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14D_w09_e01_key - Chemistry 14D Winter 2009 Exam 1...

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Chemistry 14D Winter 2009 Exam 1 Solutions Revised 02/05/06 Page 1 Statistics : High score, average, and low score will be posted on the course web site after exam grading is complete. The exam is ready to be picked up when these numbers are posted. Some questions have more than one answer, even though only one answer may be listed here. To see the projected course grade cutoffs, consult the grading scale on the Chemistry 14D course web page. 1–3. 4. Rate = k [NaI] [RBr] 5. 6. The C–H and C–C bonds (nonpolar) decrease dielectric constant. The C–O bond (polar) enhances the dielectric constant. 7. DMF Like acetone, DMF is polar and aprotic, the best combination of solvent properties for an S N 2 reaction between a negatively charged nucleophile and uncharged electrophile . 8. (a) Becomes faster. Iodide is a better leaving group than bromide . (b) Becomes slower. The carbon bearing the leaving group is changed from secondary to tertiary, causing more steric hindrance, and the S N 2 rate drops to zero. A rate of zero is slower than any other reaction rate. (c) Becomes faster. Fluoride ion is a better nucleophile than iodide ion in aprotic solvent because of fluoride’s smaller atomic radius . 9. Transition state : The highest energy structure between reactants and products for any given mechanism step.
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