Second Exam CHEM 255 Organic Chemistry I Prof. Bastin Fall 2011

Transcription

1 Second Exam CEM 255 rganic Chemistry I Prof. Bastin Fall 2011 Name Provide clear, concise answers using unambiquous, carefully drawn structures (where appropriate) for all of the questions. 1) /6 pts 2) /6 pts 3) /10 pts 4) /8 pts Total: /100 pts 5) /8 pts 6) /8 pts 7) /10 pts 8) /10 pts 9) /12 pts 10) /12 pts 11) /10 pts

2 1) (6 pts) Provide structures for the following compounds. a) (S)-3-Methylheptane b) (R)-4-ethyl-3,3-dimethyloctane c) (3R, 4S)-3,4-dimethylhexane 2) (6 pts) Provide either common or IUPAC names for the following compounds. (a) C 2 C 3 (b) C 2 C 2 C 3 C 3 (c) 3 C 6/27/2011 2

3 3) (10 pts) (a) Indicate whether each molecule is chiral or achiral. (b) Provide the relationship between all pairs (enantiomers, diastereomers, identical, constitutional isomers). 6/27/2011 3

4 4) (8 pts) Which of the following are chiral? Indicate chiral by putting an X in the space provided. (a) An iphone (b) C 2 C 3 (c) An ear (d) cis-1-methyl-4-isopropylcyclohexane 5) (8 pts) Provide all of the symmetry operations (rotation axes, mirror planes, and inversion centers) present in each of the following molecules. Assume the molecules are frozen in the conformations shown. (a) F F P F F F (b) 3 C C 3 3 C C 3 6/27/2011 4

5 6) (8 pts) Indicate whether each of the following statements is true or false. Write T or F in the blanks below. (a) Diastereomers have the same melting point. (b) Enantiomers have the same solubility in water. (c) Enantionmers rotate plane-polarized light in the same direction and to the same magnitude. (d) The (R)-enantiomer of a molecule always rotates plane-polarized light in a clockwise direction. (a) (b) (c) (d) 7) (10 pts) Provide the absolute configuration (R/S) of each stereogenic center in each of the following molecules. (a) (b) C 3 Br (c) (d) (e) C 3 C 3 C 3 Cl C 2 C 2 Cl C 2 C 3 6/27/2011 5

6 8) (10 pts) In aqueous solutions, at least three forms of fructose are in equilibrium (shown below). Given the following equilibrium ratios, calculate the relative energies of the three isomers at 25 C and sketch a reaction coordinate diagram. Is the transformation of α-d- Fructofuranose into α-d-fructopyranose exothermic or endothermic? C 2 C 2 C 2 C 2 C 2 α-d-fructofuranose (22.0%) D-Fructose (1.0%) α-d-fructopyranose (70.0%) 6/27/2011 6

7 9) (12 pts) (a) Predict the products of the following acid-base reactions using curved-arrow mechanisms to indicate electron flow. (b) Indicate the acid, base, conjugate acid, and conjugate base of each reaction. (c) Indicate whether the reactants or products are favored at equilibrium (use the pk a values that are given in the Table at the end of the exam). a) C 3 C + C 3 b) C 3 C N 6/27/2011 7

8 10) (12 pts) Which group in ascorbic acid (vitamin C, structure below) is the most acidic? Explain (be sure to include structures of each conjugate base). Explain using structures why ascorbic acid (pk a 4.17) and acetic acid (pk a 4.74) have similar acidities even though ascorbic acid lacks a carboxylic acid group. ascorbic acid acetic acid 6/27/2011 8

9 11) (10 pts) Tartaric acid [ 2 CC()C()C 2 ] was an important compound in the history of stereochemistry. Two naturally occurring forms of tartaric acid are optically inactive. ne form has a melting point of 206ºC, the other a melting point of 140ºC. The inactive tartartic acid with a melting point of 206ºC can be separated into two optically active forms of tartaric acid with the same melting point (170ºC). ne optically active tartaric acid has [α] D 25 = +12º, and the other, [α] D 25 = 12º. All attempts to separate the other inactive tartaric acid (melting point 140ºC) into optically active compounds fail. (a) Write the threedimensional structure of the tartaric acid with the melting point of 140ºC. (b) What are the possible structures for the optically active tartaric acids with melting points of 170ºC? (c) Can you be sure which tartaric acid in (b) has a positive rotation and which has a negative rotation? (d) What is the nature of the form of tartaric acid with a melting point of 206ºC? 6/27/2011 9

10 Some useful pk a (water) values Compound pk a Compound pk a Alkanes/Alkenes >40 12 N 3 38 RN + 3 ~10 Alkynes 25 C 3 S Ph 10 3 C N 2 C 3 CN 24 CN C C 2 C 3 R C 2 R ~19 C 3 C 4.8 R C 3 18 F 3.2 R N 2 (C 3 ) 3 C and (C 3 ) 2 C 17 ClC 2 C N C C 3 C 2 16 R S Cl -8 R C 2 15 Br I -10 R C 2 R ΔG = RT lnk R=1.986 cal/mol K K = [ products] [reac tants] K = C α = α cl Enantiomeric excess (ee) = %R - %S = α [ ] sample [ α] pure enant x100 6/27/