Mass spectrum 1- butanol

Three of the most intense peaks in the mass spectrum of ] 2 methyl 2 butanol appear at m/z 59 70 and 73 Explain the origin of these peaks J... 

The mass spectra of alcohols often completely lack a peak corresponding to the parent ion. This is due to extremely rapid loss of neutral fragments following initial ionization. For example, the mass spectrum of 2-methyl-2-butanol lacks a parent peak and contains strong peaks at M-15 (loss of CH3O and M-18 (loss of H2O). 

Both fragmentation modes are apparent in the mass spectrum of 1-butanol (Figure 17.14). The peak at ni/z = 56 is due to loss of water from the molecular ion, and the peak at mjz = 31 is due to an alpha cleavage. 

Figure 17.14 Mass spectrum of 1-butanol (M+ = 74). Dehydration gives a peak at mjz - 56, and fragmentation by alpha cleavage gives a peak at m/z = 31.

The major ions in the spectrum are due to the loss of the neutral fragment water, as in the case of n-butanol 1,4 elimination is probably involved.The base peak is formed via homolytic cleavage next to the OH group followed by proton transfer (Fig. 9.13). The base peak of the mass spectrum is formed as shown in Figure 9.13. 

The molecular weight of 2-methyl-2-butanol is 88. A peak in its mass spectrum at m/z 70 corresponds to loss of water from the molecular ion. The peaks at m/z 73 and m/z 59 represent stable cations corresponding to the cleavages shown in the equation. 

The observation of a peak at mlz 31 in the mass spectrum of the compound suggests the presence of a primary alcohol. This fragment is most likely H2C=OH. On the basis of this fact and the appearance of four different carbons in the 13C NMR spectrum, the compound is 2-ethyl-1-butanol. 

In addition to the elimination of water, alcohols tend to fragment by cleaving one of the bonds between the hydroxy carbon and an adjacent carbon. The resulting cations are the conjugate acids of aldehydes and ketones and are stabilized because they satisfy the octet rule at all of the atoms. Figure 15.12 shows the mass spectrum of 2-butanol. 

Sanchez and Brown have obtained a new harman derivative from Aspi-dosperma exalatum Monachino (121). The alkaloid was obtained from a butanol extract, and in the IR spectrum it showed principal absorptions at 1925 cm-1 for an immonium ion, at 1610 cm-1 for a carboxylate anion, and at 748 cm-1 for a disubstituted benzene. In the mass spectrum a molecular ion was observed at m/e 226 and a base peak at m/e 182 (M+ —44). An aromatic methyl group and five aromatic protons were observed in the PMR spectrum. Consideration of this data together with the UV spectrum led to deduction of the structure of the alkaloid as 268, harman carboxylic acid. The ethyl ester (269) was also isolated but it was probably an artifact. 

The mass spectrum of 1-butanol (Fig. 8.21) shows a very weak molecular ion peak at mie = lA, while the mass spectrum of 2-butanol (Fig. 8.22) has a molecular ion peak (rnie = 74) that is too weak to be detected. The molecular ion peak for tertiary alcohol, 2-methyl-2-propanol (Fig. 8.23), is entirely absent. The most important fragmentation reaction for alcohols is the loss of an alkyl group ... 

Figure 10.21 A mass spectrum of a secondary alcohol, 2-butanol, C4H10O.

The mass spectrum of 3-methyl-l-butanol (Figure 12-21) shows a favorable loss of water. The peak at m/z 70 that appears to be the molecular ion is actually the intense M-18 peak. The molecular ion m/z 88) is not observed because it loses water very readily. The base peak at m/z 55 corresponds to loss of water and a methyl group. 

In some cases, components in a mixture can be determined quantitatively without prior separation if the mass spectrum of each component is sufficiently different from the others. Suppose that a sample is known to contain only the butanol isomers listed in Table 10.17. It can be seen from Table 10.17 that the peak at miz = 33 is derived from butanol, but not from the other two isomers. A measurement of the miz = 33 peak intensity compared to butanol standards of known concentration would therefore provide a basis for measuring the butanol content of the mixture. Also, we can see that the abundances of the peaks at m z = 45, 56, and 59 vary greatly among the isomers. Three simultaneous equations with three unknowns can be obtained by measuring the actual abundances of these three peaks in the sample and applying the ratio of the abundances from pure compounds. The three unknown values are the percentages of butanol, 2-butanol, and 2-methyl-2-propanol in the mixture. The three equations can be solved and the composition of the sample determined. Computer programs can be written to process the data from multicomponent systems, make all necessary corrections, and calculate the results. 

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