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My textbook says:

n-Alkanes on heating in the presence of anhydrous aluminium chloride and hydrogen chloride gas isomerise to branched chain alkanes.

But no mechanism is given. After a little research, I found this: https://goenkanscienceblog.wordpress.com/2015/01/29/suggested-mechanism-of-isomerisation-of-alkanes/

However, it doesn't explain the role of the HCl gas. Is the mechanism correct and if so, what is the role of HCl?

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    $\begingroup$ Interesting question, reminded me of the synthesis of adamantane, so I googled it. The answer along with lots of discussion can be found here - pubs.rsc.org/en/content/articlepdf/1974/CS/CS9740300479 basically trace alkene is present in the alkane sample; AlCl3 abstracts Cl- from HCl, H+ protonates alkene to form carbocation which undergoes 1,2-alkyl shifts. $\endgroup$ – orthocresol Aug 22 '16 at 13:28
  • $\begingroup$ @orthocresol Wouldn't the carbocation increases the number of carbons in the original alkane? $\endgroup$ – JM97 Aug 22 '16 at 13:56
  • $\begingroup$ Don't get what you mean. The website you linked also goes via a carbocation intermediate. $\endgroup$ – orthocresol Aug 22 '16 at 14:07
  • $\begingroup$ @orthocresol There methyl shift is within the alkane whereas here trace alkene is acting as carbocation. $\endgroup$ – JM97 Aug 22 '16 at 14:09
  • $\begingroup$ e.g. in n-butane there is butene impurity - protonated to form butyl carbocation $\endgroup$ – orthocresol Aug 22 '16 at 14:19
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The mechanism for butane is shown here, instead of a general n-alkane.

Initiation

initiation

As the comments have mentioned, it probably begins by the protonation of butene impurities.

The Lewis acid, $\ce{AlCl3}$, stabilizes the chloride ion from $\ce{HCl}$ when the $\ce{H}$ is used to protonate the butene.

A secondary carbocation is formed, which undergoes methyl shift to form a primary carbocation. I have no idea why a primary carbocation can be formed.

Propagation

propagation

An alkane attacks the carbocation, and supplies a "hydride" to the carbocation to complete the carbocation.

Meanwhile, another carbocation is formed which also undergoes methyl shift to regenerate the carbocation at the beginning of this step.

Termination

termation

I am going out of my way here to suggest a isobutyl chloride (1-chloro-2-methylpropane) termination product. I can't find other Lewis bases to complete the carbocation for the termination step.

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