Page 276 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 18] ORGANIC CHEMISTRY 265

Compounds in the alkane series are chemically rather inert. Aside from burning in air or oxygen to produce
carbon dioxide and water (or carbon monoxide and water), the most characteristic reaction they undergo is
reaction with halogen molecules. The latter reaction is initiated by light. Using pentane as a typical alkane
hydrocarbon and bromine as a typical halogen, these reactions are represented by the following equations:
C 5 H 12 + 8O 2 −→ 5CO 2 + 6H 2 O
light
C 5 H 12 + Br 2 −→ C 5 H 11 Br + HBr
Because of their limited reactivity, the saturated hydrocarbons are also called the paraffins. This term is derived
from the Latin words meaning “little affinity.”

Unsaturated Hydrocarbons
The alkene series of hydrocarbons is characterized by having one double bond in the carbon chain of each
molecule. The characteristic formula for members of the series is C n H 2n . Since there must be at least two carbon
atoms present to have a carbon-to-carbon double bond, the first member of this series is ethene, C 2 H 4 , also known
as ethylene. Propene (propylene), C 3 H 6 , and butene (butylene), C 4 H 8 , are the next two members of the series.
Note that the systematic names of these compounds denote the number of carbon atoms in the chain with the
name derived from that of the alkane having the same number of carbon atoms (Table 18-2). Note also that the
characteristic ending -ene is part of the name of each of these compounds.
Owing to the presence of the double bond, the alkenes are said to be unsaturated and are more reactive than
the alkanes. The formula for propene, for example, is CH 2 =CHCH 3 . The alkenes may react with hydrogen gas
in the presence of a catalyst to produce the corresponding alkane; they may react with halogens or with hydrogen
halides at relatively low temperatures to form compounds containing only single bonds. These possibilities are
illustrated in the following equations in which ethene is used as a typical alkene:

catalyst
CH 2 CH 2 + H 2 −−−→ CH 3 CH 3
CH 2 + Br 2 −→ CH 2 BrCH 2 Br
CH 2
CH 2 CH 2 + HBr −→ CH 3 CH 2 Br
The alkyne series of hydrocarbons is characterized by having molecules with one triple bond each. They
have the general formula C n H 2n−2 and the name ending -yne. Like other unsaturated hydrocarbons, the alkynes
are quite reactive. Ethyne is commonly known as acetylene. It is the most important member of the series
commercially, being widely used as a fuel in acetylene torches and also as a raw material in the manufacture of
synthetic rubber and other industrial chemicals.
The aromatic hydrocarbons are characterized by molecules containing six-member rings of carbon atoms
with each carbon atom attached to a maximum of one hydrogen atom. The simplest member of the series is
benzene, C 6 H 6 . Using the total bond order rules discussed above, the structural formula of benzene can be written
as follows:
H H
H C H H C H
C C C C
or
C C C C
H C H H C H
H H

Such a molecule, containing alternating single and double bonds, would be expected to be quite reactive. Actually,
benzene is quite unreactive, and its chemical properties resemble those of the alkanes much more than those of
the unsaturated hydrocarbons. For example, the characteristic reaction of benzene with halogens resembles that
of the reaction of the alkanes:
C 6 H 6 + Br 2 −→ C 6 H 5 Br + HBr
Fe