Question

When n-hexane is heated with anhydrous AlCl\textsubscript{3 and HCl gas, the major product obtained is}

• A. 1-chlorohexane
• B. 2-chlorohexane
• C. 3-chlorohexane
• D. hex-3-ene
• E. mixture of 2-methylpentane and 3-methylpentane

Hint:

Whenever you see $AlCl_3$ and $HCl$ with an alkane, think Isomerization. It’s not a chlorination reaction; the $AlCl_3$ acts as a Lewis acid to "jumble" the carbon chain!

Answer 1

Answer: (E)

Concept: This reaction is an example of isomerization of alkanes.
• Reagents: Anhydrous $AlCl_3$ and $HCl$ gas act as catalysts to rearrange the carbon skeleton of straight-chain alkanes.
• Process: The linear alkane is converted into its branched-chain isomers. This process is commercially important in the petroleum industry to increase the octane rating of fuels.

Step 1: Identify the structural change. n-hexane ($CH_3-CH_2-CH_2-CH_2-CH_2-CH_3$) is a six-carbon straight chain. Under the influence of $AlCl_3/HCl$, carbocation intermediates are formed which undergo 1,2-hydride and 1,2-methyl shifts.

Step 2: Determine the products. The rearrangement leads to more stable branched structures. For hexane, the primary products are 2-methylpentane and 3-methylpentane.