Question

TRie 6-41. List several advantages of using the Craig tube to purify crystalline products in contrast to using the Hirsch funnel filtration method. 6-42. 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 the E and Z nomenclature system. 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? 6-43. The cis in-phase out-of-plane bending frequency normally is found in the range 740-680 cml and is broad. In addition, this vibrational mode can be quite variable in intensity. On conjugation with groups, the band rises to near 820 emlL (a) Can you explain the underlying cause of this observation? (Hint: See the discussion of cis double-bond group frequencies in Chapter 8.) (b) Can you explain why a broad band near 690 cm1 assigned to this out-of-plane mode is found in the cis product obtained in this reaction? 6-44. In trans-1,2-dibenzoylethylene, even though the conjugated group frequency coincides directly with the expected stretching frequency, we would not have expected to observe the latter stretching mode in the infrared spectrum. Why?

Answer 1

41. advantages of craig tube recrystallization than hirsch funnel filtration :

Craig tube is designed specifically for recrystallization of small quantities of substances. Main advantage is that it minimizes number of transfers of solid substances which  maximizes yield of crystals. Separation of crystals from the mother liquor with the Craig tube is very efficient, and little time is consumed for drying.

42. Because the cis,trans system becomes confusing with tri- and tetrasubstituted alkenes
and is not detailed enough to name all alkenes, we need E,Z system.

This system uses the priority rules to assign priority to the substituents on each carbon of a double bond. Using these rules, we decide which group on each carbon has the higher priority. If the groups of higher priority are on the same side of the double bond, the configuration of the alkene is Z . If they are on opposite sides of the double bond, the alkene is E .

Trans- 1,4-Diphenyl-2-butene-1,4-dione : E

cis- 1,4-Diphenyl-2-butene-1,4-dione : Z

: : Ph- CH HO (es) Chaus)

43. (a)  Bending vibrations are classified as in-plane and out-of-plane bending vibrations (it involves bending of a bond) , these requires less energy than stretching vibrations. Normal cis bending vibrations are in range of 720-680 cm-1. On conjugation of this double with some group other increases bond order of that due to conjugation effect, thus required energy for bending vibration, which ultimately lead leads bending vibrations frequencies to ~ 820 cm-1.