6.18 Reviewing General Principals

After surveying common compounds encountered in organic chemistry and their corresponding spectra we are able to make some generalizations about fractionation patterns.  When dealing with a compound’s molecular ion;

Unknown compounds, in the absence of a reference standard, can often be more difficult but not impossible to discern given the prevalence of multiple fragments.  The majority of compounds, however, will abide by the following rules.

Besides having a general set of guidelines that govern general fractionation, it is also important to be able to identify patterns that are indicative of particular functional groups.  As a result, a condensed table of the commonly observed fragmentation patterns is listed in the table below. 

Table 6.3 A Review of Common Fragmentation Patterns.

Functional Group

Observed Fragments

M/Z Value

Straight Chain Alkanes

CnH2n+1

43, 57, 71, …

[M – CH3]

[M – 15]

[M – CH2CH3]

[M – 29]

[M – CH2CH2CH3]

[M – 43]

Branched Alkanes

CnH2n

Various

Cyclic Alkanes

[M – H2C=CH2]

[M – 28]

Alkenes

CnH2n-1

Various

CnH2n

Various

Aromatics

91

77

56

Alcohols

[M – H2O]

[M – 18]

[M – (H2O & H2C=CH2)]

[M – 46]

[M – (CH3 & H2O)]

[M – 33]

Primary Alcohols

CH2OH

31

Ketones

43 + R

Various

Aldehydes

44

COH

29

[M – H2O]

[M – 18]

[M – H2C=CH2]

[M – 28]

[M – H2C=CH–OH]

[M – 44]

Carboxylic Acids

60

[M – OH]

[M – 17]

[M – CO2H]

[M – 45]

CnH2n-1O2

73, 87, …

Ethers

β cleavage

various

various

Esters

74, 88, …

various

R’+

various

various

Amines

various

CnH2n+2N

58, 72, …

Primary Amines

CH2NH2

30

Amides

CH2NH2

30

44

59

86

Nitriles

[M – H]

[M – 1]

41

 

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