Points: 1 / 15 / Total: 100 points /

Transcription

1 Chemistry 6372 Fall 2002 omework #6 Answer Key Name Points: 1 / 15 / 2 / 10 / 3 / 10 / 4 / 10 / 5 / 10 / 6 / 10 / 7 / 35 / Total: 100 points / Average: 73

2 1) Answer the following questions from our book: Chapter 7, questions 3 c, d, and p. C) 3 C + SCl S 3 C S attack of Cl (on the more electropositive C) other geometric isome is exactly analogous Products D) 2 C N C 2 C N C 2 NaN 2 N( ) 2 Product C 2 P) N OTs Li OTs omatic intermediate - OTs Product

3 2) The product of addition of 2 to trans-cinnamic acid can be decarboxylated to beta-bromostyrene by refluxing in water or by heating in the presence of sodium actetate in a variety of solvents. The product obtained by heating in acetone in the presence of sodium acetate is the (Z) isomer of beta-bromostyrene. In ethanol a mixture of (Z) and (E) is formed, while in water the product is predominantly the (E) isomer. Propose a mechanism a) for the exclusive formeation of the (Z) isomer in acetone b) on more polar solvents, and explain the solvent effect of the stereoselectivity of the reaction The elimination in acetone is a concerted anti elimination from the carboxylate ion. In a more polar solvent, ionization takes place. The resulting carbocation can undergo conformational change before elimination of CO 2, so the more stable product is obtained.

4 3) Concerted 1,2-eliminations occur with anti stereochemistry and 1,4 -eliminations with predominantly syn stereochemistry. Predict the stereochemistry of 1,6-, 1,8-, and 1,10-eliminations and use the curved arrow formalism to justify your predictions. X Y anti analogous for the other eactions 4) It has been proposed that there is a close analogy between the S N 1 reaction and the electrophilic addition of bromine to an alkene. Compare and contrast the two reactions in terms of a) gross mechanistis features, b) substituent effects, and c) solvent effects. In both cases, a neutral organic substrate is converted to a positively charged intermediate that adds a nucleophile. Electron releasing sub. accelerate the rates of both reactions. Both the departure of the LG in solvolysis and the departure of - are facilitated by electrophilic solvents. Just as a carbocation-carbanion pair may return to the SM in solvolysis, there is evidence that a bromonium-bromide ion pair may return to SM.

5 5) The bromination of 3-aroyloxycyclohexenes gives rise to a mixture of stereoisomeric and positionally isomeric addition products. The product composition is shown below. Account for the formation of each of the products and decribe factors that will affect the product ratio. O O 2 48% 9% 12% 30% Solution: JOC, 1987, 52, 3355.

6 6) When 2-bromo-2-methylpentane is dissolved in DMF, the formation of 2-methyl-1-pentene (A) and 2-methyl-2-pentene (B) occurs. The ratio of alkenes formed is not constant throughout the course of the reaction. Initially, the ration is 1:1 but its drops to 1:4 by the time the reaction is 25% complete and then remains fairly constant. In a similar reaction but with Na present in excess, the ratio is constant at about 1:5 throughout the reaction. Suggest an explanation. JOC, 1075, 40, Answer Key for Question #7: and-out in class