What is Sulfonation?
In chemistry, sulfonation, which is also referred to as Sulphonation, is any of the several methods by which the sulfonic acids are prepared. The reaction of aromatic hydrocarbons with sulfuric acid, chlorosulfonic acid, or sulfur trioxide; the reaction of organic halogen compounds with inorganic sulfites; and the oxidation of certain classes of organic sulfur compounds, particularly disulfides and thiols, are all important sulfonation procedures.
Sulfonation of a Few Compounds
Let us look at the sulfonation of a few compounds here.
Sulfonation of Benzene
The method of heating benzene with fuming sulphuric acid (H2SO4 + SO3) to form benzenesulfonic acid is known as sulfonation of benzene. This reaction, which is represented below, is reversible in nature.
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Sodium Xylenesulfonate
A hydrotrope is an organic compound that enhances the ability of water to dissolve other molecules. Sodium xylenesulfonate is a hydrotrope. Sodium xylene sulfonate is classified as a low-hazard substance, and the risk of adverse health effects associated with consumer and occupational use of this product is expected to be minimal.
Uses and Applications of Sodium Xylenesulfonate
Sodium xylene sulfonate can be used in shampoos and liquid household detergents, printing pastes, and degreasing compounds used in the textile industry. Also, it is a surfactant found in personal care products, mainly in shampoos, due to its ability to serve as a wetting agent or Clariant that helps a formula spread very quickly. In the paper industry, sodium xylene sulfonate can be used to remove lignin and pentosans, and in the leather industry, it can be used as a glue additive.
Sulphonation Reaction
The replacement of the hydrogen atom of an organic compound with sulfonic acid (-SO3H) functional group, often by the reaction with sulfuric acid at higher temperatures, is called Sulphonation.
“The introduction of a sulfonic acid group into an aromatic compound is referred to as sulphonation.”
A Few Examples are Listed Below
In the Case of Benzene
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In the Case of Phenols
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In the Case of Nitrobenzene
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In the Case of Naphthalene
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Dodecylbenzenesulfonic Acid
Dodecylbenzene is a kind of dodecylbenzene. Sulfonic acid is a dense liquid that ranges from light yellow to brown in color. It can be used to make detergents. Dodecylbenzene Sulfonic Acid lies on the Hazardous Substance List due to the reason it is cited by EPA and DOT. This particular chemical is present on the Special Health Hazard Substance List due to the reason it is CORROSIVE.
Sulphonation of Phenol
The reaction of the phenol with concentrated sulphuric acid is called sulphonation of phenol. The sulphonation product is determined by the operating temperature.
At a low temperature, phenol reacts with the concentrated H2SO4 to form o-phenol sulphonic acid. At a low temperature, the neighboring SO3H group and OH group interact with each other. Thus, ortho isomers predominate.
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At a high temperature, it is not possible to form any of the interactions, and thus steric repulsion overcomes the attraction. Therefore, at high temperatures, p-phenol sulphonic acid can be obtained as represented in the above figure.
Reason behind the benzene and Hexa deuterobenzene rate of reaction varies from sulphonation.
Protium contains one proton and zero neutrons in contrast to the Deuterium, which has a neutron, and hence, it is twice as heavy. That particular mass difference leads to smaller vibrations or a stronger, shorter bond.
Therefore, a Carbon-Deuterium bond is stronger in comparison with the Carbon-Protium bond.
In order to assess the impact of this difference in bond strength in the rate of an Electrophilic Aromatic Substitution, we should check the mechanism:
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As we can see, the actual electrophile in the sulfonation is given as SO3.
We have two steps: formation of the intermediate σ-complex (which is also called Wheland intermediate) and the subsequent shift of hydrogen (which is also called Deuterium) to restore the aromaticity.
The first step involves the loss of aromaticity and can be considered to be the slow step. And, in the second step, the protium or Deuterium-bond is broken. So, the second step is expected to be slower for the Deuterium in comparison with the protium.
Based on the fact that the first step is very slow, we do not expect an actual isotope effect.
It turns out that for the nitration and bromination, there is indeed no isotope effect; however—apparently — there is an isotope effect in the sulfonation reaction (although it depends on the conditions).
In order to explain this clearly, it means that the second step is the rate-determining or something else is going on:
In this way, this can be explained based on the fact - the first step can go back to the original situation. The rate constant of the inverse reaction is given as k−1.
We would not expect an actual difference in this inverse reaction between the Protium and Deuterium.
In the case where most of the intermediate product proceeds to the final reaction product k−1<k2, we would expect no isotope effect.
In contrast to the cases where the reverse reaction can be faster or about equal to the reaction towards the end product (thus, k−1>k2 ). In those cases, we would expect to notice an isotopic effect.
FAQs on Sulfonation
1. What is sulfonation in organic chemistry?
Sulfonation is a chemical reaction that introduces a sulfonic acid group (-SO₃H) into an organic compound. Most commonly, it involves treating an aromatic compound, like benzene, with a sulfonating agent to replace a hydrogen atom on the ring with the -SO₃H group. This process is fundamental in synthesising various industrial chemicals.
2. What type of reaction is the sulfonation of an aromatic ring?
The sulfonation of an aromatic ring is a classic example of an electrophilic aromatic substitution (EAS) reaction. In this process, an electrophile, which is typically sulfur trioxide (SO₃), attacks the electron-rich aromatic ring, leading to the substitution of a hydrogen atom.
3. What are the common reagents used for carrying out sulfonation?
The most common reagents, or sulfonating agents, used for sulfonation are:
- Concentrated sulfuric acid (H₂SO₄): Generally used for more reactive compounds or at higher temperatures.
- Fuming sulfuric acid (Oleum): This is a solution of sulfur trioxide (SO₃) in sulfuric acid (H₂SO₄ + SO₃). It is a much stronger sulfonating agent, ideal for reactions like the sulfonation of benzene.
- Chlorosulfonic acid (ClSO₃H): Another powerful reagent used for this process.
4. Can you explain the mechanism of sulfonation of benzene?
The sulfonation of benzene using fuming sulfuric acid involves three main steps as per the electrophilic aromatic substitution mechanism:
- Step 1: Generation of the electrophile. Sulfur trioxide (SO₃), the actual electrophile, is present in oleum or generated from the self-ionisation of sulfuric acid.
- Step 2: Attack of the electrophile. The electron-rich benzene ring attacks the SO₃ electrophile, forming a resonance-stabilised carbocation intermediate known as a sigma complex or Wheland intermediate. This step temporarily disrupts the ring's aromaticity.
- Step 3: Deprotonation. A base, such as HSO₄⁻, removes a proton (H⁺) from the sigma complex to restore the aromaticity of the ring, resulting in the final product, benzenesulfonic acid.
5. What is a key difference between sulfonation and other electrophilic aromatic substitution reactions like nitration?
A key difference is reversibility. Sulfonation is a reversible reaction, whereas other common EAS reactions like nitration and Friedel-Crafts alkylation are generally considered irreversible under typical conditions. This reversible nature allows the sulfonic acid group to be used as a temporary blocking group in organic synthesis to direct other substituents to specific positions.
6. Why is the product of the sulfonation of phenol dependent on temperature?
The product distribution in the sulfonation of phenol is temperature-dependent due to a balance between thermodynamic and kinetic control.
- At low temperatures (approx. 298 K), the reaction is kinetically controlled, and the major product is the ortho-isomer, o-phenolsulphonic acid, because it forms faster.
- At high temperatures (approx. 373 K), the reaction becomes thermodynamically controlled. The more stable para-isomer, p-phenolsulphonic acid, is the major product because the reaction is reversible, allowing the initially formed ortho-product to rearrange to the sterically less hindered and more stable para-product.
7. What is the real-world importance of sulfonation reactions?
Sulfonation is a highly important industrial process with several key applications. Its primary use is in the production of surfactants and detergents, such as linear alkylbenzene sulfonates (LAS), which are the active ingredients in most laundry and cleaning products. Other important applications include the synthesis of sulfa drugs (a class of antibiotics), various dyes, and ion-exchange resins for water purification.
8. What is the reverse of sulfonation called and how is it achieved?
Yes, the reverse of sulfonation is known as desulfonation. This process is typically achieved by heating the aromatic sulfonic acid with dilute aqueous acid (like dilute H₂SO₄) or by passing superheated steam through it. The reaction removes the -SO₃H group and replaces it with a hydrogen atom, regenerating the original aromatic compound.
9. Is there any difference between the terms 'sulfonation' and 'sulphonation'?
No, there is no chemical difference. Both 'sulfonation' and 'sulphonation' refer to the exact same chemical process. 'Sulfonation' is the spelling preferred by the International Union of Pure and Applied Chemistry (IUPAC) and is standard in American English. 'Sulphonation', with a 'ph', is the traditional British English spelling. Both correctly describe the introduction of a sulfonic acid group.
