Ketones fragmentation mass

Aldehydes and ketones generally give moderately intense signals due to their molecular ions, M+. Thus the determination of the molecular weight of a ketone by mass spectroscopy usually is not difficult. Furthermore, there are some characteristic fragmentation patterns that aid in structural identification. These are ... 

Figure 13-2 Gaschromatogram (a) and SlM-speclrum (b) of two isomers of an unsaturated ketone after selective enrichment, m/e = relative molecular or fragmental mass t = time.

In addition, to actinidine (14), Janot and co-workers also isolated a new monoterpene alkaloid from the roots of Valeriana officinalis following treatment with ammonia (25). The isolate was analyzed for the formula Ci0H8N2O, and the UV spectrum showed maxima at 261,314, and 324 nm, and the IR spectrum a band at 1680 cm-1 for an aryl ketone. The mass spectrum displayed a molecular ion at m/z 172 with fragment ions at m/z 157 (M+-15) and 129 (MM3), suggesting the presence of a methyl ketone. A methyl singlet was observed at 2.65 ppm, and a complex pattern of five... 

The mass spectrum of a ketone generally has an intense molecular ion peak. Ketones fragment homolytically at the C—C bond adjacent to the C=0 bond, which results in the formation of a cation with a positive charge shared by two atoms. The alkyl group leading to the more stable radical is the one that is more easily cleaved. 

AUcyclic ketones. The mass spectrum of 3,3,5-trimethylcyclohexanone (Figure 8.5) illustrates major additional fragmentation pathways found for cyclic ketones (Equation 9.30) these are similar to those discussed for cyclic alcohols and in Section 8.9. 

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... 

Information regarding the position of the substituents can be obtained from the mass spectra of the enamines of cyclic ketones. For instance in the case of the morpholine enamine of 3-methylcyclohexanone, which is shown to be a 2 1 mixture of/ and isomers by NMR spectroscopy, the fragmentation of the radical ion from the /) isomer results in the loss of a methyl radical from the C-3 position. The d isomer gives a complicated spectrum due to the loss of the hydrogen radical. 

An example of how information from fragmentation patterns can be used to solve structural problems is given in Worked Example 12.1. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. We ll see in the next section and in later chapters that specific functional groups, such as alcohols, ketones, aldehydes, and amines, show specific kinds of mass spectral fragmentations that can be interpreted to provide structural information. 

Molecular ion is usually not observed with perfiuorinated ketones, but may be deduced by adding 19 Daltons to the highest mass observed in the case of perfluoroacetone and 69 Daltons in the case of perfluorodiethyl ketone. A characteristic fragment ion results from a-cleavage ... 

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

The following example demonstrates the usefulness of this approach for the solution of certain mass spectrometric tasks. Ions [M — N2]+ of diazo ketones 4-1 fragment by several pathways, which require their preliminary isomerization into some other structures. The most probable among them are 4-2 and 4-3 (Scheme 5.21). 

When a ketone grows larger it does not necessarily imply that it has two identical alkyl groups at the carbonyl. In case of different alkyls at the carbonyl, Stevenson s rule may also be applied to decide which of them will dominantly be detected as part of the acylium ion and which should preferably give rise to a carbenium ion. Overall, a nonsymmetrical ketone will yield four primary fragment ions in its El mass spectrum... 

Upon extrapolation to larger ketones one can expect to observe larger acylium and alkyl fragments. The occurrence of series of homologous ions is a feature that can be very helpful to deduce structural information from mass spectra. Ions such as the acylium ion series and the carbenium ion series are also known as characteristic ions. Learning the nominal masses of the first members of each series by heart is useful (Tables 6.2 and 6.3). 

Williams, D.H. Budzikiewicz, H. Pelah, Z. Djerassi, C. Mass Spectroscopy and Its Application to Structural and Stereochemical Problems. XLIV. Fragmentation Behavior of Monocychc Ketones. Monatsh. Chem. 1964,95,166-177. 

All hydrazones show a significant absorption band due to the C —N double bond at 1600-1645 cm-1 for ketone hydrazones, and at 1600-1610 cm"1 for the aldehyde derivatives. Mass spectra are characterized by a typical fragmentation pattern and a base peak corresponding to the methoxymethyl fragment6. 

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