Reactivity in Chemistry
CO25. Summary of Elementary Steps
The reactions of carbonyls can become very complicated, involving many steps. In essence, though, the steps involve only a few, different elementary reactions.
A lone pair is donated from a nucleophile to the carbonyl carbon, forming a bond.
Protons are most often transferred from a positively charged atom to a neutral atom with a lone pair.
Protons are also easily transferred from a positively charged atom to a negatively charged atom.
Sometimes, a protos might be transfered from a neutral atom to a negatively charged one.
In many of the reactions of anionic and semi-anionic nucleophiles, these two steps complete the entire reaction mechanism. However, if an additional lone pair can be revealed at the nucleophilic atom (often by transferring a proron away from this site), additional steps occur.
In pi donation, two heteroatoms, both with lone pairs, are attached to the same carbon. A lone pair is donated to the carbon, and one of the heteroatoms is pushed off.
Many mechanisms involve a number of proton transfers and pi donations. These steps occur over and over, inching the molecule along step by step towards the product. Usually, each proton transfer helps to prepare an atom for eventual removal via pi donation.
Occasionally, if the nucleophile is neutral, these steps are preceded by an initial activation step.
Usually makes the reaction faster.
Is especially helpful when the nucleophile is uncharged, and hence less reactive.
The carbonyl is often activated by a proton (from a protic acid) but it can also be activated by a Lewis acid (such as a metal ion).
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Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License.
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This material is based upon work supported by the National Science Foundation under Grant No. 1043566.Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
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