The chemical formula of sodium dodecyl benzene sulfonate is C18H29NaO3S and its molecular weight is 384.48. It is a white or yellowish powder and is soluble in water. Sodium dodecyl benzene sulfonate is neutral, sensitive to the hardness of water, not easy to oxidation, strong foaming ability, high decontamination power, easy to mix various auxiliaries, low production cost and mature synthesis process. It has been recognized as a safe chemical raw material by international safety organizations and is an anionic surfactant.
It is widely used in the production of various detergents and emulsifiers, and can be properly used in cosmetics such as shampoo, foam bath, etc.; it can also be used as cleaning agent in textile industry, dyeing assistant, degreasing agent in electroplating industry; deinking agent in paper industry also needs to add a certain amount of sodium dodecylbenzene sulfonate. In addition, linear alkylbenzene sulfonate is very stable to oxidizer and soluble in water, so it is very suitable for washing powder with oxidizing bleach.
The following picture shows the molecular structure formula of sodium dodecyl benzene sulfonate.
Synthetic Method Analysis
At present, the common production methods of sodium dodecyl benzene sulfonate are as follows.
1. SO3 sulfonation method
The reaction is exothermic, and the reaction rate is extremely fast, almost instantaneous. There are many kinds of side reactions, and the post-treatment steps are many and complex. Due to the high reactivity of SO3, it is necessary to dilute the liquid with solvent, and dilute the gas with dry air or inert gas.
The advantages of this method are that no water is generated during sulfonation, the amount of sulfur trioxide can be close to the theoretical amount, the reaction is fast, and the waste liquid is small. However, sulfur trioxide is too active to produce sulfones and other by-products during sulfonation, so it is often diluted with air or solvent.
2. Excessive H2SO4 Sulfonation Method
The water generated by the reaction will reduce the concentration of sulfuric acid and the reaction rate, so it is necessary to use a lot of sulfonating agent to make the reaction go smoothly. Aromatics that are difficult to sulfonate should be sulfonated with oleum. At this time, the free sulfur trioxide is mainly used, so a lot of sulfonating agent should be used.
Sulfuric acid sulfonation is widely used. This method has almost no side reaction. Compared with the first method, the post-treatment is relatively simple, the reaction conditions are mild, the heat release is small, and the conversion rate is high.
3. Sulfonation of Chlorosulfonic Acid
Chlorosulfonic acid is also a common sulfonating agent, which can be regarded as SO3·HCl complex. Sulfonation with chlorosulfonic acid can be carried out in greenhouse, the reaction is irreversible, basically according to stoichiometry. The reaction rate is fast and the yield of the product is high, but chlorosulfonic acid is toxic and hydrogen chloride is produced in the reaction process.
In addition, there are azeotropic dehydration sulfonation method, aromatic primary amine baking sulfonation method and so on. Here, dodecylbenzene is used as raw material, various factors that need to be considered in the synthesis process are analyzed, the appropriate synthesis scheme is determined, and the feasibility of the scheme is tested by synthesis according to this scheme.
Analysis of Sulfonation Process
The sulfonation of dodecylbenzene is an electrophilic substitution reaction on the benzene ring. When the aromatic compound is sulfonated, the reaction is divided into two steps. First of all, the electrophilic particles attack the aromatic ring to form a σ complex, and then remove protons in the presence of alkali to obtain benzenesulfonic acid.
The obvious difference between this process and the nitration and halogenation of aromatics is that the charge of the σ complex is neutral and its stability is higher than that of the corresponding nitration or halogenation intermediate complexes with positive charge. Therefore, the second step of deprotonation of sulfonation is much more difficult than the corresponding nitration and halogenation. When sulfonated in concentrated acid, deprotonation is the control stage of reaction rate, and when sulfonated in dilute acid, the formation of σ complex is the control step of reaction rate.
The Friedel-Crafts alkylation of alkyl aromatics with long carbon chain in strong acid is reversible and the product dodecylbenzenesulfonic acid can be dealkylated. This problem is especially serious when sulfuric acid is used as sulfonating agent, it is relatively better when sulfonated in fuming sulfuric acid, and it is very smooth when SO3 is used as sulfonating agent.
Main reaction is as follow.
The sulfonation reaction is affected by the nature of the sulfonation raw material and the reaction conditions. While the main reaction is going on, there are a series of secondary reactions and parallel side reactions.
1. Formation of sulfones. The formation of sulfones is an important side reaction in the sulfonation of arenes. When a violent sulfonating agent is used, the reaction temperature is high or the reaction time is too long, it is beneficial to the formation of sulfone. The amount of sulfone produced during sulfonation of dodecylbenzene accounts for about 1%. The improvement of process conditions and equipment, such as controlling the temperature not too high or adding sodium toluenesulfonate, is beneficial to reduce the amount of sulfone.
2. Generation of polysulfonic acid. When the amount of sulfonating agent is too large, the reaction time is too long, and the temperature is too high, especially when a strong sulfonating agent is used, multiple sulfonation is prone to occur.
Synthesis Operation
Taking dodecylbenzene as raw material and sulfuric acid as sulfonating agent for sulfonation as an example.
The reaction scheme is as follows.
1. Sulfonation
In a 250mL four-port bottle equipped with agitator, thermometer, drip funnel and reflux condenser, dodecylbenzene 35mL (34.6g) was added, 98% sulfuric acid 35mL was slowly added under stirring, and the temperature was not more than 40 oC. After addition, the temperature was raised to 60~70 oC, and the reaction was carried out for 2 hours.
2. Separate Acid
Lower the temperature of the sulfonated mixture to 40~50 oC, slowly add appropriate amount of water (about 15mL), pour into the liquid separation funnel, rest for a moment, and stratify. Remove the water and inorganic salts in the lower layer and retain the organic phase in the upper layer.
3. Neutralization
Prepare a 10% sodium hydroxide solution 80mL, add it to a 250mL four-port bottle about 60~70mL, slowly add the above organic phase under stirring, control the temperature at 40~50 oC, adjust the pH=7~8 with 10% sodium hydroxide, and record the total amount of 10% sodium hydroxide.
4. Salting-out
In the above reaction system, a small amount of sodium chloride was added, the permeation ring test was clear and filtered, and the white paste product was obtained.
Precautions
When separating the acid, the temperature should not be too low, otherwise the separating funnel will be blocked by inorganic salts, making it difficult to separate the acid.
Separation and Detection
1. Post-treatment by Diluted Acid Dilution Method
The target product sodium dodecylbenzene sulfonate is colorless crystal, insoluble in cold water but slightly soluble in acetone, and easily soluble in N-dimethylformamide. Because it is insoluble in cold water, the target product will crystallize out of the system during acidification, and can be separated by filtration. The process is shown in the figure below.
Schematic diagram of post-treatment process of dilute acid method
Precautions are as follows.
1) In order to precipitate the target product as much as possible, it is usually necessary to use low-temperature cooling; secondly, adding alkali is to minimize the impact on residual sulfuric acid in the solution, and at the same time, it is also to better generate sodium sulfonate.
2) There will be a small amount of mother liquor remaining on the surface of the filtered filter residue, which generally needs to be washed and removed first. At the same time, in order to reduce the loss in the washing process, a small amount of washing should be used as much as possible. If the method of vacuum filtration is used, the effect is better. The separation device adopts a filter device.
2. Purification and Refining
Since the target product is a solid, and its solubility in solvents such as water changes significantly with temperature, recrystallization or chromatography can be considered for product purification.
3. Product Testing and Identification
It can be identified by the method of measuring melting point and infrared spectrum.
