Methyl protonation

The olefinic proton of the enol form emerges as a sharp singlet in the region 6.2 to 7.5 ppm (DMSO) (386). while the 5-methyl protons appear at approximately 2.2 ppm. 

Increased shielding of methyl protons Decreasing electronegativity of atom attached to CH3... 

Protons are equivalent to one another and have the same chemical shift when they are m equivalent environments Often it is an easy matter to decide simply by mspec tion when protons are equivalent or not In more difficult cases mentally replacing a proton m a molecule by a test group can help We 11 illustrate the procedure for a sim pie case—the protons of propane To see if they have the same chemical shift replace one of the methyl protons at C 1 by chlorine then do the same thing for a proton at C 3 Both replacements give the same molecule 1 chloropropane Therefore the methyl protons at C 1 are equivalent to those at C 3... 

Replacement of either one of the methylene protons of propane generates 2 chloro propane Both methylene protons are equivalent Neither of them is equivalent to any of the methyl protons... 

The H NMR spectrum of propane contains two signals one for the six equiva lent methyl protons the other for the pair of equivalent methylene protons... 

MHz H NMR spectrum of 1 1 dichloroethane (CI2CHCH3) showing the me thine proton as a quartet and the methyl protons as a doublet The peak multiplici ties are seen more clearly in the scale expanded insets... 

The physical basis for peak splitting mil dichloroethane can be explained with the aid of Figure 13 13 which examines how the chemical shift of the methyl protons IS affected by the spin of the methme proton There are two magnetic environments for... 

Spin of methme proton shields methyl protons from Xg Methyl signal appears at higher field... 

There are eight possible combinations of the nuclear spins of the three methyl protons m CH3CHCI2... 

FIGURE 13 14 The methyl protons of 1 1 dichloroethane split the signal of the methme proton into a quartet... 

FIGURE 13 16 The methyl ene protons of ethyl bro mide split the signal of the methyl protons into a triplet... 

The NMR spectrum of isopropyl chloride (Figure 13 17) illustrates the appearance of an isopropyl group The signal for the six equivalent methyl protons at 8 1 5 is split into a doublet by the proton of the H—C—Cl unit In turn the H—C—Cl proton signal at 8 4 2 IS split into a septet by the six methyl protons A doublet-septet pattern is char acteristic of an isopropyl group... 

Section 13 8 The methyl protons of an ethyl group appear as a triplet and the meth ylene protons as a quartet in compounds of the type CH3CH2X... 

Section 13 9 The methyl protons of an isopropyl group appear as a doublet and the methine proton as a septet in compounds of the type (CH3)2CHX... 

IS the only phosphorus isotope present at natural abundance and has a nuclear spin of The H NMR spectrum of tnmethyl phosphite (CH30)3P exhibits a doublet for the methyl protons with a splitting of 12 Hz... 

Substituent group Methyl protons Methylene protons Methine proton... 

Acetates. Because of the significant interest in selective acetylation reactions of sucrose, the need for a convenient and unambiguous method of identification has been recognized (34,35). The position of an acetyl group in a partially acetylated sucrose derivative can be ascertained by comparison of its H-nmr acetyl methyl proton resonances after per-deuterioacetylation with those of the assigned octaacetate spectmm. The synthesis of partially acetylated sucroses has generally been achieved either by way of selectively protected derivatives such as trityl ethers and cychc acetals or by direct selective acetylation and deacetylation reactions. 

Deuteration of C-methyl protons in simple methylpyrimidines and their amino and hydroxy derivatives has been studied under acidic and basic conditions. The exchange is acid/base catalyzed with, for example, a minimal rate at pH 4 for 1,4,6-trimethylpyrimidin-2(lH)-imine (67JCS(B)171). 

In theory two carbanions, (189) and (190), can be formed by deprotonation of 3,5-dimethylisoxazole with a strong base. On the basis of MINDO/2 calculations for these two carbanions, the heat of formation of (189) is calculated to be about 33 kJ moF smaller than that of (190), and the carbanion (189) is thermodynamically more stable than the carbanion (190). The calculation is supported by the deuterium exchange reaction of 3,5-dimethylisoxazole with sodium methoxide in deuterated methanol. The rate of deuterium exchange of the 5-methyl protons is about 280 times faster than that of the 3-methyl protons (AAF = 13.0 kJ moF at room temperature) and its activation energy is about 121 kJ moF These results indicate that the methyl groups of 3,5-dimethylisoxazole are much less reactive than the methyl group of 2-methylpyridine and 2-methylquinoline, whose activation energies under the same reaction conditions were reported to be 105 and 88 kJ moF respectively (79H(12)1343). 

Quinoline, amino-2-hydroxy-4-methyl-azo pigments from, 1, 334 Quinoline, 8-amino-6-methoxy-as antimalarial, 2, 517 Quinoline, 4-amino-2-methyl-protonation, 2, 341 Quinoline, anilino-3-substituted... 

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