# Can sulfuric acid attack a hydrocarbon/paraffin? Under what conditions? [closed]

Sulfuric acid can attack benzene and other hydrocarbons (when mixed with hydrogen peroxide). Also, chlorine gas and bromine can replace hydrogen(s) in hydrocarbons like methane, ethane, propane,butane etc. in a presence of sunlight and/or mild heating.

Can dil. or conc. $$\ce{H2SO4}$$ be used to attack hydrocarbons and produce sulfonates? Please cite an example.

TLDR: Sulfuric acid act as catalyst in the isomerization reaction of parafin hydrocarbons and sometimes it produce undesired sideproducts like sulfonate salts, napthenic acid, bisulfite ions etc. from incomplete oxidation. It follows "hydrogen exchange mechanism.

@andselisk cited some papers that shows that sulfuric acid can act as a catalyst for the isomerization reaction of certain paraffin hydrocarbons. It is observed that sulfuric acid act destructively towards these hydrocarbons and isomerizes and some more interesting observation were made i.e which hydrocarbons isomerizes and which don't(emphasis mine):

It is clear from an examination of the experimental results that two different reactions took place. Isomerization is the only reaction at the milder experimental conditions. Some destructive alkylation takes place with 98% sulfuric acid and higher temperatures, as indicated by the production of low and high boiling compounds. In the experiments with n-octane and neohexane there is no reaction even at the boiling point of respective hydrocarbon and 98% sulfuric acid concentration. 2,4-Dimethylpentane isomerizes mostly to 2,3-dimethylpentane, with minor amounts of 2-methylhexane and 3-methylhexane. The main products of triptane isomerization were 2-methylhexane and 2,3-dimethylpentane. These results substantiate the observations of previous investigators that sulfuric acid is a specific catalyst for shifting the methyl group along the hydrocarbon chain and that only such paraffin hydrocarbons which contain tertiary carbon atoms are susceptible to isomerization with this catalyst.(1)

The mechanism of these reaction were studied previously. Scientist proposed "hydrogen exchange mechanism" from paraffin and sulfuric acid reaction:

Ingold, Raisin, and Wilson were the first to show that hydrogen exchange occurs between paraffins and sulphuric acid. It has recently been found that such exchange occurs readily only in the cases of hydrocarbons that contain hydrogen atoms on tertiary carbon atoms (tertiary hydrogens). The mechanism of the exchange process has been established by studies of the racemisation of optically active paraffins in sulphuric and by a detailed mass-spectrographic investigation using mono-deuterated butanes and [13C]isobutane.

The initial step is the formation of a very small concentration of tertiary carbonium ions by oxidation $$\ce{R3CH + 2H2SO4 -> R3C+ + HSO4- + SO2 + 2H2O}$$, followed by a chain reaction between the carbonium ions and paraffin molecules involving exchange of hydride ions between tertiary carbon atoms. An important postulate of the mechanism is that the carbonium ion intermediate is able to undergo rapid and complete exchange of hydrogen with the sulphuric acid solvent, whereas the hydrocarbon molecules are quite unreactive in this respect. The chains are thought to be terminated by some irreversible side reactions leading to non-volatile products.(2)

In case one is wondering whether some sulfonate salts is formed as a byproduct, then it has been confirmed. The reaction do produce sulfonic acid and other byproducts during the sulfuric acid treatement of wax purification process and these byproduct proved harmful if untreated as they react undesirably(emphasis mine):

Sulfuric acid treatment serves to remove resinous materials, oil, unsaturated hydrocarbons, and other unstable substances from crude wax, as well as to remove polycyclic aromatic compounds, by treatment with 102-103% oleum and subsequent neutralization with caustic.

After sulfuric acid treatment, the wax may contain free sulfuric acid or caustic, the presence of which is explained either by incomplete neutralization of the acid after treatment or by incomplete washing of the caustic used in neutralizing the acid. The presence of either acids or alkalis in waxes is extremely undesirable, since such materials will cause corrosion of metals under the conditions of production, use, and storage. These impurities in commercial paraffin waxes are the cause of undesirable processes of oxidation, resin formation, and decomposition. Apart from free sulfuric acid, derivatives such as sulfonic acids and acid esters may be present in paraffin. These are equivalent to the mineral acid in chemical action. A negative result from the test of a petroleum product for free acid or alkali still does not guarantee that these compounds will not be formed in the petroleum product. If the product contains salts of sulfonic acids, acid esters, naphthenic acids, or byproduct compounds, then the action of high temperature or moisture (hydrolysis) will quite probably result in the formation of acidic substances of a mineral nature.(3)

References

1. Reactions of Paraffin Hydrocarbons in the Presence of Sulfuric Acid V. I. Komarewsky and W. E. Ruther Journal of the American Chemical Society 1950 72 (12), 5501-5503 DOI: 10.1021/ja01168a034
2. R. J. GILLESPIE and J. A. LEISTE "THE BEHAVIOUR OF ORGANIC COMUPOUNDS IN SULPHURIC ACID "(pdf)
3. DETERMINATION OF ACIDITY OF PARAFFIN WAX by V. S. Glukhov, O. I. Lapitskaya, E. V. Zosimov, and G. M. Grechko (pdf)
4. Hydrogen Exchange and Isomerization of Saturated Hydrocarbons with Sulfuric Acid D. P. Stevenson, C. D. Wagner, O. Beeck, and J. W. Otvos Journal of the American Chemical Society 1952 74 (13), 3269-3282 DOI: 10.1021/ja01133a017(@andselisk)