Reactivity in Chemistry

Carbonyl Addition

CO19. Biological Reduction

    Addition to a carbonyl by a semi-anionic hydride, such as NaBH4,   results in conversion of the carbonyl compound to an alcohol.  The hydride from the BH4- anion acts as a nucleophile, adding H- to the carbonyl carbon.  A proton source can then protonate the oxygen of the resulting alkoxide ion, forming an alcohol.


    Formally, that process is referred to as a reduction.  Reduction generally means a reaction in which electrons are added to a compound; the compound that gains electrons is said to be reduced.  Because hydride can be thought of as a proton plus two electrons, we can think of conversion of a ketone or an aldehyde to an alcohol as a two-electron reduction.  An aldehyde plus two electrons and two protons becomes an alcohol.

    Aldehydes, ketones and alcohols are very common features in biological molecules.  Converting between these compounds is a frequent event in many biological pathways.  However, semi-anionic compounds like sodium borohydride don't exist in the cell.  Instead, a number of biological hydride donors play a similar role.

    NADH is a common biological reducing agent.  NADH is an acronym for nicotinamide adenine dinucleotide hydride.  Insetad of an anionic donor that provides a hydride to a carbonyl, NADH is actually a neutral donor.  It supplies a hydride to the carbonyl under very specific circumstances.  In doing so, it forms a cation, NAD+.  However, NAD+ is stabilized by the fact that its nicotinamide ring is aromatic; it was not aromatic in NADH.



This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

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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.

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