Question

1/ Write a mechanism that explains the exothermicity of the α-pinene reaction, assuming a carbocation as the initial intermediate. Write an equation showing two likely products (neither
of which is a vicinal diiodide).

2/  Draw resonance structures showing all the positions at which the positive charge is delocalized on the aromatic ring in the phenonium ion. If a methoxy group (-OCH3) or a nitro group (–NO2) were substituted in the 4-position on the aromatic ring, how would the stability of the phenonium ion change? Compare methoxy substituted, nitro substituted and unsubstituted, and order them by increasing stability. Explain your answer.

3/ Benzopinacol and benzopinacolone have nearly identical boiling points. How to distinguish the two?

4/ Was the major product in the isomerization of maleic acid the cis or trans isomer? Explain why formation of one isomer is favored over the other.

5/ Why is the addition of α-pinene to iodine so exothermic? Would bromination of α-pinene be expected to be more or less exothermic? Explain.

Answer 1

1. Given below is the reaction of alpha-pinene with I2. The reaction is highly exithermic and the ring strain in released during the reaction. The reaction yields two products one by an SN1 type substituion to form product [A] and the other by the SN1 cum elimination to form iodoalkene product [B], as shown below.

2. Shown below are the ossible resonance structures for phenonium ion. When nitro is attached at 4-position, the phenonium ion is less stable as NO2 group is electron withdrawing and destabilize the +ve charge. On the other hand, OMe group is electron donating and thus stabilize the phenonium ion.

3. Benzopinacol and benzopinacolone can be separated by tretament with NaOH. benzopinacol gets deptonated and goes in water layer, whereas, benzopinacolone remains insoluble. Another method could be by derrivatization. Benzopinacolone is treated with 2,4-DNP reagent, forms a 2,4-DNP derrivative which is solid and can be easily separated by filtration. Clevage of the derrivative will yield benzopinacolone. Benzopincaol does not form any derrivative and thus can be separated easily.

4. Isomerization of maleic acid would yield trans fumaric acid isomer, which is less sterically hindered and thus is more stable as compared to the cis-isomer of maleic acid.

5. Iodination of pinene allows the ring to expand from 4 to 6 membered, rellief of ring strain is associated with energy release and thus the process is exothermic.