Inorganic chemistry,sulfonic acid (orsulphonic acid) refers to a member of the class oforganosulfur compounds with the general formulaR−S(=O)2−OH, where R is an organicalkyl oraryl group and theS(=O)2(OH) group asulfonyl hydroxide.[1][2] A sulfonic acid can be thought of assulfuric acid with one hydroxyl group replaced by an organicsubstituent. Theparent compound (with the organic substituent replaced by hydrogen) is the parent sulfonic acid,HS(=O)2(OH), atautomer ofsulfurous acid,S(=O)(OH)2.[a]Salts oresters of sulfonic acids are calledsulfonates.
Most sulfonic acids and, indirectly, most sulfonate salts are produced by treatment of organic compounds withsulfur trioxide. One large scale application of this method is the production ofalkylbenzenesulfonic acids:[3]
In this reaction, sulfur trioxide is anelectrophile and thearene is the nucleophile. The reaction is an example ofelectrophilic aromatic substitution.[1]
In a related process, terminal alkenes react with sulfur trioxide to giveα-olefin sulfonic acids (and hydroxysulfonic acid):
A third large-scale reaction of sulfur trioxide to give organic sulfonic acids starts simply with saturated hydrocarbons. Calledsulfoxidation, alkanes are irradiated with a mixture ofsulfur dioxide andoxygen. This reaction is employed industrially to produce alkyl sulfonic acids, which are used assurfactants.[3]
Direct reaction of alkanes with sulfur trioxide is used for the conversionmethane tomethanedisulfonic acid.
Carboxylic acids react with sulfur trioxide to give the sulfonic acids.[4]
From terminal alkenes, alkane sulfonic acids can be obtained by the addition ofbisulfite.
Bisulfite can also bealkylated byalkyl halides:[3]
Sulfonic acids can be prepared by oxidation ofthiols:
Typical oxidants includepotassium permanganate, chlorine (followed by hydrolysis), andnitric acid[5]The biosynthesis oftaurine proceeds by oxidation of the thiol.
Many sulfonic acids are prepared by hydrolysis ofsulfonyl halides and related precursors. Thus,perfluorooctanesulfonic acid is prepared by hydrolysis of the sulfonyl fluoride, which in turn is generated by theelectrofluorination 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 theReed reaction.
Vinylsulfonic acid is derived by hydrolysis ofcarbyl sulfate, (C2H4(SO3)2), which in turn is obtained by the addition of sulfur trioxide toethylene.
Sulfonic acids are strong acids. They are around a million times stronger than the correspondingcarboxylic acid. For example,p-Toluenesulfonic acid andmethanesulfonic acid havepKa values of −2.8 and −1.9, respectively,[6] while those ofbenzoic acid andacetic acid are 4.20 and 4.76, respectively. The pKa of methanesulfonic acid has been reported to be as high as −0.6[7] or as low as −6.5.[8] Sulfonic acids are known to react with solid sodium chloride (salt) to form the sodiumsulfonate and hydrogen chloride.[9] 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.[citation needed] They are also usually colourless and nonoxidizing,[10] 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.[3]
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 ofsulfuric acid.[11]
Both alkyl and aryl sulfonic acids are known, most large-scale applications are associated with the aromatic derivatives. Often, e.g. fordetergents,[12] dyes,[13], andion exchange resins (water softening), they are converted to thesulfonate salts, not the acid.
Being strong acids, sulfonic acids are also used ascatalysts. The simplest examples aremethanesulfonic acid, CH3SO2OH andp-toluenesulfonic acid, which are regularly used inorganic chemistry as acids that are lipophilic (soluble in organic solvents). Polymeric sulfonic acids are also useful.Dowex resin are sulfonic acid derivatives ofpolystyrene and is used as catalysts and for ion exchange (water softening).Nafion, a fluorinated polymeric sulfonic acid is a component of proton exchange membranes infuel cells.[14]
Sulfa drugs, a class of antibacterials, are produced from sulfonic acids.
The reactivity of the sulfonic acid group is extensive. Many reactions entail conversions first to the sulfonate salt.[15]
Although strong, the (aryl)C−SO3− bond can be broken by nucleophilic reagents. Such conversions sometimes calledalkaline 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.[13] An early method for producingphenol involved the base hydrolysis of sodiumbenzenesulfonate, which can be generated readily from benzene.[16]
The conditions for this reaction are harsh, however, requiring 'fused alkali' or molten sodium hydroxide at 350 °C for benzenesulfonic acid itself.[17] 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 a14C labeling, despite the lack of stabilizing substituents.[18] Sulfonic acids with electron-withdrawing groups (e.g., with NO2 or CN substituents) undergo this transformation much more readily.
Arylsulfonic acids are susceptible to hydrolysis, the reverse of the sulfonation reaction:
Whereas benzenesulfonic acid hydrolyzes above 200 °C, many derivatives are easier to hydrolyze. Thus, heating aryl sulfonic acids in aqueous acid produces the parent arene. This reaction is employed in several scenarios. In some cases the sulfonic acid serves as a water-solubilizing protecting group, as illustrated by the purification of para-xylene via its sulfonic acid derivative. In the synthesis of2,6-dichlorophenol, phenol is converted to its 4-sulfonic acid derivative, which then selectively chlorinates at the positions flanking the phenol. Hydrolysis releases the sulfonic acid group.[19]
Sulfonic acids can be converted toesters. This class oforganic compounds has the general formula R−SO2−OR.Sulfonic esters such asmethyl triflate are considered goodalkylating agents inorganic synthesis. Such sulfonate esters are often prepared byalcoholysis of the sulfonyl chlorides:
Sulfonyl halide groups (R−SO2−X) are produced by chlorination of sulfonic acids usingthionyl chloride. Sulfonyl fluorides can be produced by treating sulfonic acids withsulfur tetrafluoride:[20]
Arylsulfonic acids react with two equiv of butyl lithium to give the ortho-lithio derivatives, i.e.,ortho-lithiation. These dilithio sulfonates are suited for reactions with many electrophiles.[15]
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