Aldehydes mass spectrum

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 mass spectrum of 2-methylbenzaldehyde suggests an aromatic compound because of the intensity of the molecular ion and peaks at m/z 39, 51, and 65 (see Figure 6.2). The loss of hydrogen atoms and loss of 29 Daltons from the molecular ion indicate that this is an aromatic aldehyde. Looking up m/z 91 in Part III suggests the following structure ... 

Compound 37a showed the absence of an aldehydic proton and the singlet around 8.15 ppm was assigned to the ethylenic proton located p with respect to the electron-withdrawing cyano and ester groups. The benzofuranyl coumarins 38 exhibited the carbonyl-stretching band around 1690 cm in the IR spectra (Table 6). PMR data for 13 compounds are given in Table 2. The El mass spectrum of 36a showed a molecular ion peak at m/z 324 (41%). 

Fig. 11.4. Electron ionization mass spectrum of nonanal. Unlike the previous example (toluene, Fig. 11.3), this 9-carbon alkyl aldehyde displays extensive fragmentation and a very low abundance molecular ion at mlz 142. The extensive degree of fragmentation exhibited by many compounds under El conditions makes manual interpretation complex and tedious. Consequently, computerized searches of spectral libraries find extensive use in compound identification.

Fig. 2.9.15. APCI-FIA-MS-MSMS-MS(+) (CID) product ion mass spectrum of selected [M + NH4]+ parent ion (m/z 266) compound was identified as metabolite (aldehyde compound) of degradation product of non-ionic surfactant compound PPG (general formula H0-(CH(CH3)-CH2-0)x-CH(CH3)-CH0 ( = 3) fragmentation behaviour under CID presented in the inset [22].

Identification. Identification of the carbonyl PFBOA derivatives was performed by mass spectrometry using electron impact ionization running in the scan mode. It was confirmed that fragment m/z 181 was the main fragment of all analyzed aldehydes (6). Figure 1 shows as an example the mass spectrum of the PFBOA derivative of methi-onal. To increase the selectivity of the method, all aldehyde analyses were run in the... 

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. 

McLafferty Rearrangement of Ketones and Aldehydes The mass spectrum of butyraldehyde (Figure 18-4) shows the peaks we expect at m/z 72 (molecular ion), m/z 57 (loss of a methyl group), and m/z 29 (loss of a propyl group). The peak at m/z 57 is from cleavage between the /3 and y carbons to give a resonance-stabilized carbocation. This is also a common fragmentation with carbonyl compounds like the other odd-numbered peaks, it results from loss of a radical. 

But in one particular case, the reaction between pentan-2-one and 4-chlorobenz-aldehyde, a different product is formed. The mass spectrum shows that two aldehydes have reacted with one ketone as usual, but that only one molecule of water has been lost. 

Treatment of 2 with 3 equiv of 3-phenyl-2-propenal in refluxing toluene-cfe while the reaction progress was monitored by H NMR spectroscopy resulted in the disappearance of the aldehyde hydrogen peak (5 1.56). The IR spectrum of 4 shows a new absorption due to a vC-o stretch at 1448 cm 1. The mass spectrum of the product shows a molecular ion at m/z 538. To our surprise, an X-ray study of 4 showed it to be the insertion product of the two carbonyl ligands into the C-Si bond in 2. The reaction has the potential for developing a new method for double C-C bond formation between the carboranyl unit and carbonyl compounds. Such an insertion of the carbonyl functionality into the o-carborane has been observed in Yamamoto s work on the chemoselective addition of o-carborane to the aldehyde groups by a palladium-catalyzed9 or a fluoride-promoted reaction.10... 

Compound B has M 140/142 (3 1) and a carbonyl group (IR 1700 cm ) and looks i vr C7H5CIO. It is an aldehyde (5h 9.9) and again still has the disubstituted benzene. The structure 3 even easier this time We have included the fragmentation in the mass spectrum just to remind that this starts with a radical reaction too. Occasionally in the mass spectrum (gas phase, era vacuum) you get the very rare loss of H, a reaction that never occurs in solution. Here it does because of the great stability of the acylium ion (139/140) (p. 554). 

A second alkaloid without the oxazolidine ring, but with the carbino-lamine system is cycloneosamandaridine (XXVII). The IR-spectrum evidences a five-membered lactone ring (band at 1780 cm i) as well as the carbinolamine group (bands at 1050/1180 cm i). The IR-spectrum of the X-acetyl compound no longer shows these hands at 1050-1180 cm i, hut an additional carbonyl frequency at 1735 cm i for an aldehyde group. The mass spectrum confirms the molecular formula as well as structure XXVII(id). 

---