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What does this spectrum tell us? Two peaks = two chemical environments One chemical environment contains 3 hydrogen atoms, the other 1 hydrogen Using the.

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Presentation on theme: "What does this spectrum tell us? Two peaks = two chemical environments One chemical environment contains 3 hydrogen atoms, the other 1 hydrogen Using the."— Presentation transcript:

1 What does this spectrum tell us? Two peaks = two chemical environments One chemical environment contains 3 hydrogen atoms, the other 1 hydrogen Using the table on page 250: The chemical shift value 9.7 is a hydrogen from a –CHO (aldehyde) The chemical shift value 2.2 are those of a -COCH 3 So the compound is likely to be ETHANAL

2 Interpreting proton ( 1 H) nmr spectra Aims What causes spin-spin coupling? What is the n+1 rule? How can 1 H nmr spectra be interpreted?

3 High resolution proton NMR – spin-spin coupling The high resolution 1 H nmr spectra of many compounds reveal that many absorption peaks are split into particular patterns. This happens because the applied magnetic field felt by any hydrogen atom is affected by the small magnetic fields produced by the hydrogen atoms on the adjacent/’next door’- carbon atoms. Ethanol

4 δ/ppmType of hydrogen Number of Hydrogens Splitting pattern Inference CH 3 -CH 2 O -O-H Triplet Quartet Singlet -CH 2 -CH 3 CH 3 -CH 2 -O- -O-H

5 The CH 3 signal is split into three peaks, a triplet, whereas the CH 2 signal appears as four peaks, a quartet. In general, the splitting of absorption peaks into more complex patterns, called spin-spin coupling, is due to interaction between neighbouring nuclear spins. Thus, the spin of one proton can couple with the spins of adjacent protons, but only if the “next door” protons have a different chemical shift. The splitting of an absorption signal is described by the (n + 1) rule – n hydrogens on a adjacent carbon atom will split a peak into n+1 smaller peaks The splitting pattern is useful in that it gives us information about the number of neighbouring hydrogens Note 1: In the example above, the CH 3 protons must be next door to a CH 2 group as the signal appears as a triplet (2 + 1), and the CH 2 protons must be next door to a CH 3 group as the signal appears as a quartet (3 + 1). Note 2: The OH absorption is seen as a single peak, devoid of any splitting. Since it is adjacent to a CH 2 group it should appear as a triplet. However, the weakly acidic OH protons ionise rapidly and transfer between other ethanol molecules and also to water molecules normally present in trace amounts. This is called chemical exchange. Absorptions of this type are said to be decoupled.

6 Predicting 1H NMR spectra Example: Ethylethanoate CH 3 COOCH 2 CH 3 Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern CH 3 -CO Singlet O-CH 2 -R Quartet R-CH Triplet

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8 Ethanal Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern CH 3 -CO doublet -CO-H quartet

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10 Propanoic acid Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern CH 3 -CH Triplet CH 3 -CH 2 -COOH Quartet R-COOH Singlet

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12 Propanone Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern Type of hydrogen Number of hydrogens Predicted δ/ppm Number of adjacent hydrogens Predicted splitting pattern CH 3 -CO-CH singlet

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14 To do 1.A compound with molecular formula C 3 H 6 O 2 gives the following peaks in its proton nmr spectrum: Identify the molecule and account for the chemical shifts, splitting and integration factors of all three peaks. 2.A compound with molecular formula C 5 H 10 O 2 gives the following peaks in its proton nmr spectrum: Chemical shiftSplittingIntegration factor 1.1Triplet3 2.2Quartet2 11.8Singlet1 Chemical shiftSplittingIntegration factor 1.2Triplet3 1.3Triplet3 2.3Quartet2 4.1Quartet2 Identify the molecule and account for the chemical shifts, splitting and integration factors of all four peaks.

15 Given the molecular formula of the compound is C 4 H 8 O 2, can you work out the structure given the high resolution nmr spectra above?

16 Three different environments for the hydrogens. The hydrogens in those three environments are in the ratio 2:3:3. Since there are 8 hydrogens altogether, this represents a CH 2 group and two CH 3 groups. The CH 2 group at about 4.1 ppm is a quartet. That tells you that it is next door to a carbon with three hydrogens attached - a CH 3 group. The CH 3 group at about 1.3 ppm is a triplet. That must be next door to a CH 2 group. This combination of these two clusters of peaks - one a quartet and the other a triplet - is typical of an ethyl group, CH 3 CH 2. Finally, the CH 3 group at about 2.0 ppm is a singlet. That means that the carbon next door doesn't have any hydrogens attached.


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