This page explains how the M+2 peak in a mass spectrum arises from the presence of chlorine or bromine atoms in an organic compound. It also deals briefly with the origin of the M+4 peak in compounds containing two chlorine atoms.
One chlorine atom in a compound
The molecular ion peaks (M+ and M+2) each contain one chlorine atom - but the chlorine can be either of the two chlorine isotopes, 35Cl and 37Cl.
Halogen Atomic
Any halogen atom can substitute for a hydrogen atom in an organic compound, giving rise to a halogenated organic compound, thus an organic halide. The substitution is usually effected by the attack of a hydrogen halide such as HCl upon the double bond of an alkene. Halogen atoms may be one of the four differing substituents on a sp3 hybridized carbon atom, resulting in chirality and optical isomerism with R and S enantiomers: Halogen atoms also may be one of the differing substituents on a sp 2 hybridized carbon atom of.
The molecular ion containing the 35Cl isotope has a relative formula mass of 78. The one containing 37Cl has a relative formula mass of 80 - hence the two lines at m/z = 78 and m/z = 80.
Notice that the peak heights are in the ratio of 3 : 1. That reflects the fact that chlorine contains 3 times as much of the 35Cl isotope as the 37Cl one. That means that there will be 3 times more molecules containing the lighter isotope than the heavier one.
So . . . if you look at the molecular ion region, and find two peaks separated by 2 m/z units and with a ratio of 3 : 1 in the peak heights, that tells you that the molecule contains 1 chlorine atom.
You might also have noticed the same pattern at m/z = 63 and m/z = 65 in the mass spectrum above. That pattern is due to fragment ions also containing one chlorine atom - which could either be 35Cl or 37Cl. The fragmentation that produced those ions was:
Two chlorine atoms in a compound
The lines in the molecular ion region (at m/z values of 98, 100 ands 102) arise because of the various combinations of chlorine isotopes that are possible. The carbons and hydrogens add up to 28 - so the various possible molecular ions could be:
28 + 35 + 35 = 98
28 + 35 + 37 = 100
28 + 37 + 37 = 102
If you have the necessary math, you could show that the chances of these arrangements occurring are in the ratio of 9:6:1 - and this is the ratio of the peak heights. If you don't know the right bit of math, just learn this ratio! So . . . if you have 3 lines in the molecular ion region (M+, M+2 and M+4) with gaps of 2 m/z units between them, and with peak heights in the ratio of 9:6:1, the compound contains 2 chlorine atoms.
Compounds containing bromine atoms
Bromine has two isotopes, 79Br and 81Br in an approximately 1:1 ratio (50.5 : 49.5 if you want to be fussy!). That means that a compound containing 1 bromine atom will have two peaks in the molecular ion region, depending on which bromine isotope the molecular ion contains. Unlike compounds containing chlorine, though, the two peaks will be very similar in height.
The carbons and hydrogens add up to 29. The M+ and M+2 peaks are therefore at m/z values given by:
29 + 79 = 108
29 + 81 = 110
Hence, if two lines in the molecular ion region are observed with a gap of 2 m/z units between them and with almost equal heights, this suggests the presence of a bromine atom in the molecule.
Contributors and Attributions
Jim Clark (Chemguide.co.uk)
Halogens as Functional Groups
James Richard Fromm
Halogen Elements
If atoms other than carbon and hydrogen are substituted for part of a hydrocarbonmolecule, the chemical reactivity of the hydrocarbon is generally increased. Thenonhydrocarbon part of the molecule is called a FUNCTIONAL GROUP. Most of the chemical reactivity of the substituted hydrocarbon is due to thefunctional group attached to it.
One family of substituted hydrocarbon molecules has a halogen atom substituted for ahydrogen atom. For example, if we substitute a bromine atom for a hydrogen atom onmethane, we obtain
CH3Br
Bromomethane (methylbromide)
In subsequent discussions of compounds, we will represent any hydrocarbon radical by R-and any halogen atom by -X. The general formula for the halogen-substitutedhydrocarbon compounds is R-X. As one might expect, it is possible to have more thanone hydrogen atom replaced by a halogen atom. In the compound
CCl4
Tetrachloromethane (carbon tetrachloride)
Halogens On The Periodic Table
tetrachloromethane, more commonly called carbon tetrachloride, four chlorine atoms aresubstituted for the four hydrogen atoms in a methane molecule. Trichloromethane(CHCl3), or chloroform, which is used extensively as a solvent andformerly was employed as an anesthetic, is another example of a multisubstitutedhydrocarbon.
Trichloromethane(Chloroform)
Organic halides are organic compounds in which one or more hydrogen atomshave been substituted by a halogen atom. The IUPAC name for halides is the same asbranched chain hydrocarbons. The branch is named by shortening the halogen name to fluoro-,chloro-, bromo-, or iodo-. The halide(s) are treated as branched groups and arelocated on the continuous chain of carbons as you would locate and name any alkyl branch.
| Fluorine | Fluoro- |
| Chlorine | Chloro- |
| Bromine | Bromo- |
| Iodine | Iodo- |
Prefixes for the first four members of the halogen family
We number the carbon atoms to avoid any ambiguity in naming the compounds. Thus,
HClC=CClCH2CH3 or CH3CH2CCl=CHCl
Halogen Atom List
Cl
|
CH=C-CH2-CH3
|
Cl
1,2-dichloro-1-butene
In multisubstituted aromatic compounds, it is necessary to indicate the relativepositions of the various substituent groups on the ring. If only two substituentgroups are attached, the compound can be named using a prefix to designate the position ofthe substituents. The three possible relative positions of two substituent groups,and the corresponding prefixes are:
Halogen Atomic Radius
For example, the molecule
may be called ortho-dichlorobenzene or 1,2-dichlorobenzene.
If more than two substituents are attached to the benzene ring, it is necessary toassign position numbers to the carbon atoms of the ring. The atoms in the benzene ring arenumbered so as to give the smallest position numbers to the substituents. For example:
is 1,3-dibromobenzene rather than 1,5-dibromobenzene. In thenaphthalene molecule, the 1-position is next to the atom without a hydrogen atom attached. There are four 1-positions possible in each molecule of naphthalene. The1-position which gives the lowest numbers to substituents is always used. Thenumbering system for naphthalene requires that carbon number 1 must begin at one of fourpositions. Those positions are either one of the two uppermost positions or lowermost positions as represented on the molecule below. Once that position isidentified all carbons furthest from the center of the molecule are number in order thatcarbons 9 and 10 are always those which form the bridge creating the 'appearance'of two benzene rings.
Any halogen atom can substitute for a hydrogen atom in an organic compound, giving riseto a halogenated organic compound, thus an organic halide. The substitution isusually effected by the attack of a hydrogen halide such as HCl upon the double bond of analkene. The attack of a halogen molecule such as chlorine upon the double bond of analkene usually leads to attachment of both halogen atoms, one to each carbon of the doublebond, rather than single substitution.
H2FCCHFCH2CH3 is 1,2-difluorobutane
F
|
2HF + CH=CH-CH2-CH3 CH2-CH-CH2-CH3 + H2
|
F
Hydrofluoric Acid + 1-Butene 1,2-Difluorobutane + Hydrogen Gas
CH3CHFCHFCH3 is 2,3-difluorobutane
H F
| |
2HF + CH3-CH=CH-CH3 CH3-C-C-CH3 + H2
| |
F H
Hydrofluoric Acid + 2-Butene 2,3-Difluorobutane + Hydrogen Gas
Organic halides are stable compounds. The halogen-carbon bond is polar, with the moreelectronegative halogen being more negative as expected. Organic halides are namedas their parent unsubstituted compound with the substituent and its location indicated. Other examples include: H2FCCH2CHFCH3,
F
|
CH2-CH2-CH-CH3
|
F
1,3-difluorobutane, and
H2FCCH2CH2CH2F.
F
|
CH2-CH2-CH2-CH2
|
F
1,4-difluorobutane.
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Copyright 1997 James R. Fromm