Chemistry 25b M. Walker
ANSWER KEY for Practice Problem Set #3
1. What would be the major product(s) of the following reactions?
2. How might you effect the following transformations in a practical manner? Give reagents, catalysts and (if requested) solvents. You may separate ortho from para.
3. How might you carry out the following short multi-step conversions?
4. Explain using resonance forms why phenol reacts more rapidly at the para position than at the meta position.
The Wheland intermediate resulting from para attack is more stable than from meta attack, suggesting the para transition state is more stable, thus the para attack has a lower activation energy and happens more rapidly.
An alternative explanation might use resonance forms of neutral phenol to show that there is a higher electron density para over meta due to +R, this answer is also OK.
5. Which of the following are aromatic?
6. Explain the following phenomena:
(a) Why does compound X fail to react as a diene in the Diels Alder reaction?
It is locked in the s-trans conformation, it cannot attain the s-cis conformation necessary for the Diels-Alder reaction.
(b) Why does Y give mainly Z in the reaction shown, rather than a simple substitution product? The weak nucleophile implies an SN1 reaction, not SN2, going via the carbocation shown. At low temps. the major product will be the kinetic product, formed from the more stable resonance form (the 3o cation), i.e. product Z.
(c) Why does 1,3,5-hexatriene absorb UV light at a longer
wavelength than does ethane?
In the triene, the energy difference between HOMO (highest occupied MO) and the
LUMO (lowest unoccupied MO) is smaller. This means that the energy required to go
from pi --> pi* is less, so it absorbs at a longer wavelength. See p 608 of
Vollhardt & Schore.
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