Sulfonation of benzene, as well as other aromatic hydrocarbons, is one of the most important reactions in organic chemistry, since its products are widely used in industry. To obtain sulfonic acids, benzene is treated with sulfuric acid or oleum. Sulfonation can also be carried out stepwise to create several functional groups.
Sulfonation principle
Sulfonation of benzene is the introduction of the sulfo group SO 3 H — its substitution of a hydrogen atom in an organic compound. As a result, the resulting substances acquire acidic properties and become soluble in water. In the future, by cleavage of the sulfo group, other compounds can be synthesized - aminobenzene (aniline), halogenobenzene, nitrobenzene, hydroxybenzene (phenol).
Not only hydrocarbons themselves, but also their derivatives are subjected to this process. Homologous benzene compounds (toluene and xylene) are sulfonated more easily, and the most active reaction is with aniline and phenol.
The benzene sulfonation reaction, along with nitration, is one of the most characteristic for aromatic hydrocarbons. Substitution of hydrogen in such substances is much easier than in aliphatic organic compounds.
Sulfonating agents
The following substances can be used as sulfonating agents:
- sulfuric acid H 2 SO 4 in various concentrations;
- oleum (fuming sulfuric acid);
- chlorosulfonic acid - HSO₃Cl;
- sulfuric anhydride;
- vitriol oil (H 2 SO 4 at a concentration of 93% or more);
- sulfuric acid monohydrate;
- sulfuryl chloride SO₂Cl₂;
- alkaline salts of sulphurous acid;
- polysulfates;
- acid salts of sulfuric acid.
The most frequently used oleum sulfonation mechanism is a solution of SO 3 in 100% sulfuric acid. Varieties of oleum are selected those in which the concentration of sulfur trioxide is about 65%, since these compositions remain liquid under ordinary conditions. When using this substance, the benzene sulfonation reaction mechanism occurs at a high rate.
Sulfonation Process: Description
The reaction of benzene with oleum occurs as follows:
C 6 H 6 + H 2 SO 4 ---> C 6 H 5 -SO 3 H + H 2 O.
The structural order of the benzene sulfonation reaction is as follows:
The target product is sulfonic acid. When treated with 92-94% sulfuric acid, their yield is 90-96%.
Sulfonation with a 3-fold excess of concentrated sulfuric acid proceeds according to the following scheme:
Since the process is slow (although heat is released), heating is required.
The disadvantages of sulfonation of benzene with sulfuric acid include:
- low concentration of electrophiles;
- a rapid drop in the reaction rate upon dilution of the acid with water;
- aggressiveness of the reaction medium;
- the need for the use of neutralizing reagents, a large amount of waste (environmental aspect).
Product properties
The chemical properties of benzene during sulfonation are characterized by the following features:
- colorless or light yellow fusible crystalline substance with a pungent odor;
- good solubility in water, almost complete dissociation;
- high absorption capacity with respect to water vapor; crystals disperse in air;
- pronounced acidic character;
- the compound is decomposed by water with the release of toxic gases;
- vapors are heavier than air;
- acidity constant is in the range of 5.0-8.0.
One of the most important properties of sulfonic acids is their ability to exchange a sulfo group for other functional groups, the direct introduction of which into the structure of aromatic hydrocarbons is difficult.
The introduction of several sulfo groups
The substitution of two or three hydrogen atoms is carried out stepwise, by a phased change of the following parameters:
- sulfonating agent;
- the concentration or amount of this substance;
- temperature.
Obtaining meta-benzenedisulfonic acid is carried out in 2 stages:
- Sulfonation of C 6 H 6 with concentrated acid H 2 SO 4 , resulting in the formation of monosulfonic acid.
- Processing the product of the previous reaction with an excess of 20% oleum at a temperature of 100 ° C (or 60% at 60-80 ° C). Since the initial introduction of the sulfo group significantly slows down the subsequent sulfonation (approximately 10,000 times), the second reaction is carried out under more severe conditions.
Further sulfonation to 3- and 5-benzenesulfonic acids is possible only when exposed to oleum at 60% concentration at a temperature of 250 ° C in the presence of a mercury sulfate catalyst.
Under normal conditions, the sulfonation reaction is reversible, but if oleum with a high content of anhydride is used, the process becomes irreversible. The same is observed when benzene is treated with chlorosulfonic acid.
Temperature effect
An increase in temperature during benzene sulfonation not only increases the reaction rate, but also leads to the formation of by-products: polysulfonic acids, sulfones, and oxides. When heated above 160 ° C, the appearance of disulfonic acids is noted.
Therefore, each reaction should be carried out at a certain temperature. The site of attachment of the SO 3 H group (regioselectivity) in aromatic hydrocarbons also depends on it.
Sulphonation "in pairs"
Under production conditions, one of the most economical methods for producing benzenesulfonic acid is to conduct the reaction in “vapors”. This technology reduces the need for H 2 SO 4 by almost 2 times, compared with the process in which the liquid phase of this substance is used. The disadvantages of the reaction include a decrease in the rate of sulfonation.
Elimination of the disadvantages associated with the use of H 2 SO 4 in a liquid state is possible by removing water from the reaction mass or by using oleum (increasing the concentration of SO 3 ).
The technology of benzene sulfonation in this case proceeds as follows:
- Benzene from the tank is fed into the piping of the evaporator using a pump. In this device, the substance is heated by steam passing through external pipes. Then the pairs are adjusted to t = 150 ° C.
- In a vaporous state, benzene is fed in excess to the reactor, where it passes through a layer of concentrated sulfuric acid at a temperature of 150 ° C.
- In a refrigerator, C 6 H 6 and H 2 O condense and cool to a temperature of 30 ° C.
- In the separator, the mixture is stratified, and recycled benzene is returned to sulfonation.
Residues of benzene in the sulfomass are removed by blowing air. Then the mixture enters the neutralization in the apparatus, lined with acid-resistant tiles inside. This reaction is carried out with the participation of an aqueous solution of Na₂SO₃. After processing, the mass is sent to the column to eliminate sulfur dioxide. The resulting product is used to obtain phenol by alkaline melting (currently this process has lost its industrial value, phenol is synthesized from isopropylbenzene) or evaporated to dry residue sodium benzenesulfonate.
other methods
Other benzene sulfonation techniques are also used:
- Liquid hydrocarbon is passed countercurrently through a layer of H 2 SO 4 . The resulting product is first dissolved in benzene, and then washed with water.
- Sulfonation with low concentration oleum at an elevated temperature of 190-250 ° C and a pressure of 1-3 MPa. By-products are suppressed by the administration of sodium benzenesulfonate.
Isolation of sulfonic acids
Since the reaction products are readily soluble, they are isolated in the form of salts. To do this, neutralizing agents are introduced into the sulfomass, with constant stirring, as which the following substances are used:
- sodium sulfite;
- a piece of chalk;
- technical soda;
- lime.
Free sulfonic acids are released by reaction with ion-exchange resins or by acidification with hydrogen chloride.