CK-12-Chemistry Intermediate

25.3. Functional Groups http://www.ck12.org

Alkyl Halides

Analkyl halideis an organic compound in which one or more halogen atoms are substituted for one or more
hydrogen atoms in a hydrocarbon. The general formulas for organic molecules with functional groups use the letter
R to stand for the rest of the molecule outside of the functional group. Because there are four possible halogen atoms
(fluorine, chlorine, bromine, or iodine) that can act as the functional group, we can use the general formula R−X to
represent an alkyl halide. The rules for naming simple alkyl halides are listed below.

1. Name the parent compound by finding the longest continuous carbon atom chain that also contains the halogen.
   Add a prefix for the particular halogen atom. The prefixes for each of the four halogens arefluoro-,chloro-,
   bromo-, andiodo-. If more than one kind of halogen atom is present, put them in alphabetical order. If there
   is more than one of the same halogen atom in a compound, use the prefixesdi-,tri-, ortetra-before the prefix
   for the halogen.


2. As with branched hydrocarbons, start numbering the parent carbon chain on the end that will give the sub-
   stituents the lowest possible numbers. If there are multiple substituents, it is the sum of the numbers that
   should be considered. If different halogens are present in equivalent positions, give the lower number to the
   one that comes first in alphabetical order.


3. Add the numerical prefix into the name before the halogen prefix.


4. Separate numbers with commas and separate numbers from names or prefixes with a hyphen. There are no
   spaces in the name.

Shown below (Figure25.16) are some examples of names and structural formulas for a few alkyl halides.

FIGURE 25.16

Note that for the structure based on methane, no number needs to be used since there is only one carbon atom. In
the third example, the chloro substituent is listed first alphabetically, and the chain is numbered so that the sum of
the numbers is as low as possible.

Properties of Alkyl Halides

Alkyl halides are seldom found in nature but can be prepared from their parent hydrocarbons. A class of simple
alkyl halides called chlorofluorocarbons (CFCs) was once very widely used in aerosol sprays and as refrigerants.
One such example is CCl 3 F, called trichlorofluoromethane. Unfortunately, CFCs are harmful to the ozone layer of
our upper atmosphere. Ozone is critical in limiting the amount of damaging ultraviolet radiation that reaches Earth.
A CFC molecule breaks down in the atmosphere when exposed to solar radiation.

CCl 3 F(g)

solar radiation

−→ CCl 2 F(g) + Cl(g)

The free chlorine atoms react readily with molecules of ozone, creating chlorine monoxide and molecular oxygen.

Cl(g) + O 3 (g)→ClO(g) + O 2 (g)

The ClO is then able to react with isolated oxygen atoms that are also present in the atmosphere, regenerating the
free chlorine atoms.