Q16P - Organic Chemistry (Mcmurry)

Question

How could you use 1H NMR to determine the regiochemistry of electrophilic addition to alkenes? For example, does addition of HCl to 1-methylcyclohexene yield 1-chloro-1-methylcyclohexane or 1-chloro-2-methylcyclohexane?

Step-by-Step Solution

Answer

By using proton NMR we can decide the regiochemistry of the addition of HCl to 1-methylcyclohexene.

Step 1:Signals given by different types of protons

Homotopic and enantiotopic protons give one signal.

If the asymmetric center is present then the protons of -CH₂ are diastereotopic. Diastereotopic protons give different signals.

Step 2: Addition of HCl to 1-methylcyclohexene using the Markovnikov rule

The addition reaction takes place in two ways by using the Markovnikov rule or anti-Markovnikov rule.

If addition takes place by the Markovnikov rule then the negative part of the reagent that is chlorine goes to that carbon atom which carries less number of hydrogen atoms. The product obtained is 1-chloro-1-methylcyclohexane.

Hence in the above structure, we get 11 signals of different protons.

Step 3: Anti-Markovnikov addition

If addition takes place by the anti-Markovnikov rule then the negative part of the reagent that is chlorine goes to that carbon atom which carries more number of hydrogen atoms. The product obtained is 1-chloro-2-methylcyclohexane.

In the above structure, we get 6 signals of different protons.Hence by using proton NMR we can differentiate 1-chloro-1-methylcyclohexane from 1-chloro-2-methylcyclohexane