Chemistry 21 Homework set 9

Chem 21 Homework set 9 End-of-chapter problems from Hornback: Ch 8: 23, 25, 26, 27b,c,h,i, 28-30, 32-36, 38-40, 44, 46-48, 50, 53. Ch 9: 17d,m, 18, 19, 26, 28. 1. Draw the products of the E2 reaction of each of the following compounds with CH3O– in CH3OH (solvent). Pay attention to stereochemistry. For a and b, be sure to consider the proper conformation around the C2-C3 bond. For c and d, you'll need to draw the chair conformations. (a) (b) Ph (c) Ph Br Br (d) Ph Ph I I 2. Write a mechanism for each the following SN1 solvolyses, and draw the products, including stereochemistry. A small amount of E1 occurs also, but let's forget about E1 for now. Your mechanism should show each step clearly and include curved arrows indicating how electron pairs move in each step. (yes, there is a reaction in b.) (a) (b) EtOH CH3OH Cl I (c) For this one you’ll need to draw the structure of solvent, acetic acid to have any hope of drawing a sensible product structure (the product is an ester, not a carboxylic acid). Think also about which oxygen of acetic acid adds to the cation. (hint: which oxygen is protonated by a strong acid?) One adds to make a much more stable CH3CO2H intermediate, although both possibilities actually lead to the tBu Br same product. 3. Draw the products from the following SN1 solvolyses, paying attention to stereochemistry. In each case, state whether the products are enantiomers, diastereomers, or constitutional isomers. (a) (b) Br EtOH CH3OH CH3 I (c) cis-1-bromo-4-methylcyclohexane in formic acid (HCO2H). (Hint: structure, structure, structure; no swarms of letters.) (d) note the mixed solvent — only water can react with the carbocation to make a stable product. water (S)-3-iodo-3-methylhexane acetone (e) MeOH (R)-1-chloro-3-bromobutane (f) Small amounts of alkenes are also produced in each of these reactions. Draw the alkenes produced in parts a, b, and c via E1 reactions. 4. Write a mechanism for each the following SN1 solvolyses. Let’s ignore stereochemistry this time. Let's also ignore the structure of that thing over the arrow in part a... NOT. (a) CH3CO2H OCOCH3 Br (b) I H2O acetone OH (c) Cl Et–OH O (d) Sketch a reaction coordinate diagram for the rearrangement in part c. 5. Here's something a little different. When the optically active tosylate below is solvolyzed in methanol, a substitution occurs, but with a rather unusual stereochemical outcome, as shown below — only the trans-dimethoxy product is formed; not the cis. Furthermore, the product is racemic. One more interesting bit of evidence is that the trans-tosylate (below) solvolyzes much faster than the cis stereoisomer. Write a mechanism that accounts for these observations. OTs OCH3 CH3OH OCH3 OCH3 (this is a continuation of the non-graded part; ignore the boxes in the header above) 6. The reactions below may proceed by SN2, E2, or SN1/E1 pathways, or not at all. Draw the products, and indicate which reaction mechanism(s) is/are operative, or write NR for "no rxn". (If you're confident that you understand all the reaction mechanisms, but you're having trouble with these, it's probably because you're not taking a systematic approach — use the step-by-step procedure outlined in class.) Br (a) KOEt EtOH (b) CH3CO2– Na+ I DMF Ph (c) CH3OH Cl (d) (e) I ! HC C NaOCH3 TsO acetone Br (f) tBuO– K+ DMSO (g) I CH3O– Na+ CH3OH Na+ (h) HCO2Na acetonitrile Br (i) Br CH3CO2H ! (j) KOH OMs Br (k) Ph CO2H Br tBu NaOEt DMF (l) (m) DMSO CH3OK Et2O Br hexane ! Chem 21 Fall 2009 HW set 9 30 points; due Wed, Nov 11 Name _________________________________________ 1. (a) Think carefully about the possible conformations of its cyclohexane ring, and then draw the alkene(s) formed via the E2 reaction of the compound below with potassium ethoxide in ethanol. KOEt Br EtOH (b) Would you expect SN2 reaction to occur as well? If so, draw the product. (c) If the bromide above is instead dissolved in ethanol, with no base present, SN1 and E1 reactions occur. What are the products of these reaction pathways? 2. Explain why the E2 reaction of 1-bromo-1-methylcyclobutane produces methylenecyclobutane rather than 1-methylcyclobutene. Note that the two alkenes have virtually identical enthalpies of formation, so product stability isn't an issue in this case. (Yes, the second one has a trisubstituted double bond, but it also has a wee bit more ring strain.) Br NaOCH3 CH3OH not 3. Write the mechanism of the SN1 solvolysis below. Two constitutionally isomeric cyclopentenes are formed (let's not worry about stereochemistry for this problem). Write the mechanism and then draw the products. CH3–OH I 4. Same instructions as for #6 of the non-graded part. (a) I CH3CO2H (b) Br SNa OMs DMSO CH3O (c) CO2tBu Ph KOtBu tBuOH Br