chemistry

Sulfonic acid

Sulfonic acid is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sulfonic acid rather than just read about it. In short: In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2(OH) group a sulfonyl hydroxide. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent.

Key takeaways

Sulfonic acid belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sulfonic acid to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sulfonic acid from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula R−S(=O)2−OH, where R is an organic alkyl or aryl group and the S(=O)2(OH) group a sulfonyl hydroxide. A sulfonic acid can be thought of as sulfuric acid with one hydroxyl group replaced by an organic substituent. The parent compound (with the organic substituent replaced by hydrogen) is the parent sulfonic acid, HS(=O)2(OH), a tautomer of sulfurous acid, S(=O)(OH)2. Salts or esters of sulfonic acids are called sulfonates.

Preparation

Most sulfonic acids and, indirectly, most sulfonate salts are produced by treatment of organic compounds with sulfur trioxide. One large scale application of this method is the production of alkylbenzenesulfonic acids:

RC6H5 + SO3 → RC6H4SO3H In this reaction, sulfur trioxide is an electrophile and the arene is the nucleophile. The reaction is an example of electrophilic aromatic substitution. In a related process, terminal alkenes react with sulfur trioxide to give α-olefin sulfonic acids (and hydroxysulfonic acid):

RCH2CH=CH2 + SO3 → RCH=CHCH2SO3H Likewise, carboxylic acids react with sulfur trioxide to give the sulfonic acids. A third large-scale industrial process starts simply with saturated hydrocarbons. Sulfoxidation, similar to the Reed reaction, irradiates a mixture of alkanes, sulfur dioxide and oxygen:

RH + SO2 + 1/2 O2 → RSO3H UV (or higher-energy) light is usually necessary, but titanium dioxide catalyzes the reaction with visible light. In accord with its free-radical mechanism, the reaction favors secondary positions and produces mixtures. The direct reaction of alkanes with sulfur trioxide is used to convert methane to methanesulfonic and methanedisulfonic acids. Sulfonic acids can be prepared by oxidation of thiols:

RSH + 3/2 O2 → RSO3H Typical oxidants include potassium permanganate, chlorine (followed by hydrolysis), and nitric acid The biosynthesis of taurine proceeds by oxidation of the thiol. Alternatively, bisulfite adds as a nucleophile to terminal alkenes:

HSO−3 + RCH=CH2 → RCH2CH2SO−3 Halides can also alkylate bisulfite directly:

HSO−3 + RBr → RSO3H + Br−

Hydrolyses Many sulfonic acids are prepared by hydrolysis of sulfonyl halides and related precursors. Thus, perfluorooctanesulfonic acid is prepared by hydrolysis of the sulfonyl fluoride, which in turn is generated by the electrofluorination of octanesulfonic acid. Similarly the sulfonyl chloride derived from polyethylene is hydrolyzed to the sulfonic acid. These sulfonyl chlorides are produced by free-radical reactions of chlorine, sulfur dioxide, and the hydrocarbons using the Reed reaction. Vinylsulfonic acid is derived by hydrolysis of carbyl sulfate, (C2H4(SO3)2), which in turn is obtained by the addition of sulfur trioxide to ethylene.

Properties Sulfonic acids are strong acids. They are around a million times stronger than the corresponding carboxylic acid. For example, p-Toluenesulfonic acid and methanesulfonic acid have pKa values of −2.8 and −1.9, respectively, while those of benzoic acid and acetic acid are 4.20 and 4.76, respectively. The pKa of methanesulfonic acid has been reported to be as high as −0.6 or as low as −6.5. Sulfonic acids are known to react with solid sodium chloride (salt) to form the sodium sulfonate and hydrogen chloride. This observation implies an acidity greater than that of HCl. Because of their polarity, sulfonic acids tend to be crystalline solids or viscous, high-boiling liquids. They are also usually colourless and nonoxidizing, which makes them suitable for use as acid catalysts in organic reactions. Their polarity, in conjunction with their high acidity, renders short-chain sulfonic acids water-soluble, while longer-chain ones exhibit detergent-like properties. The structure of sulfonic acids is illustrated by the prototype, methanesulfonic acid. The sulfonic acid group, RSO2OH features a tetrahedral sulfur centre, meaning that sulfur is at the center of four atoms: three oxygens and one carbon. The overall geometry of the sulfur centre is reminiscent of the shape of sulfuric acid.

Applications

Both alkyl and aryl sulfonic acids are known, most large-scale applications are associated with the aromatic derivatives. One primary application is as surfactants. Often, e.g. for detergents, dyes,, and ion exchange resins (water softening), they are converted to the sulfonate salts, not the acid.

Acid catalysts Being strong acids, sulfonic acids are also used as catalysts. The simplest examples are methanesulfonic acid, CH3SO2OH and p-toluenesulfonic acid, which are regularly used in organic chemistry as acids that are lipophilic (soluble in organic solvents). Polymeric sulfonic acids are also useful. Dowex resin are sulfonic acid derivatives of polystyrene and is used as catalysts and for ion exchange (water softening). Nafion, a fluorinated polymeric sulfonic acid is a component of proton exchange membranes in fuel cells.

Drugs Sulfa drugs, a class of antibacterials, are produced from sulfonic acids.

Reactions The reactivity of the sulfonic acid group is extensive. Many reactions entail conversions first to the sulfonate salt.

Hydrolysis to phenols Although strong, the (aryl)C−SO3− bond can be broken by nucleophilic reagents. Such conversions sometimes called alkaline fusion. Of historic and continuing significance is the α-sulfonation of anthroquinone followed by displacement of the sulfonate group by other nucleophiles, which cannot be installed directly. An early method for producing phenol involved the base hydrolysis of sodium benzenesulfonate, which can be generated readily from benzene.

C6H5SO3Na + NaOH → C6H5OH + Na2SO3 The conditions for this reaction are harsh, however, requiring 'fused alkali' or molten sodium hydroxide at 350 °C for benzenesulfonic acid itself. Unlike the mechanism for the fused alkali hydrolysis of chlorobenzene, which proceeds through elimination-addition (benzyne mechanism), benzenesulfonic acid undergoes the analogous conversion by an SNAr mechanism, as revealed by a 14C labeling, despite the lack of stabilizing substituents. Sulfonic acids with electron-withdrawing groups (e.g., with NO2 or CN substituents) undergo this transformation much more readily.

Hydrolytic desulfonation

Arylsulfonic acids are susceptible to hydrolysis, the reverse of the sulfonation reaction:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Sulfonic acid

Start with the simplest possible case. Write down what Sulfonic acid claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sulfonic acid before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sulfonic acid ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sulfonic acid

In research: Sulfonic acid appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sulfonic acid in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sulfonic acid is common in secondary-school and first-year university syllabi. It links to neighbouring topics Functional groups, Sulfonic acids, so understanding it makes those chapters shorter.
In everyday life: Look for Sulfonic acid outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Sulfonic acid in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sulfonic acid means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sulfonic acid out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sulfonic acid in simple terms?

Why does Sulfonic acid matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sulfonic acid?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sulfonic acid.