From the molecular ion

Mass Spectrometry Aldehydes and ketones typically give a prominent molecular ion peak m their mass spectra Aldehydes also exhibit an M— 1 peak A major fragmentation pathway for both aldehydes and ketones leads to formation of acyl cations (acylium ions) by cleavage of an alkyl group from the carbonyl The most intense peak m the mass spectrum of diethyl ketone for example is m z 57 corresponding to loss of ethyl radi cal from the molecular ion... 

The peak at rn/z 70 corresponds to loss of water from the molecular ion The peaks at m/z 59 and 73 correspond to the cleavages indicated... 

Characteristic Low-Mass Neutral Fragments from the Molecular Ion... 

Electron impact mass spectrometry has been employed to study the fragmentation patterns of isoxazolylmethyl- and bis(isoxazolylmethyl)-isoxazoles and the results are in agreement with proposed pathways (79AC(R)8l). Electron impact studies of nitrostyryl isoxazole (6) show fragmentation in a variety of ways. The standard loss of NO2 from the molecular ion... 

W as an important factor which influenced the fragmentation pattern. 2-Furylpyrido[4,3-d]pyrimidin-4(3i7)-one (154, Rj=furyl, R2 = H,) showed six different primary breakdown peaks originating from the molecular ion. These corresponded to the loss (in decreasing order of probability) of H-, C1H3O, -CN, CO, HCN, C4H3O-, and HCNO. 

In three of the compounds (154, R2= H) examined the commonest loss from the molecular ion was the cyanide RjCN to give the most predominant ion at m/e= 120 in each case. The M-0 peak (M-16), was observed in cyclic hydroxamic acids (154, R = OH). 

There has as yet been no systematic work on the mass spectra of cyclic hydroxamic acids, but from the limited information available the direct loss of 0 or OH from the molecular ion is to be expected. The fragmentation behavior of the 0-alkyl derivatives is rather unpredictable, although again processes involving fission of the N—0 bond are generally important. Table II shows the prominent first-generation fragment ions from a few hydroxamic acids and their ethers. 

Tlie mass spectra of the parent 3-nitro-l,X-naphthyridines (X = 5,6,7, and 8) and their amino and chloro derivatives feature as main fragmentation pathway the loss of a molecule of NO2 from the molecular ions and further consecutive expulsion of two molecules of HCN. A second fragmentation pathway, although of much smaller intensity, is the loss of NO from the molecular ions, followed by consecutive expulsion of CO and HCN (82MI1 89MI1). 

The electron impact positive ion spectrum of l,2,5-oxadiazolo[3,4-/]quinoline IV-oxide 46 shows the loss of N2O2 from the molecular ion, a process that must be followed by a substantial rearrangement to enable the observed loss of propyne-nitrile. This remarkable result apparently arises through a series of H-atom shifts which relocate the dehydroaromatic moiety in the heteroring (890MS465). 

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. 

The peak representing the loss of water from the molecular ion can easily be mistaken for the molecular ion. The spectrum is similar to an olefin below the [M - H20]+ peak except that the peaks at m/z 31, 45, and 59 indicate an oxygen-containing compound. 

A primary alcohol is indicated when the m/z 31 peak is intense and will be the base peak for C,-C4 straight-chain primary alcohols. C4 and higher straight-chain primary alcohols lose 18, 33, and 46 Daltons from the molecular ion. Branched aliphatic alco-... 

Fragmentation Characteristic losses from the molecular ions areas follows ... 

Generally the acid or protonated acid is observed. The aromatic alcohols can be differentiated by the loss of 18 Daltons from the molecular ion. 

The molecular ion typically is not observed but can be deduced by adding 31 to the highest-mass peak observed (even in the case of methyl cyanoacetate). An intense ion in the mass spectra of methyl esters is m/z 74, formed in a McLafferty rearrangement. Also present is m/z 87, but it is of low intensity. Characteristic losses from the molecular ion are 31, 40 (small), 59, and 73 Daltons. 

The methyl esters of aromatic cyano acids show intense molecular ions, but the intensity decreases as the length of the side chain increases. Losses from the molecular ion are 31 and 59 Daltons. 

Fragmentation Loss of 105 Daltons from the molecular ion is usually observed. 

Alkyl halides are more easily identified than the dichloro halides. Figure 17.1 shows the mass spectra of two dichlorohexanes. Long-chain saturated halides may also lose an alkyl portion from the molecular ion, such as 15, 29, 43, 57, 71, 85, and 99 Daltons. These can be identified as halogenated compounds, but it is difficult to deduce their molecular weights without Cl or negative Cl. 

Molecular ion Molecular ions of cyclic ketones are relatively intense. Characteristic fragment ions of cyclic ketones occur ai m/z 28, 29, 41, and 55. Cyclic ketones also lose CO and/or C>H (m/z 28) from the molecular ion (Q and higher). Low-abundance ions corresponding to loss of H20 are frequently observed. Keto-steroids are a special class of cyclic ketones and have abundani molecular ions. 

The aliphatic mononitriles may not show molecular ions, but M - 1, M - 27, or M - 28 are usually observed. Sometimes a loss of 15 Daltons may also be observed. If CH3 is replaced by CF3, as in the case of CF3CH2CH2CN, a fluorine is first lost from the molecular ion, followed by the loss of HCN (from the M -F ion). This influence of the CF3 group diminishes as the alkyl chain length increases. 

The o-nitrotoluene isomer is easy to identify because of the loss of OH from the molecular ion. All the nitrotoluenes lose 30 Daltons from their molecular ions. The m- and p-isomers can be distinguished from each other by the relative abundances of the m/z 65 ion versus the molecular ion, particularly if both isomers are present (see Figure 22.1). 

The molecular ion is the base peak in the spectra of the dinitroanilines. Other important ions occur at m/z 153,137, 107, and 9]. The m/z 137 ion is very weak in the 2,6-isomer, which is also characterized by the loss of 17 Daltons from the molecular ion. 

Dichloronitrotoluenes are indicated by the presence of an odd molecular ion with chlorine isotopes showing two chlorine atoms and losses of 30 and 46 Daltons. Again, when the chlorine atoms are on the benzene ring, the loss of chlorine from the molecular ion does not occur. An M - Cl ion indicates that at least one of the chlorines is on the alkyl group. 

Trichlorofon loses HC1 from the molecular ion, producing a spectrum identical to that 0 dichlorovos. These pesticides can be differentiated by preparing a TMS derivative. 

Fragment ions The losses of 28 and 29 Daltons from the molecular ions are characteristic. Methylphenol can be distinguished from dimethylphenol by comparing the M - 1 and the M - 15 peaks. Methylphenol has an intense M - 1 ion, whereas the M - 15 for dimethylphenol is more abundant. Methylphenol can be distinguished from benzyl alcohol by the m/z 107 ion. which is characteristic of alkylphenols. 

Daltons below the molecular ion. Typically, this ion corresponds to M - (R + 42), where 42 is C3H6. If there is no side chain, as in the cases of estrone, estradiol, and estriol, the substitution on the D-ring can be determined using the following losses from the molecular ions ... 

B. Fragment Ions Common losses from the molecular ions include 90 and 105 Daltons. (See Figure 31.2 for the mass spectrum of the TMS derivative of cholesterol.)... 

Fragmentation Characteristic losses from the molecular ion include 26, 33, and 44 (C=S) Daltons. Ions characteristic of the phenyl group are also observed. 

M - 14 Possible Precursor Compounds Not observed from a molecular ion however, a mixture of two different compounds may be present. The higher mass ion is M—H. The apparent loss of 14 would be the loss of 15 from the molecular ion. 

Examine fragment ions to determine the mass of the neutral fragments that were lost from the molecular ion, even though these high-mass peaks may be of low abundances. Compare the neutral loss from the molecular ion with the neutral losses tabulated in Part III to see if these losses agree with the suspected structural type. 

The mass spectra of aliphatic aldehydes show m/z 29 (CHO) for C] -C3 aldehydes and m/z 44 for C4 and longer chain aldehydes. Characteristic losses from the molecular ion ... 

Aldehydes are distinguished from alcohols by the loss of 28 and 44 Daltons from the molecular ion. The M — 44 ion results from the McLafferty rearrangement with the charge remaining on the olefinic portion. 

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