preface to fifteenth edition

7.126 SECTION 7
7.8.2 Mass Spectra and Structure
The mass spectrum is a fingerprint for each compound because no two molecules are fragmented
and ionized in exactly the same manner on electron-impact ionization. In reporting mass spectra the
data are normalized by assigning the most intense peak (denoted as base peak) a value of 100. Other
peaks are reported as percentages of the base peak.
A very good general survey for interpreting mass spectral data is given by R. M. Silverstein, G.
C. Bassler, and T. C. Morrill, Spectrometric Identification of Organic Compounds, 4th ed., Wiley,
New York, 1981.
7.8.2.1 Initial Steps in Elucidation of a Mass Spectrum
1. Tabulate the prominent ion peaks, starting with the highest mass.
2. Usually only one bond is cleaved. In succeeding fragmentations a new bond is formed for each
additional bond that is broken.
3. When fragmentation is accompanied by the formation of a new bond as well as by the breaking
of an existing bond, a rearrangement process is involved. These will be even mass peaks when
only C, H, and O are involved. The migrating atom is almost exclusively hydrogen; six-membered
cyclic transition states are most important.
4. Tabulate the probable groups that (a) give rise to the prominent charged ion peaks and (b) list
the neutral fragments.
7.8.2.2 General Rules for Fragmentation Patterns
1. Bond cleavage is more probable at branched carbon atoms: tertiary secondary primary. The
positive charge tends to remain with the branched carbon.
2. Double bonds favor cleavage beta to the carbon (but see rule 6).
3. A strong parent peak often indicates a ring.
4. Saturated ring systems lose side chains at the alpha carbon. Upon fragmentation, two ring atoms
are usually lost.
5. A heteroatom induces cleavage at the bond beta to it.
6. Compounds that contain a carbonyl group tend to break at this group; the positive charge remains
with the carbonyl portion.
7. For linear alkanes, the initial fragment lost is an ethyl group (never a methyl group), followed
by propyl, butyl, and so on. An intense peak at mass 43 suggests a chain longer than butane.
8. The presence of Cl, Br, S, and Si can be deduced from the unusual isotopic abundance patterns
of these elements. These elements can be traced through the positively charged fragments until
the pattern disappears or changes due to the loss of one of these atoms to a neutral fragment.
9. When unusual mass differences occur between some fragments ions, the presence of F (mass
difference 19), I (mass difference 127), or P (mass difference 31) should be suspected.
7.8.2.3 Characteristic Low-Mass Fragment Ions
Mass 30 Primary amines
Masses 31, 45, 59 Alcohol or ether