7. For Diels-Alder cycloaddition reactions to take place most rapidly and in hig

Question

7. For Diels-Alder cycloaddition reactions to take place most rapidly and in highest yield the diene must: A. be able to adopt an S-trans conformation B. be substituted with electron-withdrawing groups C. be substituted with electron-donating groups D. be able to adopt an S-cis conformation E. none of these 8. 200-400 nm in its UV spectrum? Which of the following structures WILL NOT show an absorbance between A. B. C. D. A &B; E. A &C; -9. Free-radical halogenation of alkanes requires A. light or heat as an initiator B. peroxides as an initiator C. an ether as a solvent D. choices A, B, & C E. choices A & B

Explanation / Answer

7. In Diels Alder reaction, if the diene is in s-cis conformation, the reaction will wrok rapidly with the dienophile.

That's because in s-cis conforation, the double bonds are connected to the carbons which face the same side of the dienophile while connects to the diene later, due to better conjugation of electrons at the same side, the s-cis conformation is preferred in comparison to the s-trans where pi orbitals are not on the same side, making the conjugation of electrons difficult.

Therefore, the answer is (D)

8. (B)

That's because in the structures of A and C, conjugation in atleast one ring, with homoannular arrangement is present, that is responsible for the higher wavelength of absirption in the UV region, whereas in B, the placement of double bonds is such that there is no conjugation at all. Thus, B will have a much lower wavelength than A and C and wont absorb in the range of 200-400 nm.

9. (E)

The Peroxides structures possess a weak oxygen-oxygen bond which are responsible for initiating the free radical chain mechanism for Heating leads to homolytic fragmentation of this bond – that is, the bond breaks such as to leave one unpaired electron on each atom. Strong sources of light : SUnlight, UV, help breaking this bond.

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