Chemistry Reference
In-Depth Information
necessary to open a ''door'' that will give chemical access to more of the tools of
the synthetic trade. If one of the terminal hydrogen atoms on the chain is exchanged
for an
OH group (path 1, Figure 2.1), the new material, 1-dodecanol, still has rela-
tively low solubility in water, but it increases substantially relative to the parent
hydrocarbon:
CH
3
ð
CH
2
Þ
10
CH
2
OH or
n-C
12
H
25
OH
While such a modification seems pretty simple on paper, the actual process can be
pretty messy, involving a rather random process of halogenation followed by sub-
stitution to form the alcohol. For purposes of the current discussion, however, we
will pretend to live in a simpler world and assume that only the desired product is
produced. If the alcohol functionality is placed internally on the hydrocarbon chain,
as in 3-dodecanol (path 1a), the resulting material will be similar to the primary
alcohol but will have slightly different solubility characteristics:
CH
3
ð
CH
2
Þ
2
CH
ð
OH
Þð
CH
2
Þ
5
CH
3
Solubility differences among chemical isomers will generally be evident in other
functional modifications. They may also be evident in changes in chemical reacti-
vity and substitution patterns in subsequent chemical reactions. The effects of the
position of substitution on surfactant properties can be quite significant and will be
discussed in more detail later. Once formed, the alcohol can be sulfated (path 2) to
produce dodecane sulfuric acid ester, a strongly acidic compound with good water
solubility. When the sulfuric acid ester is neutralized with alkali, certain alkaline-
earth metals, or organic amines, the material becomes highly soluble in water and
an excellent surfactant. It is, in fact, probably the most extensively studied and best
understood surfactant known to science—sodium dodecylsulfate (SDS):
OSO
3
Na
þ
n-C
12
H
25
OH
þ
SO
3
!
n-C
12
H
25
OSO
3
H
þ
NaOH
!
n-C
12
H
25
n-dodecylsulfuric acid ester
SDS
If the original n-dodecanol is treated with ethylene oxide (OE) and base catalyst
(path 3), the material obtained is a docedylpolyoxyethylene (POE) polyether.
n-C
12
H
25
OH
þ
nOE
!
n-C
12
H
25
ð
OCH
2
CH
2
Þ
n
OH
Such molecules can have widely varying solubility characteristics, depending on
the value of n, the number of OE groups added to the molecule. Because of the
nature of the reaction, the value of n will always be an average with a relatively
large distribution of values. If n
10, the material will be soluble in water and
will show good surfactant properties. If n is as small as 5, its water solubility
will decrease significantly, as will its usefulness as a surfactant. If n is taken to
20 or higher, high water solubility is attained, but most of the good surfactant
¼
