1- Pentanol mass spectrum

The mass spectrum of 2-methyl-3-pentanol is shown in Figure 12.8. What fragments can you identify ... 

Purify the crude product by silica gel TLC (Merck Kieselgel 60 GF254) (developing solvent, ethyl acetate hexane, 1 4) and pure (2/ ,3fl)-5-phenyl-2,3-methano-1-pentanol is obtained in quantitative yield. Identify it by 1H NMR, IR, and mass spectrum. 

Mass spectrum of 2-methyl-3-pentanol, Practice Problem 12.3. 

A daughter ion that is 18 mass units less than the molecular ion is associated with loss of a water molecule (H2O m/2 = 18). This observation, of course, raises the question of which organic molecrdes can lose a molecule of water. One answer is that alcohols contain an OH unit and lose water in the mass spectrum in most cases. The daughter ion is the M-18 peak. Loss of water often means that the molecular ion of alcohols may be very weak and, with tertiary alcohols, is often missing altogether. The mass spectnim of 3-pentanol (see Figiue 14.11) shows essentially no molecular ion at m/2 89, but there is a weak M-18 peak at m/2 70. The base peak at m/2 59 corresponds to M-29 (loss of an ethyl group). A detectable M-18 peak is often indicative of an alcohol. 

Identify two peaks that are expected to appear in the mass spectrum of 3-pentanol. For each peak, identify the fragment associated with the peak, and show a mechanism for its formation. 

The mass spectrum of straight-chain pentanol isomers, 1-pentanol (Fig. 4.27), 2-pentanol (Figure 4.28), and 3-pentanol (Fig. 4.29) all exhibit very weak molecular ion peaks at miz = 88, while the molecular ion in the mass spectrum of the tertiary alcohol 2-methyl-2-butanol (Fig. 4.30) is entirely absent. The most important fragmentation reaction for alcohols is the loss of an alkyl group via a-cleavage. As discussed earlier, the largest alkyl group is most readily lost. In the spectrum of... 

A second common mode of fragmentation involves dehydration. The importance of dehydration increases as the chain length of the alcohol increases. While the fragment ion peak resulting from dehydration (m/z = 70) is very intense in the mass spectrum of 1-pentanol, it is quite weak in the other pentanol isomers. Dehydration may occur by either thermal dehydration prior to ionization or by fragmentation of the molecular ion. Thermal dehydration is especially troublesome for alcohol samples analyzed by GC-MS. The injection port of the gas chromatograph is usually maintained at more than 200°C, and many alcohols, especially tertiary or allylic/benzylic, will dehydrate before the sample molecules even reach the GC column and certainly before the molecules reach the ion source of the mass spectrometer. Thermal dehydration is a 1,2-eUmination of water. If the alcohol molecules reach the ion source intact, however, dehydration of the molecular ion can still occur, but in this case it is a 1,4-elimination of water via a cyclic mechanism ... 

Alcohols containing four or more carbons may undergo the simultaneous loss of both water and ethylene. This type of fragmentation is not prominent for 1-butanol but is responsible for the base peak at m/z = 42 in the mass spectrum of 1-pentanol (Fig. 4.27). 

Following are mass spectra for the constitutional isomers 2-pentanol and 2-methyl-2-butanol. Assign each isomer its correct spectrum. 

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