Chem 336 - Spring 1999 - Organic Chemistry III
Portland State University - Dr. Carl C. Wamser
Exam 2 Answers
1. (15 points) Write the repeat unit of the polymer that would be expected from each of the following:
a) 
b) 
c) 
d) 
e) 
2. (10 points) Unsaturation in fatty acids or fats can be detected quantitatively by addition of bromine. Using oleic acid (cis-octadec-9-enoic acid) as an example, illustrate the products expected from addition of one equivalent of Br2, in particular indicating the stereochemistry expected (in terms of R and S).
3. (10 points) Allose is the C-3 epimer of glucose. Write the structure for D-allose in an open-chain Fischer projection, and as an a-pyranose in a flat-ring Haworth projection and in both possible chair conformations. Predict the more stable chair form.
4. (10 points) Explain why vinyl polymerizations usually give head-to-tail polymer, e.g., polystyrene is formed with a phenyl group on exactly every other carbon. Show two steps of a typical polymerization to explain this preference.
Every addition occurs with the same regiochemistry so that the intermediate (radical, cation, or anion) is the more stable possibility (benzylic in this case).
5. (15 points) A method of degrading an aldose to the next smaller analog involves the following steps. Identify the compounds that would be formed in each step.
Consider running this sequence of reactions in reverse in order to create a larger aldose. Indicate the steps you would need and identify alternatives if some steps would not work well.
Conversion from a nitrile to oxime will be difficult. A better strategy would be to find a method that converts the nitrile directly to an aldehyde. Perhaps DIBALH would do it directly (with hydrolysis), or the nitrile could be hydrolyzed to a carboxylic acid, converted to an ester, and the ester treated with DIBALH.
6. (15 points)
a) Oxidation of D-glucose with nitric acid gives the same aldaric
acid as another sugar. Identify that sugar by writing its structure
in the standard Fischer open-chain form.
b) Starches often involve some branching between the 1 and 6 positions. Illustrate such a structure on a typical unit of a starch polymer.
c) Poly(vinyl alcohol) is a water-soluble polymer made by hydrolyzing poly(vinyl acetate). Why could it not be made by polymerizing vinyl alcohol?
Vinyl alcohol is an unstable enol. The polymer is made from vinyl acetate, then the acetate groups are hydrolyzed to alcohols.
7. (10 points) Natural rubber is the all-cis isomer of polyisoprene, which polymerizes in a 1,4-addition (i.e., head-to-tail, as in terpenes). The all-trans isomer of polyisoprene is called gutta percha, and is a hard solid. Explain the difference in properties based on the structure. Include a description of the expected crystalline or amorphous character of these two polymers.
Natural rubber is soft and flexible because the chains do not pack well and the overall polymer is mainly amorphous.
Gutta-percha is hard because the trans chains pack well together, leading to a more crystalline structure.
8. (15 points) Write the structures of the products that would be expected upon complete hydrolysis of the following compounds:
a) a lecithin
b) sucrose
c) methyl-alpha-D-fructofuranoside
