Halogens as Functional Groups

James Richard Fromm

If atoms other than carbon and hydrogen are substituted for part of a hydrocarbon molecule, the chemical reactivity of the hydrocarbon is generally increased.  The nonhydrocarbon part of the molecule is called a FUNCTIONAL GROUP.   Most of the chemical reactivity of the substituted hydrocarbon is due to the functional group attached to it.

One family of substituted hydrocarbon molecules has a halogen atom substituted for a hydrogen atom.  For example, if we substitute a bromine atom for a hydrogen atom on methane, we obtain


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-substituted hydrocarbon compounds is R-X.  As one might expect, it is possible to have more than one hydrogen atom replaced by a halogen atom.  In the compound


Tetrachloromethane  (carbon tetrachloride)

tetrachloromethane, more commonly called carbon tetrachloride, four chlorine atoms are substituted for the four hydrogen atoms in a methane molecule. Trichloromethane (CHCl3), or chloroform, which is used extensively as a solvent and formerly was employed as an anesthetic, is another example of a multisubstituted hydrocarbon.

Chloroform.gif (285 bytes)

Trichloromethane (Chloroform)

Organic halides are organic compounds in which one or more hydrogen atoms have been substituted by a halogen atom.  The IUPAC name for halides is the same as branched 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 are located 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,




In multisubstituted aromatic compounds, it is necessary to indicate the relative positions of the various substituent groups on the ring.  If only two substituent groups are attached, the compound can be named using a prefix to designate the position of the substituents.  The three possible relative positions of two substituent groups, and the corresponding prefixes are:

ortho- meta- para-
1,2- 1,3- 1,4-

For example, the molecule

dichlorobenzene.gif (689 bytes)

may be called ortho-dichlorobenzene or 1,2-dichlorobenzene.

If more than two substituents are attached to the benzene ring, it is necessary to assign position numbers to the carbon atoms of the ring. The atoms in the benzene ring are numbered so as to give the smallest position numbers to the substituents. For example:

dichlorobenzene.gif (689 bytes)

is 1,3-dibromobenzene rather than 1,5-dibromobenzene. In the naphthalene 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.  The 1-position which gives the lowest numbers to substituents is always used.  The numbering system for naphthalene requires that carbon number 1 must begin at one of four positions.  Those positions are either one of the two uppermost positions or lower most positions as represented on the molecule below.  Once that position is identified all carbons furthest from the center of the molecule are number in order that carbons 9 and 10 are always those which form the bridge creating the "appearance" of two benzene rings.

naphthalene.gif (471 bytes)

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.  The attack of a halogen molecule such as chlorine upon the double bond of an alkene usually leads to attachment of both halogen atoms, one to each carbon of the double bond, rather than single substitution.

H2FCCHFCH2CH3 is 1,2-difluorobutane

2HF + CH=CH-CH2-CH3 arrow2.gif (113 bytes) CH2-CH-CH2-CH3 + H2
       Hydrofluoric Acid + 1-Butene arrow2.gif (113 bytes) 1,2-Difluorobutane + Hydrogen Gas

CH3CHFCHFCH3  is 2,3-difluorobutane

                                                 H   F
                                                 |    |
2HF + CH3-CH=CH-CH3 arrow2.gif (113 bytes) CH3-C-C-CH3 + H2
                                                 |    |
                                                 F   H
       Hydrofluoric Acid + 2-Butene arrow2.gif (113 bytes) 2,3-Difluorobutane + Hydrogen Gas

Organic halides are stable compounds. The halogen-carbon bond is polar, with the more electronegative halogen being more negative as expected.  Organic halides are named as their parent unsubstituted compound with the substituent and its location indicated.   Other examples include: H2FCCH2CHFCH3,

     1,3-difluorobutane, and



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Copyright 1997 James R. Fromm