Chem 3003, Organic Chemistry I, Midterm II Prof. Ramig November 2017 1.) (12 poi

Question

Chem 3003, Organic Chemistry I, Midterm II Prof. Ramig November 2017 1.) (12 points) Draw the structures of the following molecules: (a) (R)-1-fluoro-L22-trimethylcyclobutane (b) (his-4-methyl-2-o tene (c) (S)-4-cyclopropyl-1 2.) (16 points) Name the following molecules: -pentyne CH3 3.) (14 points) For the molecule in question 2b: (a) Draw all possible stereoisomers, labelling each chiral atom with the proper descriptor (R or S). (byLabel pairs of enantiomers and diastereomers. (c) If the optical rotation of the R.R isomer is +13.3, what is the optical rotation of the SS isomer? What is the optical rotation of the R isomer? (d) If you were to distill a mixture of all possible stereoisomers with the aim of separating all of them, how many fractions would you expect and what would be in each? 4) (4 points) For the molecule in question 2c: (a) What is the stereochemical relationship between it and its ring-flip isomer? (b) Would it be possible to separate it and its ring-flip isomer by physical means, for example by distillation? Explain your answer. 5.) (24 points) Give the products: 1) Hg(OAch H20 Li 2) NaBH4 NH, 1) BH3 2) H20,/H-C Pd/Pb H2 Pd 2) 2n/HOAc 2 HCI 1) Mg 2) H2O 1) Li 2 Br2 d) 2) Cul 1) NaNH2 2) CH3CH2l

Explanation / Answer

4. When cyclohexane in its most-stable conformation called chair-form undergoes a "ring-flip", all the substitutents in the ring, relatively axial and equitorial also switch positions and this, for a disubstituted chiral cyclohexane as the one given, results in the formation of diastereomer. Diastereomers can be seen as stereoisomers of chiral compounds with more than one chiral centers having different configuration at more than one (not all) chiral center(s). Therefore, for a) we can say that the two ring-flip isomers are diastereomers.

b) Diastereomers, unlike enantiomers are not mirror-images of one another and possess considerably different properties. Therefore, they can be separated by any physical method used to separate different molecules from a mixture like recrystallization, column chromatography, etc. In the former, due to different solublities of diastereomers in the same solvent, there will be two distinct sets of crystals. A more efficient method is column chromatography, in which the differential interactions of diastereomers with the stationary phase is exploited to separate them. Distillation however, can be used only when a fractionating column is used in combination with the distillation apparatus as diastereomers often have boiling points difference less than 25 degrees. If there is a higher difference, conventional distillation can also be used.

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