story I have also told in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which I here quantify with some calculations.
(more…)
Tags:Berry mechanism, Elimination reaction, energy, energy barrier, energy profile, free energy, Leaving group, lower energy orientation, Molecular geometry, Organic reactions, overall free energy, Pseudorotation, search query, SN2 reaction, Stereochemistry, Trigonal bipyramidal molecular geometry
Posted in reaction mechanism | No Comments »
Wednesday, November 12th, 2014
In London, one has the pleasures of attending occasional one day meetings at the Burlington House, home of the Royal Society of Chemistry. On November 5th this year, there was an excellent meeting on the topic of Challenges in Catalysis, and you can see the speakers and (some of) their slides here. One talk on the topic of Direct amide formation – the issues, the art, the industrial application by Dave Jackson caught my interest. He asked whether an amide could be formed directly from a carboxylic acid and an amine without the intervention of an explicit catalyst. The answer involved noting that the carboxylic acid was itself a catalyst in the process, and a full mechanistic exploration of this aspect can be found in an article published in collaboration with Andy Whiting’s group at Durham.[1] My after-thoughts in the pub centered around the recollection that I had written some blog posts about the reaction between hydroxylamine and propanone. Might there be any similarity between the two mechanisms?
(more…)
References
-
H. Charville, D.A. Jackson, G. Hodges, A. Whiting, and M.R. Wilson, "The Uncatalyzed Direct Amide Formation Reaction – Mechanism Studies and the Key Role of Carboxylic Acid H‐Bonding", European Journal of Organic Chemistry, vol. 2011, pp. 5981-5990, 2011. http://dx.doi.org/10.1002/ejoc.201100714
Tags:Andy Whiting, Dave Jackson, dielectric, Durham, energy profile, free energy barrier, London, non-polar solution, PDF, Royal Society of Chemistry
Posted in reaction mechanism | 6 Comments »
Thursday, September 19th, 2013
Paul Schleyer sent me an email about a pattern he had spotted, between my post on F3SSF and some work he and Michael Mauksch had done 13 years ago with the intriguing title “Demonstration of Chiral Enantiomerization in a Four-Atom Molecule“.[1] Let me explain the connection, but also to follow-up further on what I discovered in that post and how a new connection evolved.
(more…)
References
-
P.V.R. Schleyer, and M. Mauksch, "Demonstration of Chiral Enantiomerization in a Four‐Atom Molecule", Angewandte Chemie International Edition, 2000. http://doi.org/d8g2nw
Tags:energy, energy profile, head, low energy barrier, lowest energy route, Michael Mauksch, Paul Schleyer, reactant/product
Posted in Interesting chemistry | 3 Comments »