Alpha effect

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The alpha effect ( α effect ) describes the increased nucleophilicity of a molecule due to the presence of a nearby free electron pair . The molecule does not necessarily show increased basicity compared to a similar molecule without the nearby electron donor. The α-effect can be justified by various theoretical explanations, although it is not yet clear which approach best explains the effect.

The effect was first observed by Jencks and Carriuolo in a series of experiments on the kinetics of the reaction of the ester p -nitrophenyl acetate with a number of nucleophiles. Nucleophiles such as fluoride - anion , pyridine or ethylenediamine reacted as their corresponding expected by the pK a value specified basicity. Other ions, such as B. hydroxylamine , the hypochlorite anion or the hydroperoxide anion , however, reacted much faster than expected.

In 1962 Edwards and Pearson named this effect the "α-effect". They suggested that this effect could be caused by a stabilization of the transition state (TS): When entering the TS, the lone pair of electrons moves away from the nucleus, causing a partial positive charge , which can be stabilized by a nearby lone pair, such as it occurs, for example, in carbocations .

The α-effect is also unpredictably dependent on the solvent: it can become stronger or weaker, or even cross maxima.

literature

  • Michael B. Smith, March's Advanced Organic Chemistry , 7th Edition, Wiley, Hoboken, New Jersey 2013, pp. 430-431.

Individual evidence

  1. ^ Chemical Reactivity. ( Memento of the original from July 21, 2006 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. July 14, 2006. Michigan State University. July 27, 2006. @1@ 2Template: Webachiv / IABot / www.cem.msu.edu
  2. ↑ William P. Jencks, Joan Carriuolo, Reactivity of Nucleophilic Reagents toward Esters . In: Journal of the American Chemical Society . tape 82 , no. 7 , April 1, 1960, ISSN  0002-7863 , pp. 1778–1786 , here p. 1778. , doi : 10.1021 / ja01492a058 .
  3. ↑ William P. Jencks, Joan Carriuolo: General Base Catalysis of the Aminolysis of Phenyl Acetate1 . In: Journal of the American Chemical Society . tape 82 , no. 3 , February 1, 1960, ISSN  0002-7863 , p. 675–681 , here p. 675 , doi : 10.1021 / ja01488a044 .
  4. ^ John O. Edwards, Ralph G. Pearson: The Factors Determining Nucleophilic Reactivities . In: Journal of the American Chemical Society . tape 84 , no. 1 , January 1, 1962, ISSN  0002-7863 , p. 16–24 , here p. 16. , doi : 10.1021 / ja00860a005 .
  5. ↑ Erwin Buncel, Ik-Hwan Um: The α-effect and its modulation by solvent . In: Tetrahedron . tape 60 , no. 36 , August 30, 2004, p. 7801-7825 , here p. 7801 , doi : 10.1016 / j.tet.2004.05.006 .