Chemistry 210 Organic Chemistry I Winter Semester 2002 Dr. Rainer Glaser

Transcription

1 hemistry 210 rganic hemistry I Winter Semester 2002 Dr. Rainer Glaser Examination #6 Reactions of Alkenes & Alkyne hemistry. Friday, March 26, 2002, 9 9:50 am Name: Answer Key Question 1. alogenation Reactions. 20 Question 2. Alcohol and Diol Syntheses. (News #8) 20 Question 3. Redox hemistry of Alkenes. 20 Question 4. arbene Additions. 20 Question 5. Alkynes. (V #9) 20 Total 100 1

2 Question 1. alogenation Reactions. (20 points) (a) onsider the electrophilic addition of to 2-methylpropene. Draw the structures of the Markovnikov product and of the anti-markovnikov product. ircle the major product. Draw the structures of the cationic intermediates that lead to each of these products. (8 points) Markovnikov Product Anti-Markovnikov Product 3 3 ircle this one! 3 3 ationic Intermediate leading to M-Product ationic Intermediate leading to AM-Product (b) The bromination of propene results in 1,2-dibromopropane (give name of product). The addition involves the formation of a bromine cation ion followed by the addition of a bromide. In the bromonium ion, one (cation, radical, anion) is bonded to both -atoms. omplete the resonance forms that describe the bridged bromonium ion by adding all lone pairs and formal charges as needed. ircle the resonance form that contributes the most! (6 points) + (c) omination of E-2-butene. The two atoms will be added in a (trans,cis) fashion. Draw the product(s) of this reaction. If stereoisomers are formed, then draw each of these and indicate their stereochemical relationship (e.g. enantiomers,...). (6 p.) R S one product, not chiral 2

3 Question 2. Alcohol and Diol Syntheses. (20 points) (a) Show the oxymercuration/demercuration reaction of butene. The reaction exhibits (Markovnikow, anti-markovnikow) regiochemistry. Give the reagent. Draw the structures of the organomercurial alcohol and of the alcohol obtained after reduction with NaB 4. (7 p.) +gac 2 -Ac gac NaB 4 3 (b) Suggest a preparation of butanol from butene (a terminal alkene) by hydroboration. Provide the structures of butene and of butanol. Specify all reagents needed. If the reaction involves several steps, then provide reagents for each step and draw the structure(s) of the intermediate(s). (7 points) TF. B B 2 (c) The article NEW USES FR LD DRUGS (Fort Worth Star-Telegram, Nov. 19, 1998) talked about minoxidil. The drug is applied as a solution in ethanol and propylene glycol. Let s consider some aspects of the diol propylene glycol, aka 1,2-propanediol, 3 -()- 2. ow can you prepare this diol from propene via epoxidation? Provide the structure and the name of a reagent to form the epoxide, draw the epoxide formed, and specify the reaction conditions to convert the epoxide to 1,2- propanediol. (6 points) 3 l MPBA 3 2 +, 2 or -,

4 Question 3. Redox hemistry of Alkenes. (20 points) (a) 1,2-Propanediol also can be made by way of oxidation of propene with osmium tetroxide. Draw the structure of the osmate ester. Provide the reagent for the oxidation of the osmate ester to the diol with simultaneous reformation of osmium tetroxide. (6 points) s s 3 3 (b) zonolysis of 1-butene. Draw the structures of the primary ozonide, of the ozonide, and of the two products obtained by reductive work-up. Specify the reducing reagent. (10 points) ozone + 2 Zn/l (c) ydrogenations of alkenes require a catalyst and most hydrogenations are carried out as heterogeneous reactions. Give an example of a heterogenous hydrogenation catalyst and briefly describe what the catalyst accomplishes. (4 points) Many possible answers. ommon answer: Pt The catalyst achieves the homolysis of the - bond and provides atomic hydrogen. 4

5 Question 4. arbene Additions. (20 points) (a) arbenes were first discovered when chloroform was treated with strong base, e.g. K. Provide a mechanism for the formation of the dichlorocarbene from chloroform. This reaction is an (alpha, beta, gamma) elimination. (4 points) l l 3 - l 3 - l 2 + l - (b) Draw the Lewis-Kekule structure of diazomethane. Show all lone pairs. Show all formal charges if any. Draw two resonance forms. (4 points) N N N N (c) onsider the cyclopropanation of trans-butene with diazomethane. Trans-butene is (E or Z) butene. Draw the structure of trans-butene. Then draw the two products of the carbene addition to trans-butene where the carbene adds from the top and from the bottom. The methyl groups in the cyclopropane are (cis, trans) in both products. These two products are (the same, enantiomers, diastereoisomers). Provide complete name(s) of the product(s), that is, include any stereochemical descriptors that might be needed (such as R or S). (12 points) R S R S (R,R)-trans-dimethylcyclopropane (S,S)-trans-dimethylcyclopropane 5

6 Question 5. Alkynes. (20 points) (a) Provide the name the molecule shown on the right and draw its Lewis-Kekule structure. (4 p.) cyclooctyne (b) Identify the bond length of the bond in alkynes. (2 points) (1) 1.10 Å (2) 1.20 Å (3) 1.30 Å (4) 1.40 Å (5) 1.50 Å (c) Which orbital description best describes the sigma-bond in acetylenes? (2 points) (1) (s) (s) (2) (sp) (sp) (3) (sp 2 ) (sp 2 ) (4) (sp 3 ) (sp 3 ) (d) The acetylide Li + is treated with 1-bromobutane. Draw the organic product of this reaction and name it. The reaction mechanism of this reaction is SN2_. (6 points) 2-heptyne, (e) Suggest a synthesis of - - from =. Give the reagents needed in each step and draw the products obtained in each step. (6 points) + 2 Na -2 Do not worry about stereochemistry, E- or Z-alkene K. Accept any possible base. 6