Chemistry Reference
In-Depth Information
grease separates from surface
and is held in suspension
water
molecules
The calcium and magnesium salts of this detergent
molecule are water soluble, so the problem of scum
is solved. Very many of our washing powders (and
liquids) contain this type of substance.
soap
molecules
with
agitation
Manufacture of soapless detergents
Soapless detergents are detergents that can be used
more effectively than soap, particularly in hard water
areas (see Chapter 11, p. 188). They are fairly cheap
to make and are rapidly replacing soaps.
The general process involves, initially, the reaction
of a long, straight-chain alkene, such as dodecene
(CH
3
(CH
2
)
9
CH
grease
Figure 15.17
Soap dissolves grease like this.
On the other hand, the hydrophilic ionic head group
is not attracted to the grease but is strongly attracted
to the water molecules. When the water is stirred, the
grease is slowly released and is completely surrounded
by the soap molecules. The grease is, therefore,
'solubilised' and removed from the dish. The soap is
able to remove the grease because of the combination
of the covalent and ionic bonds present.
R
CH
2
), with benzene.
benzene + dodecene → dodecylbenzene
C
6
H
6
(
l
) + CH
3
(CH
2
)
9
CHRCH
2
(
l
) → C
6
H
5
(CH
2
)
11
CH
3
(
l
)
The molecular formula of dodecylbenzene is C
18
H
30
.
This compound is then reacted with concentrated
sulfuric acid to give a compound that is known as a
sulfonic acid.
Soapless detergents
In Chapter 11, p. 188, we discussed the way in
which, in hard water areas, an insoluble scum
forms when soap is used. This problem has been
overcome by the development of synthetic
soapless
detergents
. These new substances do not form
scum with hard water since they do not react with
Ca
2+
and Mg
2+
present in such water. Furthermore,
these new soapless detergent molecules have been
designed so that they are biodegradable. Bacteria
readily break down these new molecules so that they
do not persist in the environment.
Sodium alkyl benzene sulfonates were developed
in the early 1970s. The structure of sodium
3-dodecylbenzene sulfate, C
18
H
29
SO
3
Na, is given
below.
dodecylbenzene + sulfuric → dodecylbenzene + water
acid
sulfonic acid
C
18
H
30
(
l
)
+ H
2
SO
4
(
l
) → C
18
H
29
SO
3
H(
aq
) + H
2
O(
l
)
Finally, this is reacted with the alkali sodium
hydroxide, NaOH.
dodecylbenzene +
sodium →
sodium
+ water
sulfonic acid
hydroxide
dodecylbenzene
sulfonate
+ NaOH(
aq
) → C
18
H
29
SO
3
−
Na
+
(
aq
)
C
18
H
29
SO
3
H(
aq
)
+ H
2
O(
l
)
(soapless detergent)
These days, soapless detergents such as this are to be
found in most washing powders and liquids.
Questions
1
What class of organic compound do substances like
glyceryl stearate belong to?
2
What do you understand by the terms:
a
hydrophobic?
b
hydrophilic?
c
saponifi cation?
3
What is the main advantage of detergents over soaps?
SO
3
Na
