Hexanol, mass spectrum

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). 

Fig. 6.41. El mass spectrum of 2-hexanol, Mr = 102. Spectmm used with permission by NIST. NIST 2002.

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). 

Like alkyl halides and ethers, alcohols undergo a cleavage. Consequently, the mass spectrum of 2-hexanol shows a base peak dXmlz = 45 (a cleavage leading to a more... 

Figure 6.5. El mass spectrum of 1-hexanol (MW — 102 u). The M ion is missing, but the loss of H2O from it is conspicuous.

Predict the order of relative abundances of the a-cleavage products in the El mass spectrum of 3-methyl-3-hexanol. 

The mass spectrum of 2-hexanol. The molecular ion peak at m/z = 102 is small because alcohols fragment so easily. 0 -0... 

Explain clearly why 2-hexanol gives rise to a mass spectrum with key peaks at m/z - 102, 84, and 45. 

The i-butane chemical ionization mass spectra of a number of saturated mono-hydroxylic alcohols have been determined to establish the general patterns of the spectra of this class of compounds. The spectrum of 2-hexanol is given as a typical example in Table X, and it may be seen that the following ion types comprise large fractions of the spectra of alcohols the alkyl ion formed from the hydrocarbon portion of the molecule R ) the ion formed by abstraction of the hydride from the molecule, (M l)" the protonated molecule, (M + 1) the association complex of the molecule with the mje = 39 ion of the i-butane plasma, (M -h 39) the association complex of the molecule with the mje = 57 ion of the /-butane plasma, (M -h 57) and the protonated dimer of the molecule, (2M -h 1). The intensity of this ion in the spectrum of 2-hexanol is so small that it is not included in Table X. 

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