Sulfonation. It is the introduction of a sulfonic acid group (-SO 3 H) in an organic compound in order to produce, for example, an aromatic sulfonic acid from the corresponding aromatic hydrocarbon.
[ hide ]
- 1 Sulfonating agent
- 2 Sulfonation reaction
- 3 Separation of the two isomers
- 4 Feasibility of continuous operations
- 5 Sources
The most widely used sulfonating agent is concentrated sulfuric acid , although sulfur trioxide , chlorosulfonic acid, metal sulfates, and sulfamic acid may also occasionally be used . However, due to the nature and properties of sulfuric acid, it is highly desirable to use it to carry out nucleophilic substitution whenever possible.
For each substance that is being sulfonated, there is a critical concentration of the acid below which the sulfonation ceases. The removal of the water formed in this reaction is therefore essential. Using a very large excess of acid , while very expensive, can maintain an essentially constant concentration as the reaction progresses. It is not easy to volatilize the water contained in the concentrated solutions of sulfuric acid, although sometimes azeotropic distillation can help to do this.
The sulfonation reaction is exothermic, but it is not highly corrosive, so the sulfonation can be carried out in steel , stainless steel or cast iron sulfonators . A jacket may be used to heat the contents of the reactor enough to start the reaction, and then to extract the reaction heat generated during the reaction. As heating agents, hot oil or steam can be used . The equipment must also be equipped with a good agitator, a condenser and a gas and smoke control system.
1- and 2-naphthalenesulfonic acids are formed simultaneously when naphthalene is sulfonated with concentrated sulfuric acid. The isomers should be separated if it is desired to obtain α- or β-naphthol from the product mixture.
Variations in reaction time , temperature , sulfuric acid concentration, and acid / hydrocarbon ratio alter performance to favor a particular type of isomer, but a pure single substance will never form. Using equal acid / hydrocarbon ratios for each run, sulfonation at 40 ° C temperature will yield 96% α-isomer (alpha-isomer) and 4% β-isomer (beta-isomer), while at 160 ° C the proportions obtained will be 15% α-naphthol and 85% β-naphthol.
The α-sulfonic acid can be hydrolyzed to naphthalene by passing water vapor at 160 ºC over the sulfonated mass. The naphthalene thus formed is transported by the steam and is recovered in a subsequent operation. The pure β-sulfonic acid left behind can be hydrolyzed by caustic fusion to obtain relatively pure β-naphthol.
Separation of the two isomers
In general, the separation of the two isomers is based on some of the following considerations:
- Variations in the rateof hydrolysis of the two isomers.
- Variations in the solubility of different saltsin water.
- Solubility differences in solvents other than water.
- Differences in solubility accentuated by the effect of thecommon ion (additions of salt).
- Differences in the physico-chemical properties of the derivatives obtained.
- Differences based on molecular size, such as the use of molecular or absorption screens.
Sulphonation reactions can be carried out in reactors batch or continuous reactors. Continuous sulfonation reactions are feasible only when organic compounds possess certain physical and chemical properties, and are practical only in a few industrial processes. Most commercial sulfonation reactions are performed in batch operations.
Feasibility of continuous operations
- The organic compound ( benzeneor naphthalene) can be volatilized.
- Reaction rates are high (as occurs in chlorosulfonation of paraffinsand sulfonation of alcohols)
- Production is large (as in the manufacture of detergents, such as alkyl aryl sulphonates)
Reaction water is formed during most sulfonation reactions, and unless a method is devised to prevent excessive dilution of the reaction mixture due to water formation, the sulfonation rate will be reduced. From the economic point of view with regard to the consumption of sulfuric acid, it is advantageous to chemically remove or combine this reaction water. For example, the use of reduced pressures to remove the reaction water has some technical advantages in the sulfonation operations of phenol and benzene .
The use of distillation to pressure part shall be justified by the ability to have the compound diluent, or by the presence of excess reagent volatile chemical, removing the water as it is formed, so to remain so high concentration of sulfuric acid in the reaction mixture. If this concentration is maintained, the need to use extra amounts (excess) of sulfuric acid will be eliminated, since the sole objective of this excess is to keep the concentration of the acid above the desired value. The azeotropic removal of the reaction water during the sulfonation of benzene can be carried out using an excess of vaporized benzene.
The use of oleum (H 2 SO 4 .SO 3 ) is a practical procedure to maintain the concentration of the sulfur trioxide contained in the sulfonation mixture at the desired levels. Preferably, oleum and organic compounds should be added gradually, upstream, to a large volume of acid, to receive the water as quickly as possible as it forms in the reaction. Sulfur trioxide should be added intermittently to the sulfonation reactor to maintain its concentration above the required value in order to obtain the desired degree of sulfonation.