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.
Long answer:
@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
- 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
- R. J. GILLESPIE and J. A. LEISTE "THE BEHAVIOUR OF ORGANIC COMUPOUNDS IN
SULPHURIC ACID "(pdf)
- DETERMINATION OF ACIDITY OF PARAFFIN WAX by V. S. Glukhov, O. I. Lapitskaya,
E. V. Zosimov, and G. M. Grechko (pdf)
- 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)