Alkanes proton chemical shifts

Table 13.1 collects chemical-shift information for protons of various types. Within each type, methyl (CH3) protons are more shielded than methylene (CH2) protons, and methylene protons are more shielded than methine (CH) protons. These differences are small—only about 0.7 ppm separates a methyl proton from a methine proton of the same type. Overall, proton chemical shifts among common organic compounds encompass a range of about 12 ppm. The protons in alkanes are the most shielded, and O—H protons of carboxylic acids are the least shielded. 

From the 13C chemical shift data collected in Table 4.1, Grant and Paul [85] deduced their additivity rule for the 13C chemical shifts of alkanes. The signal assignments for the alkanes, also given in Table 4.1, are based on signal intensities and proton decoupling experiments. 

When there are many hydrogens and small chemical-shift differences, as in alkanes, the proton nmr spectra may have so many closely spaced resonance lines that they merge together to give a series of smooth, more-or-less feature-... 

In proton nmr spectra, the chemical shifts of alkenic hydrogens are toward lower fields than those of alkane hydrogens and normally fall in the range 4.6-5.3 ppm relative to TMS (see Section 9-10E and Table 9-4). Spin-spin couplings of alkenic hydrogens are discussed in Section 9-10G and 9-10J. 

Saturated compounds. The position of absorptions of methyl, methylene, meth-ine and quaternary carbon atoms in the alkanes is shown in Fig. 3.49. Within each group the exact position of absorption is determined by the number and nature of substituents on the p and y carbons. Replacement of a proton by CH3 results in a downfield shift of c. 8 p.p.m. at C-l, and c. 10 p.p.m. at C-2, and an upfield shift at C-3 of c. 2 p.p.m. Polar substituents result in a downfield shift in the position of absorption Table A3.12 in Appendix 3 shows the effect on 13C chemical shifts of replacing a methyl group by various polar substituents. 

Cycloalkanes and Saturated Hetero-cyclics The chemical shifts of the CH2 groups in monocyclic alkanes are given in Table 4.7. The striking feature here is the strong shift to the right of cyclopropane, analogous to the shift of its proton absorptions. 

To estimate the chemical shift of protons that are deshielded by two groups, add the chemical shifts you would expect with each deshielding group individually, and subtract 1.3 (the 8 for an alkane CH2 group) from the result. 

Different types of protons and carbons in alkanes tend to absorb at similar chemical shifts, making structure determination difficult. Explain how the 13C NMR spectrum, including the DEPT technique, would allow you to distinguish among the following four isomers. 

Table 14.1 illustrates that absorptions for a given type of C-H bond occur in a narrow range of chemical shift values, usually 1-2 ppm. For example, all sp hybridized C-H bonds in alkanes and cycloalkanes absorb between 0.9 and 2.0 ppm. By contrast, absorptions due to N-H and O-H protons can occur over a broader range. For example, the OH proton of an alcohol is found anywhere in the 1-5 ppm range. The position of these absorptions is affected by the extent of hydrogen bonding, making it more variable. 

The temperature of zeolite samples containing various adsorbed molecules was switched from room temperature to 500-600 K within 30-40 seconds by means of a laser beam. Catalytic n-alkane cracking and H-D exchange with deuterated cyclohexane were monitored by IH MAS NMR in time steps of down to one second. A two-dimensional representation of the chemical shift and the chemical reaction of the species will be given, allowing a good characterization of reaction steps. At low temperature a weak proton transfer without chemical reaction can be observed, whereas at 430 K and 530 K the proton transfer is accompanied, respectively, by an isomerization or a decomposition to methane and coke. In addition to the effect of high temperature, the laser radiation itself can force the conversion of alkanes to methane and coke. 

Thiols (RSH) behave in a similar way to alcohols but are not so deshielded, as you would expect from the smaller electronegativity of sulfur (phenols are all about 5.0 p.p.m., PhSH is at 3.41 p.p.m.). Alkane thiols appear at about 2 p.p.m. and arylthiols at about 4 p.p.m. Amines and amides show a big variation, as you would expect for the variety of functional groups involved, and are summarized below. Amides are slightly acidic, as you saw in Chapter 8, and amide protons resonate at quite low fields. Pyrroles are special—the aromaticity of the ring makes the NH proton unusually acidic and they appear at about 10 p.p.m. chemical shifts of NH protons... 

In alkanes (aliphatic or saturated hydrocarbons) aU of the CH hydrogen absortions are typically found from about 0.7 to 1.7 ppm. Hydrogens in methyl groups are the most highly shielded type of proton and are found at chemical shift values lower (0.7-1.3 ppm) then methylene (1.2-1.2 ppm) or methine hydrogens (1.4-1.7 ppm). 

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). 

Obtain the NMR spectrum of a straight-chain alkane, such as n-octane. Identify the methyl and methylene peaks. Note the chemical shift and the spin-spin splitting. Measure the total area of the methyl and methylene peaks and correlate this with the number of methyl and methylene protons in the molecule. 

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