The first really interesting reaction seen by Organic Chemistry students is the Diels-Alder [4+2] cycloaddition, an example of a concerted process where all the bond making and breaking take place concomitantly. The SN2 is also a concerted process, however the Diels-Alder reaction is a gem because of the high density of relative stereochemistry that can be established in a single synthetic step. The most interesting and unique of all organic reactions learned fall within the narrow category of sigmatropic rearrangements, intramolecular pericyclic processes wherein one σ-bond is exchanged for another σ-bond. Example reactions are the Cope and Claisen rearrangements.
Few reactions have the capacity to introduce densely packed relative stereochemistry to a molecule in one single step as does the Diels-Alder reaction, a concerted [4+2] cycloaddition. This highly synthetically useful reaction permits facile entry into complex fused ring systems endemic to some natural products exhibiting medicinal properties. This article focuses on the rudimentary elements of the Diels-Alder reaction. Students interested in a more detailed understanding of the dynamics of the reaction are encouraged to engage in the suggested readings.
When basicity and/or steric effects outweigh nucleophilicity, and a good leaving group (LG) is present, we observe elimination chemistry leading to olefins (alkenes) instead of substitution. The E2 reaction (elimination bimolecular) takes place via a concerted mechanism, and involves an antiperiplanar alignment of orbitals – proton and LG lined up across from one another however in the same plane. Continue reading
Whereas the SN2 reaction is a concerted process, the SN1 reaction (substitution, nucleophilic, unimolecular) proceeds via a stepwise mechanism. The rate determining step for the SN1 reaction involves formation of a stable (trigonal planar) carbocation intermediate. Carbocations, in general, are high energy intermediates, and hence the term “stable” is relative, not absolute.