2-Methylpropane chemical shifts

The spectra can be predicted for the alkanes butane and isobutane (or 2-methylpropane). The peaks should appear in the 1-1.5 ppm chemical shift region according to Table 3.3. Butane, CH3CH2CH2CH3, has two types of protons as noted in Fig. 3.15(a). Isobutane also has two types of protons, shown in Fig. 3.15(b). Therefore both spectra should have two absorption peaks. In butane, the methyl protons should be split by the adjacent methylene protons into a triplet the methylene protons would be split by the methyl protons into a quartet. We would predict that the proton NMR spectmm of butane would look like the schematic spectrum in Fig. 3.15(a), with the relative peak areas shown. Isobutane would show a very different splitting pattern. There are nine chemically equivalent protons (marked b on the structure) on the three methyl groups the peak for these nine protons will be split into a doublet by the single a type proton on the middle carbon. The peak for the single proton will be split into (9 + 1) = 10 peak multiplet by the b type protons, with the relative peak areas as shown schematically in Fig. 3.15(b). 

So, how can we choose between these The solution is in the H NMR spectrum, which is shown below. There are only two peaks visible one at 3.3 and one at 1.1 ppm. It s quite common in NMR spectra not to see signals for protons attached to O or N (you will see why in Chapter 13) so we can again rule out all structures with more than two different types of H attached to C. Again, we are left with A and B, confirming our earlier deductions. But the chemical shift of the signal at 5 3.3 teUs us more it has to be due to H atoms next to an oxygen atom because it is deshielded. The industrial emulsifier must therefore be A 2-amino-2-methylpropan-l-ol. 

Problem 3.1. Sketch the NMR spectrum of the two isomers of butane (n-butane and 2-methylpropane), showing the approximate chemical shifts (relative to tetra-methylsilane [TMS]), the integrated areas, and the multiplicities expected. Assume 7 = 7.0Hz. 

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