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NMR Spectroscopy of Epoxides: Analyzing Structural Characteristics

This lecture focuses on the application of 1H-NMR spectroscopy to determine the structure of epoxides based on their unique splitting patterns in the aromatic and epoxide ranges. It discusses challenges in analyzing spectra from complex mixtures containing epoxides, aldehydes, and other compounds. The presence of CDCl3 is noted in the spectra, revealing important signals at specific chemical shifts. Various compounds, such as 4-methylstyrene oxide and 4-methylbenzacetaldehyde, are examined through their NMR spectra, highlighting peak characteristics and integration aspects.

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NMR Spectroscopy of Epoxides: Analyzing Structural Characteristics

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  1. Lecture 5c NMR Spectroscopy of Epoxides

  2. Introduction • 1H-NMR spectroscopy is used to determine the structure of the epoxide based on characteristic splitting patterns in the aromatic range and the epoxide range • When analyzing the spectrum, it will become much more difficult if the submitted sample is a mixture of many compounds i.e., epoxide, aldehyde, water (d=1.56 ppm), ethyl acetate (d=1.26 ppm, 2.05 ppm and 4.12 ppm), hexane (d=0.88 ppm, 1.26 ppm), etc. (see SKR, p. 260) • The proton spectrum will exhibit a singlet at d=7.26 ppm due to the presence of CDCl3 if the concentration of the epoxide is very low • The carbon spectrum will show a “triplet” at d=77 ppm due to the presence of CDCl3

  3. 4-Methylstyrene oxide • 1H-NMR spectrum (integration in blue) 4 CH3 3 H1, dd H2, dd H3, dd 1 1 1

  4. 4-Methylstyrene oxide • 13C{1H}-NMR spectrum • Seven signals total • Epoxide carbons at ~ 50-60 ppm • Four signals in the aromatic range • The size of the peakfor CDCl3depends on the concentration of the sample CDCl3

  5. 4-Methylbenzacetaldehyde • 1H-NMR spectrum (J3(CH2-CHO)= 2.56 Hz) CHO, “s” CH3 1 3 CH2, “s” 2 4

  6. 4-Methylbenzacetaldehyde • 13C{1H}-NMR spectrum • Aldehyde: ~200 ppm • Methylene: 45-50 ppm • Methyl group: ~30 ppm CH3 CHO CH2 CDCl3

  7. 4-Methylacetophenone • 1H-NMR spectrum • Two doublets in the aromatic range, one of then significantly shifted downfield due to the adjacent carbonyl function • Two singlets in the d= 2-2.5 ppm rangedue to the two methylgroups 3 3 2 2

  8. 4-Methylacetophenone • 13C{1H}-NMR spectrum • Carbonyl: ~195 ppm (small) • Methyl groups: 20-30 ppm CH3 CDCl3 CO

  9. What is that? • Interpret the following 1H-NMR spectrum

  10. How about that one? • Interpret the following 13C{1H}-NMR spectrum

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