Methine/methylene/methyl groups

In this simplified example of phenylalanine, in the first iteration the methyl groups arc given a value of I in the first classification step because they contain a primaiy C-atom, The methylene group obtains a value of 2, and the methine carbon atom a value of 3. In the second step, the carbon atom of the methyl group on the left-hand side obtains an extended connectivity (EC) value of 2 because its neighboring atom had a value of 2 in the first classification step. 

The least sterically hindered p hydrogen is removed by the base in Hofmann elimination reactions. Methyl groups are deprotonated in preference to methylene groups, and methylene groups are deprotonated in preference to methines. The regioselectivity of Hofmann elimination is opposite to that predicted by the Zaitsev rule (Section 5.10). Elimination reactions of alkyltrimethylammonium hydroxides are said to obey the Hofmann rule they yield the less substituted alkene. 

Acetal 5 contains only one type of methyl group, while acetal 4 has two different types. Thus the spectrum recorded after 3 min corresponds to acetal 5, while that recorded after 7.5 min results from acetal 4. The methylene groups (between 3.6 and 4.2 ppm) and the methine protons (at 4.8 and 5.4 respectively) confirm these assignments. 

Thus, for a terminal methyl group only one doublet will be observed while methylene, methine and quaternary carbons can have a maximum of two, three or four doublets respectively, depending on the structure. The 2-D INADEQUATE spectrum of geraniol is shown in Figure 9.47. 

The structure was determined by NMR spectral analysis including a variety of two-dimensional NMR techniques. The 500-MHz XH NMR spectrum of 77 taken in CDCI3 (Figure 26) revealed the presence of 5 aromatic protons, 15 olefinic protons, a methoxy (63.65), an allylic methyl (62.14) and a tertiary methyl group (61.33). The 13C NMR spectrum showed signals due to all 34 carbons, which were assigned to 7 quaternary carbons, 23 methines, 1 methylene and 3 methyls by DEPT experiments. The 13C and XH NMR spectral data are summarized in Table 27. 

Figure la shows the proton NMR spectrum of PMS. A similar spectrum was also obtained from the P-p-MS s. The broad peak from 1.35 to 1.60 ppm arises from the protons in the polymer main chain although it is not possible to distinguish between the methylene and methine protons from these spectra. The intense peak at 2.16 ppm is attributed to the protons of the methyl group substituted on the benzene ring. The aromatic protons appear as a broad multiplet with peaks centered at 6.42 and 6.9 ppm. The small sharp peak appearing at 7.25 ppm is due to proton impurities in CDC13. 

At 55 kHz field, where relaxation times should indicate molecular motion, the relaxation times of the methyl groups showed a temperature dependence between —30 °C and 50 °C. An unresolved peak containing methylene and methine resonances showed a very weak temperature variation. Garroway et al. 62) concluded that the observed C-13 Tle values for fields above 40 kHz were not dominated by spin-spin effects for the DGEBA-PIP system. 

Furthermore, a methyl group on a quaternary carbon is more deshielded than a methyl attached to a methine, which is more deshielded than a methyl attached to a methylene group. We can see examples of all three of these methyl groups in the C chemical shifts of two C6H14 isomers, 3-methylpentane and 2,2-dimethylbutane (Figure 4.30). 

As just noted (Section 2.7), CH3 is called a methyl group. In addition to having methyl groups at both ends, n-butane contains two CH2, or methylene groups. Isobutane contains three methyl groups bonded to a CH unit. The CH unit is called a methine group. 

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