Mass Spectra of Alcohols

Electrostatic potential map for 2-methyl-2-butanol radical cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

Spin density for 2-methyl-2-butanol radical cation shows location of unpaired electron. 

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). 

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Exercise 9-48 The mass spectra of alcohols usually show peaks of (M —18), which correspond to loss of water. What kind of mechanisms can explain the formation of (M — 18) peaks, and no (M — 19) peaks, from 1,1-dideuterioethanol and 1,1,1,3,3-pentadeuterio-2-butanol ... 

The mass spectra of alcohols may not always show strong molecular ions. The reason is that the M+ ions readily fragment by a cleavage. The fragment ions are relatively stable and are the gaseous counterparts of protonated aldehydes and ketones ... 

C Fragmentation Splitting Out a Small Molecule Mass Spectra of Alcohols... 

The ESI mass spectra of alcohol dehydrogenase under native form obtained from (A) horse liver and (B) yeast. Reproduced from Loo J.A., J. Mass Spectrom., 30, 180, 1995, with permission. 

From electron impact ionisation mass spectra of alcohols and amines, it is well known that onium ions - oxonium ions (CR1R2OR3)+ and immonium ions (CR1R2NR3R4)+, respectively - play important roles in determining fragmentation and thereby the detailed appearance of the spectra. Onium ions are usually the results of a-cleavages of the molecular ions, and they may fragment further... 

The molecular ions obtained from alcohols fragment so readily that few of them survive to reach the collector. As a result, the mass spectra of alcohols show small molecular ion peaks. Notice the small molecular ion peak at m/z = 102 in the mass spectrum of 2-hexanol (Figure 13.8). 

Explain the following observations that can be made about the mass spectra of alcohols ... 

Both the radical site and the charge site of the hydroxyl group are intermediate in their capability for reaction initiation, so that mass spectra of alcohols exhibit many of the types of reactions outlined in Chapters 4 and 8. Tertiary alcohols contain the largest, and primary alcohols the smallest, total abundance of oxygen-containing ions in their spectra. In all but the spectra of 1-alkanols, a-cleavage (Equation 4.17) is the most useful characteristic reaction. In the spectrum of 3-methyl-3-heptanol (Figure 9.5) this produces the important peaks at... 

Interpreting the mass spectra of sulfur compounds is aided by the observation of an M+2 peak because of the presence of the mass 34 isotope of sulfur The major cleav age pathway of thiols is analogous to that of alcohols... 

Polarimetric analysis of sorbitol and mannitol in the presence of each other and of sugars is possible because of their enhanced optical rotation when molybdate complexes are formed and the higher rotation of the mannitol molybdate complex under conditions of low acidity (194). The concentration of a pure solution of sorbitol may be determined by means of the refractometer (195). Mass spectra of trimethylsilyl ethers of sugar alcohols provide unambiguous identification of tetritols, pentitols, and hexitols and permit determination of molecular weight (196). 

The intensity of the m/z 31 ion is sufficient to suggest the presence of oxygen. Masses 44 and 57 are usually present, and an M - 18 peak is also detectable. Mass 44 usually suggests an aldehyde unbranched on the a-carbon, but this ion is also prominent in the mass spectra of cyclobutanol, cyclopentanol, cyclohexanol, and so forth. Mass 57 (C3H5O) is also fairly intense for C5 and larger cyclic alcohols. If an aldehyde is present, M - 1, M - 18, and M - 28 peaks are observed. 

The molecular ion is slightly more intense in the mass spectra of secondary alcohols than in tertiary alcohols, but even in secondary alcohols, the molecular ion intensity is very small. 

Many low-molecular weight (tertiary alcohols exhibit no M - 18 peaks. C8 and higher secondary alcohols exhibit M - 18 peaks. The M - 46 peak is usually missing in the mass spectra of secondary and tertiary alcohols. In secondary and tertiary alcohols, the loss of the largest alkyl group results in intense fragment ions. 

The electron ionization (El) mass spectra of TMS ethers and esters are generally characterised by weak or absent molecular ions. The [M—15]+ ion formed by loss of a methyl radical is generally abundant and in the case of alcoholic functions, the loss of a trimethylsilanol molecule [M—90]+ is also diagnostic. The peak at mJz 73, corresponding to the TMS group, is important in nearly all the TMS-derivative mass spectra. Figure 8.2 shows the fragmentation of TMS esters and ethers in mass spectrometric analyses. 

Establishing the elemental composition based on the isotopic peaks may be problematic if, for example, the sample contains impurities with the masses in the region of the molecular ion cluster. In the El mass spectra of amines, alcohols, acids, and some other classes of organic compounds there is often a peak of [M + H]+ ion. It distorts the isotopic picture. It is worth mentioning as well that in real experimental conditions the peak intensity may vary slightly in each... 

Example The El mass spectra of 1-hexanol, Mj = 102, and 1-hexene, Mr = 84, show close similarity because the molecular ion peak is absent in the mass spectrum of hexanol (Fig. 6.40). However, a more careful examination of the hexanol spectrum reveals peaks at m/z 18, 19, 31, and 45 that are absent in the hexene spectrum. These are due to H20, HsO and to oxonium ions (H2C=OH and H3CCH=0H in this case) which are reliable indicators of aliphatic alcohols and ethers (Table 6.8). 

Note Trimethylsilylether (TMS) derivatives are frequently employed to vola-tize alcohols, [14,203] carboxylic acids, [204,205] and other compounds [206,207] for mass spectrometry, and for GC-MS applications in particular. The El mass spectra of TMS derivatives exhibit weak molecular ion peaks, clearly visible [M-CHa]" signals and often [Si(Me)3], m/z 73, as the base peak. 

Rosen, R.T. Hartmann, T.G. Rosen, J.D. Ho, C.-T. Fast-Atom-Bombardment Mass Spectra of Low-Molecular-Weight Alcohols and Other Compounds. Evidence for a Chemical-Ionization Process in the Gas Phase. Rapid Commun. Mass Spectrom. 1988, 2, 21-23. 

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