Would you predict that the heat of hydrogenation of the of cycloheptatriene or i

Question

Would you predict that the heat of hydrogenation of the of cycloheptatriene or its cation would be higher? Explain your prediction. Which of the following molecules undergoes SN1 hydrolysis at a faster rate? Explain your answer. Hydrogenation of the following molecules gives two different products. One is a chiral molecule, one is a meso compound. Determine which starting material gives which product. What charge would be needed to make cycloundecapentaene need to make it aromatic? Which of the following molecules are aromatic?

Explanation / Answer

1. the heat of hydrogenation of the compounds directly relates to its stability. As we can see, cyclheptatriene is a non-aromatic molecule with an sp3 carbon in it. On the other hand cycloheptatriene cation fully satisfies the Huckels rule for aromaticity and thus is more stable than the neutral molecule. The heat of hydrogenation would be higher for cycloheptatriene cation as compared to cyclheptatriene.

2. For an SN1 reaction, the stability of the resultant carbocation determines the rate of reaction. In the first compound, the bromo-tert-butyl group stays our of the plane to that of benzene with two Me groups on it due to steric crowding. The resultant carbocation will not be not in resonance with the bezene ring and would be less stable. On the other hand, the second compound bromo-tert-butylbenzene, the tert-butyl does not have steric hindrance as was the case for the first compound and thus the resultant carbocation is in the same plane as benzene. The carbocation is stabilized by resoance effect. thus the rate of SN1 reaction would faster for second compound as opposed to the first compound.

3. Hydrogenation of the first compound would yield a chrial product whereas, with the second would give a meso compound. Hydrogenation mechanism is syn-fashion, such that both the Hydrogens add to the same side of the alkene. The second compound is symmetrical and thus hydrogenation would lead to an meso derrivative.

4. A charge of +2 would make cycloundecapentene aromatic.

5. Of the given molecules the Ist and the IInd structures are aromatic, they satisfy the 4n+2 pi electron requirement with the lone pair of N as pat of the aromatic structure.  

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