Introduction to Soil Chemistry

organic compounds 59
and may contain smaller amounts of other elements such as the halogens,
phosphorus, and silicon. They can be straight-chained, branched, or cyclic, and
may contain single, double, and triple bonds in any except the simplest molecules.
There are nine principal organic functional groups, as shown in Table
3.2. There are also four important functional groups derived from the acid
functional group as shown in Table 3.3.
These functional groups along with the general structure of the molecule
will determine its solubility and its ease or resistance to degradation.Typically,
as the number of groups capable of forming hydrogen bonds or associating
with hydrogen on the water molecule increases, the more soluble an organic
molecule will be. Conversely, the longer the straight-chain hydrocarbon
portion of a molecule, the less soluble it is. Low-molecular-weight alcohols,
aldehydes, ketones, acids, and amines are soluble, while high-molecular-weight
members of these families are insoluble. These represent a very large group
of compounds with simple and complex structures, including cyclic and multicyclic
compounds containing several functional groups and having a wide
range of solubilities.
Cyclic organic structures are widespread and common. They can contain
any of the functionalities listed in Tables 3.2 and 3.3. Because of bonding angles
and steric effects, some will be highly unstable while others will be particularly
stable and resistant to decomposition.
Table 3.2. The Organic Functional Groups
Family Structure and
Name Composition IUPAC a Name
Alkanes Carbon, hydrogen-bonded with
single bonds
Alkenes Carbon, hydrogen, single bonds
and containing at lease one
double bond
Alkynes Carbon, hydrogen, single bonds
and containing at least one
triple bond
Alcohols Carbon, hydrogen, single bonds
with at least one —OH
(may contain double and triple
bonds and other structures)
H H H
H C C C H
H
H H
Propane
H H H
H C
H
C C H
Propene
H
H C
H
C C H
Propyne
H
H H H
C C C OH
H H H
1-Propanol