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Useful Examples for Discussion of Proton-NMR Spectroscopy: N-Substituted alpha-Aminobenzenebutanenitriles. Anisotropy of Diastereotopic Methylene Protons

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John Almy , R. Martinez Alverez , A. Herrera Fernandez and A. Sanchez Vazquez

Universidad Complutense, Departamento de Química Orgánica I, Facultad de Ciencias Químicas, E-28040 MadridSPAIN

J. Chem. Educ., 1997, 74 (12), p 1479

DOI: 10.1021/ed074p1479

Publication Date (Web): December 1, 1997

Section:

History, Education, and Documentation

Abstract

Undergraduate texts commonly discuss the anisochrony (non-coincidence of NMR signals) of methylene protons near chiral center using projections in which the chiral center and the carbon bearing the methylene protons are separated by a single bond. An analysis of the various rotational conformers about this bond demonstrate the non equality of the protons in question and reinforces the fact that the chemical shift for a given proton is the weighted average for the proton in its various conformations. N-substituted derivatives of the alpha-aminobenzenebutanenitrile system and alpha-methyaminopropanenitrile are obtained by easy one step syntheses. During the characterization of these compounds, which have two consecutive methylene groups attached to their chiral center, we were surprised to observe that although both pairs of methylene protons are diastereotopic and therefore anisochronous, only the gamma protons actually displayed different chemical shifts. The ß protons, which are separated from the chiral center by fewer bonds, gave no measurable chemical shift difference. The degree of anisochrony is not always a function of the distance from a chiral center and will vary with the solvent used in the analysis. We report here a summary of the study we made to explore this phenomenon.

Keywords (Audience):

Upper-Division Undergraduate

Keywords (Domain):

Organic Chemistry

Keywords (Subject):

NMR Spectroscopy

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This article has been cited by 1 ACS Journal articles (1 most recent appear below).

Discovery-Oriented Approach To Organic Synthesis: Tandem Aldol Condensation-Michael Addition Reactions. Identifying Diastereotopic Hydrogens in an Achiral Molecule by NMR Spectroscopy
Nanette Wachter-Jurcsak and Kendra Reddin
Journal of Chemical Education2001 78 (9), 1264
- Discovery-Oriented Approach To Organic Synthesis: Tandem Aldol Condensation-Michael Addition Reactions. Identifying Diastereotopic Hydrogens in an Achiral Molecule by NMR Spectroscopy
  Nanette Wachter-Jurcsak and Kendra Reddin
  Journal of Chemical Education2001 78 (9), 1264
  We have found a beautiful example of anisochrony of diastereotopic acyclic methylene hydrogens in a symmetric diketone, synthesized by techniques traditionally performed in an introductory organic laboratory course. Synthesis of the diketone is high-...
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