Question

Write an equation to show the proton transfer between each alkene or cycloalkene and HCl. Where two carbocations are possible, show each. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) C1=CCCCC1 2-Pentene Cyclohexene

Short answer

Answer: When HCl reacts with 2-Pentene, two possible carbocations are formed: (a) CH3CH2CH+CH2CH3 and (b) CH3CH2+CH2=CHCH3. The most stable carbocation in the case of 2-Pentene is (a) because it is a secondary carbocation. On the other hand, when HCl reacts with Cyclohexene, there is only one possible carbocation formed, which is Cyclohexyl+. This carbocation is secondary and equally stable.

Step-by-step solution

Proton transfer with 2-Pentene

We'll start by drawing the structure of 2-Pentene (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\)) with the double bond between carbons C2 and C3, and transfer a proton from HCl to one of the double-bonded carbon atoms. There will be two possible carbocations: (a) Proton transfer to the C2 carbon: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3} + \mathrm{HCl} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}^{+}\mathrm{CH}_{2}\mathrm{CH}_{3} + \mathrm{Cl}^{-}\) (b) Proton transfer to the C3 carbon: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3} + \mathrm{HCl} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2}^{+} \mathrm{CH}_{2}=\mathrm{CHCH}_{3} + \mathrm{Cl}^{-}\) The most stable carbocation is (a) because it is a secondary carbocation.

Proton transfer with Cyclohexene

Now, let's consider the proton transfer between HCl and Cyclohexene. As Cyclohexene is a six-membered ring with one unsaturated bond, there is only one possible carbocation: \(\mathrm{Cyclohexene} + \mathrm{HCl} \rightarrow \mathrm{Cyclohexyl}^{+} + \mathrm{Cl}^{-}\) In this case, the carbocation formed is secondary and equally stable.