Sulfuric acid, fuming hydrocarbons

The sulfonation of alkyl aromatic hydrocarbons as typified by dodecyl benzene is a relatively simple chemical operation which can be successfully carried out with 98% sulfuric acid, fuming sulfuric acid, or any combination of the two. 

Introduction. It will be recalled that one of the most common methods of distinguishing between aromatic and aliphatic hydrocarbons is the difference in the rates of their reactions with sulfuric acid. Aromatic hydrocarbons readily form sulfonic acids when heated with concentrated sulfuric acid at temperatures varying from 80 to 200 . Saturated paraffin hydrocarbons, on the other hand, do not react with sulfuric acid under comparable conditions. A number of saturated paraffins are sulfonated directly by using fuming sulfuric acid and heating under pressure, but the sulfonic acids of the lower paraffin hydrocarbons are prepared by reacting alkyl halides with alkali sulfites. The sulfonic acids of the aromatic hydrocarbons are of much greater importance than the sulfonic acids of paraffins. 

Substitution of the sulfonic acid (—S03H) group for a hydrogen of an aromatic hydrocarbon can be carried out. by heating the hydrocarbon with a slight excess of concentrated or fuming sulfuric acid ... 

Mixtures with sulfur or sulfuric acid (used in commercial reactions) may explode if heated to 120°C. Initiation temperature depends on the quality of the dinitronaphthalene. When heated to decomposition it emits toxic fumes of NOx. See also NITRO COMPOUNDS of AROMATIC HYDROCARBONS. 

DOT CLASSIFICATION 6.1 Label Poison SAFETY PROFILE Poison by ingestion, intravenous, and intraperitoneal routes. Moderately toxic by intramuscular route. Mutation data reported. Acute symptoms of exposure are headache, nausea, vomiting, weakness and stupor, cyanosis and methemoglobinemia. Chronic exposure can cause liver damage. Experimental reproductive effects. Combustible when exposed to heat or flame. See NITRATES for explosion and disaster hazards. To fight fire, use water spray or mist, foam, dry chemical, CO2. Vigorous reaction with sulfuric acid above 200°C. Reaction with sodium hydroxide at 130°C under pressure may produce the explosive sodium-4-nitrophenoxide. When heated to decomposition it emits toxic fumes of NOx. See also m-NITROANILINE, o-NITROANILINE, NITRO COMPOUNDS OF AROMATIC HYDROCARBONS, and ANILINE DYES. 

Benzene and toluene are insoluble in sulfuric acid, whereas the sulfonic acids are readily soluble completion of reaction is indicated simply by disappearance of the hydrocarbon layer. When shaken with fuming sulfuric acid at room temperature, benzene reacts completely within 20 to 30 minutes, whereas toluene is found to react within only a minute or two. 

Aryl halides show much the same response to characterization tests as the hydrocarbons from which they are derived insolubility in cold concentrated sulfuric acid inertness toward bromine in carbon tetrachloride and toward permanganate solutions formation of orange to red colors when treated with chloroform and aluminum chloride dissolution in cold fuming sulfuric acid, but at a slower rate than that of benzene. 

Sulfonation by Means of Fuming Sulfuric Acid. (Demonstration) Use n-hexane, cyclohexane and benzene. Place 0.5 ml of each hydrocarbon in separate marked test tubes. Add about 1 ml of fuming sulfuric acid (20-25 per cent SO3), and shake cautiously from time to time over a period of 15 minutes. Add the contents of each tube very slowly and carefully to a 150 ml beaker containing 25-30 ml of water. When all the acid mixture has been added stir the contents of the beaker, and then note whether a layer of hydrocarbon separates on the top of the diluted acid. If the hydrocarbon is completely sulfonated, the product is soluble. Record your data. 

Concentrated or fuming sulfuric acid (oleum) is widely used for the direct sulfonation of aromatic compounds (see Chapter 7, p. 97).5,6 The active sulfonating agent in sulfuric acid is the electrophile sulfur trioxide, and the sulfonating power of sulfuric acid is proportional to the concentration of S03. Consequently, fuming sulfuric acid, which contains excess sulfur trioxide, is a more powerful sulfonating agent than concentrated sulfuric acid. The sulfonation of an aromatic hydrocarbon is depicted in Scheme 26. 

---