Chapter 9:Nucleophiles & Substitution Reactions

Transcription

1 1. a. Place the following nucleophiles in order of strength (1= strongest; 3 = weakest). i. ii. b. Place the following in order of leaving group ability (1= best; 7 = worst). (A pka table may help you!) c. Label each as a strong (better) or weak (ineffective) nucleophile. d. Rank the following in order of increasing reaction via a S N 1 mechanism (1= fastest; 5 = slowest). What would be your expected order if you were discussing S N 2 reactivity? SN1: SN2: Page 1 of 5

2 2. Each carbocation below is capable of rearranging, through either a hydride or methyl shift, to a more stable carbocation. Limiting yourself to a single 1,2 shift, determine the structure for the more stable rearranged carbocation. Be able to explain clearly why the new carbocation would be of higher stability than what is given AND how to use arrows to demonstrate this shift. a. H b. c. H d. H e. 3. Provide structures for each of the following solvents. Identify each of the following solvents as either polar protic, polar aprotic or non polar aprotic. Why can you not really have a non polar protic solvent? DMS DMF toluene acetic acid ammonia polar aprotic polar aprotic non polar aprotic non polar protic polar protic S H N Acetone diethyl ether hexane benzene methanol polar aprotic non polar aprotic non polar aprotic non polar aprotic polar protic H NH 3 CH 3 H 4. Draw the mechanisms for the following reactions, and provide correct stereochemical product(s) as necessary. SN2: SN1: Page 2 of 5

3 5. For the following reaction, indicate which reaction mechanism (i.e. write SN1 and/or SN2 in the blank below) the following observations would support. Start by writing the rate law for an SN1 and an SN2 reaction, and draw products for the reaction if the reaction proceeds via SN1 or SN2. A. the reaction rate increased when the [Nucleophile] was increased SN2 B. the reaction rate increased when the [RX] was increased SN1/SN2 C. the reaction rate increases in the presence of an aprotic solvent SN2 D. the reaction rate increases in the presence of a polar protic solvent SN1 E. the reaction showed a rearranged product SN1 F. the reaction involved a carbocation intermediate SN1 G. the products showed inversion of configuration SN2 6. Consider the following methyl ester cleavage reaction: LiF DMF a. Identify the reactant, reagent, solvent and the product. b. Identify the electrophile, the nucleophile and the leaving group. electrophile nucleophile LiF (reagent) DMF (solvent) product reactant leaving group c. Based on the following evidence, determine the mechanism for the reaction. SN2 o the reaction occurs much faster in DMF (aprotic) than in ethanol (protic) o the corresponding t butyl ester reacts approximately 1000 times more slowly than the methyl ester (steric hindrance slows down the reaction) d. Based on your proposed mechanism for this reaction, draw the mechanism. F LiF, DMF e. What other kinds of experimental evidence could one gather to support your proposed mechanism? Vary the concentrations of the reagent and nucleophile to observe rate dependence, vary the identity of the solvent. Page 3 of 5

4 7. Substitution Reactions. Predict the SUBSTITUTIN mechanism for the following reactions and draw the appropriate product(s). Indicate clearly the correct stereochemistry in your product(s). Page 4 of 5

5 8. Determine the anticipated mechanisms for the reactions below. Using correct mechanistic arrows, provide the step by step mechanism that shows how the following substitution reactions proceed. You do not need to show stereochemistry while providing the mechanism, but provide correct stereochemistry in the final product(s). Page 5 of 5