Lubricant Additives

128 Lubricant Additives: Chemistry and Applications

somewhat less-effi cient sulfonation but better oil solubility of the fi nal sulfonate detergent. The com-
mon reagents used to sulfonate alkylaromatics are sulfur trioxide, fuming sulfuric acid or oleum,
and chlorosulfonic acid [35]. Oleum is 15–30% sulfur trioxide dissolved in concentrated sulfuric
acid. In general, the alkylate is dissolved in a hydrocarbon solvent, such as hexane or heptane, and
reacted with the sulfonating reagent. One obtains a mixture of a monosulfonic acid and disulfonic
or higher sulfonic acids. The latter must be removed because of the high polarity of their metal salts,
hence potentially lower oil solubility. This can be easily achieved by water washing. The disulfonic
and higher sulfonic acids are also undesired because, when reacted with polyvalent metals, they
have the tendency to make polymeric salts that are usually of low lubricant solubility as well.
Not all components of the alkylate are sulfonatable. In the case of alkylbenzenes, the spe-
cies that do not sulfonate easily include polyalkylated benzenes, such as trialkylbenzene, or highly
branched dialkylbenzenes. Their sulfonation diffi culty is primarily a consequence of the steric
crowding of the sulfonatable positions. Monoalkylbenzenes do not suffer from this drawback and
hence sulfonate easily. In general, the sulfonation of branched alkylbenzenes is slower than linear
alkylbenzenes. In the case of alkylbenzenes, Figure 4.4 shows structures that are sulfonatable and
those that are not.

FIGURE 4.3 Synthesis of sulfonic acid substrates.

+ Olefin or alkyl halide Acid

Acid

R

Benzene Alkylbenzene

+ Olefin

R

R R

R

Naphthalene Alkylnaphthalene

+ SO 3 or oleum

SO 3 H

Alkylbenzene Alkylbenzenesulfonic acid

FIGURE 4.4 Alkylbenzene structures.

R R R

R

R

R

RR

Monoalkyl

Sulfonatable

Dialkyl Trialkyl

Not sulfonatable if R group

is highly branched

Not sulfonatable

Alkylbenzene constituents