8 questions (L3)

16. What properties should an ideal recrystallization solvent have?




17. What is meant by the term solvent pair when it is employed in reference to
the recrystallization of solids?




18. What is the purpose of adding powdered charcoal to the solvent system
during a recrystallization sequence?


19. Why do we obtain larger crystals if a solution of an organic compound is
allowed to cool slowly?


20. List several advantages of using the Craig tube to purify crystalline
products in contrast to using the Hirsch funnel filtration method.


21. The stereochemistry of the more highly substituted alkenes is difficult to
define using the cis and trans designations. Therefore, a more systematic
manner of indicating stereochemistry in these systems has been
developed that uses an E and Z nomenclature.
Draw the structures of the E and Z stereoisomers of 1,4-diphenyl-2-
butene-1,4-dione used in this experiment. In this case, which is cis and
which is trans?


22. In trans-1,2-dibenzoylethylene, even though the conjugated C=O group
frequency coincides directly with the expected C=C stretching frequency,
we would not have expected to observe the latter stretching mode in the
infrared spectrum. Why?



23. Explain how this photochemical isomerization allows the production of the
thermodynamically less stable cis isomer. In other words, why is the trans
isorner exclusively converted to the cis isomer during short reaction
periods and not vice versa? Is it possible, under these conditions, that the
trans and cis isomers are in equilibrium with one another?