# Mechanism of reaction of ketones with isoalkanes under conc. sulfuric acid and heat

Taken from the book of GOC by Dr. O.P Tandon, Himanshu Pandey, Dr. A.K. Virmani,

Can anyone elaborate on the mechanism?

• It seems like an interesting reaction, if it is correct; I've never seen alkanes reacting in such a manner. But surely acetone should undergo acid-catalysed aldol, it seems weird that the book has not mentioned that at all. Jul 28 at 13:32
• @TRC Under "saturated aliphatic hydrocarbons"
– Neha
Jul 28 at 13:40
• @AshishAhuja Oh yes, of course it can. I meant that the final product depicted here certainly isn't acetone's self-aldol product, rather it looks like a sort of condensation between the two compounds.
– TRC
Jul 28 at 13:40
• Due to lack of much complexity there is only really 1 plausible way to look at this, you have to generate a nucleophile to make any headway. This acidic media makes it impossible to do that. $\ce{H2SO4}$'s most extreme reaction is charring action (It never generates nucleophiles anyway). I don't see anyway to facilitate any further reaction. Self aldol looks way more plausible. Jul 28 at 14:08
• Jul 28 at 18:24

This was going to be a comment but it got too long.

TL;DR: - This is not an answer, rather a justification for why the question is (probably) wrong.

This paper (linked by @Rishi) gives us experimental evidence that under action of concentrated $$\ce{H2SO4}$$ hydrogens are exchanged from paraffins in the following fashion:

\begin{align} \ce{(CH3)2CHCH3 + 2H2SO4 -> (CH3)C^{+}OSO3H^{-} + SO2 + 2H2O & \tag{R1}}\\ \ce{(CH3)3C+ + (CH3)3CH -> (CH3)3CH + (CH3)3C+ & \tag{R2}}\\ \end{align}

R1 is the slow step and chain initiating step. R2 is the chain propagating step (the hydride transfer here creates a chain of approximately 20 tert-butyl carbocation.

Conc. $$\ce{H2SO4}$$ is a well known dehydrating and oxidising agent and tert-butyl carbocation has considerable stability due to extensive hyperconjugation. So it is possible that the reaction mechanism is of the following manner:

From the paper by Otvos et.al.1:

Chains are terminated by some irreversible side reactions of carbonium ions leading to nonvolatile products. This picture would account for the lack of exchange between t-isobutane hydrogen atoms and sulfuric acid, since any molecule dissolving in acid, reacting and re-entering the vapor phase, would have received a new tertiary hydrogen from another isobutane molecule.

The chain termination step is where we run into a problem. Carbonyl group cannot act as a nucleophile at the carbon center. There is one way to do this but it is extremely unlikely

This is not a good mechanism due to the excess of $$\ce{H^+}$$ present. In fact, any mechanism for the given product wouldn't be satisfactory because any nucleophile produced at carbonyl carbon would be would be quickly protonated (tert-butyl carbocation can't compete with $$\ce{H+}$$ due to steric reasons). Also it is highly unlikely that $$\ce{SO2}$$ would participate in the reaction rather than escape to atmosphere (I suppose you can use sealed tubes).

Further problems are the much better suited side reactions such as acid-catalyzed aldol and attack of $$\ce{C=O:}$$ lone pair on the carbocation.

Citation

1. The Behavior of Isobutane in Concentrated Sulfuric Acid1 J. W. Otvos, D. P. Stevenson, C. D. Wagner, and O. Beeck Journal of the American Chemical Society 1951 73 (12), 5741-5746 DOI: 10.1021/ja01156a075
• I fixed some punctuation and citation. Feel free to roll back if you don't like the edit. Tip regarding formatting: Instead of spamming ~, use \begin{align}. It looks pleasant. Aug 3 at 8:27