Chemistry 210 Organic Chemistry I Winter Semester 2001 Dr. Rainer Glaser

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1 hemistry 210 rganic hemistry I Winter Semester 2001 Dr. Rainer Glaser Examination #3 Alkenes and Alkynes. Structure, Synthesis and Reactions. Friday, April 20, 2001, 9:00-9:50 Name: Answer Key Question 1. Alkenes I. Structures & Properties. 20 Question 2. Alkenes II. Addition Reactions. (V #8) 20 Question 3. Alkenes III. Reductions and xidations. (V #8) 20 Question 4. Alkynes I. Strs., Props. & Preps. (incl. V #9) 20 Question 5. Alkynes II. Reactions. (incl. News #9) 20 Total 100 Allowed: Periodic System of the Elements and Molecular Model Set. Not Allowed: alculator. 1

2 Question 1. Alkenes I. Structures & Properties. (20 points) (a) The molecular model is shown of an alkene. This molecule is (cis, trans, neither). Provide a suitable IUPA name of the molecule. (3 points) Name: 2-methyl-2-butene (b) Draw the Lewis-Kekule structures of the (E,E)-stereoisomer of 2,4-hexadiene. (4 points) 3 3 (c) Alkene synthesis by way of elimination from secondary alkyl halides employs bulky bases to reduce competition by nucleophilic substitution. ne such base is called TEA. Provide the full name and the structure of TEA. (3 points) 2 5 triethylammonia N (d) Identify the approximate bond length (Å) of the = bond in alkenes. (2 points) (1) 1.15 (2) 1.25 (3) 1.35 (4) 1.45 (5) 1.55 (e) eats of hydrogenation allow one to determine the relative stabilities of alkenes. The heat of hydrogenation always is (exothermic, endothermic) because 2 σ-bonds are formed while one bond and one = π-bond are broken. The more stable alkene has the (more, less) negative heat of hydrogenation. (4 points) (f) ircle the most stable alkene. (2 points) (1) butene (2) 2-methylpropene (3) cis-2-butene (4) trans-2-butene (g) Isobutylene, aka 2-methylpropene, is a (1) mono- (2) di- (3) tri- (4) tetrasubstituted alkene. (2 points) 2

3 Question 2. Alkenes II. Reactions. (20 points) (a) onsider the addition of Br to butene. In principle, this reaction could give the products Br and 3 2 Br 3. Indicate which of these products is the Markovnikov product and which one is the anti-markovnikov product. Indicate which product is the major product in this addition reaction. The addition reaction starts with the addition of a proton to butene. Draw the two possible carbenium ions that lead to the two possible products. (10 points) Br 3 2 Br 3 Markovnikov or Anti-M.? A-M M Mark the major product. Draw the carbenium ion that would lead to the product shown. Major! (+) 3 2 (+) 3 Number of chiral -atoms? None! 1 Product optically active? (Y,N) ourse not! No, racemate formed! (b) The bromination of propene results in 1,2-dibromopropane. The addition involves the formation of a bromonium ion and the addition of a bromide to the bromonium ion. In the bromonium ion, one Br (cation, radical, anion) is bonded to both -atoms. omplete the resonance forms that describe this bridged bromonium ion by adding all lone pairs and formal charges as needed. ircle the resonance form that contributes the most! (5 points) Br + Br best! Br (c) xymercuration/demercuration of 1-butene, 3 2 = 2. The reaction shows (Markovnikow, anti-markovnikow) regiochemistry. Draw the structures of the organomercurial alcohol and of the alcohol obtained after reduction with NaBD 4 (note the deuterium!) (5 p.) g(ac) NaBD 4 D 3

4 Question 3. Alkenes III. Reductions and xidations. (20 points) (a) onsider the metal-catalyzed hydrogenation of butadiene to butane (name of product). Suggest a metal that is suitable as a hydrogenation catalyst: Ni, Pd, Pt. The reaction is a (homogeneous, heterogeneous) catalytic hydrogenation. (3 points) (b) onsider the thermal cyclopropanation of cis-butene with diazomethane. is-butene is (E or Z) butene. Draw the structure of cis-butene. Draw the product of the carbene addition to cis-butene. Name the product. The methyl groups in the cyclopropane are (cis, trans). (6 points) cis-1,2-dimethylcyclopropane 2 (c) onsider the ozonolysis 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 employed in the reduction of the ozonide. (6 points) + (d) onsider the transformation of propene, 3 = 2, into acetone, 3 3, by oxidation with air. Aside from air, the reaction requires the presence of Pdl 2 and of ul 2. (2 p.) (e) A good way to cleave = bonds completely involves a sequence of two reactions. First, the alkene is converted into a diol and this reaction is best done with s 4 (give the formula of the oxidation reagent). Subsequently, the diol is reacted with I 4 (give the formula of the diol cleaving reagent) to achieve the cleavage of the diol. If this reaction sequence is applied to 2,3- dimethyl-2-butene, ( 3 ) 2 =( 3 ) 2, then the product formed will be acetone (name of product). (3 points) 4

5 Question 4. Alkynes I. Structures, Properties, and Preparations. (20 points) (a) Identify the formula of cyclooctyne. (2 points) (1) 8 12 (2) 8 14 (3) 8 16 (4) 8 18 (5) 8 20 (b) Identify the bond strength (kcal/mol) of the bond in alkynes. (2 points) (1) 100 (2) 125 (3) 150 (4) 175 (5) 200 (c) Which orbital description best describes the σ-bond-m in acetylene? (2 points) (1) (s) (s) (2) (sp) (sp) (3) (sp 2 ) (sp 2 ) (4) (sp 3 ) (sp 3 ) (d) The protonation of propyne, 3, in principle can lead to two vinyl cations. Draw the complete Lewis structures of these two vinyl cations and indicate which one of these vinyl cations is the more stable one. (4 points) Isomer 1: + Isomer 2: More stable + (e) The acetylide Na + is treated with chloroethane. Draw the complete Lewis structure of the organic product of this reaction and name it using IUPA nomenclature (V #9). The reaction mechanism type of this reaction is _SN2_. The inorganic product of this reaction is _Nal_. (6 points) Na + + l Nal Name of product: 3-hexyne (f) onsider the synthesis of propyne from 1,2-dibromopropane. Draw the structures of the starting material and of the product. Give the reagents needed to afford the elimination. (4 points) Br 2 Br + 3 NaN 2-3 N 3-2 NaBr Na l, -Nal 5

6 Question 5. Alkynes II. Reactions. (20 points) (a) onsider the hydration of acetylene. This hydration of alkynes requires the presence of a special catalyst and this catalyst is gs 4. The hydration initially results in the formation of a product A. This product A then undergoes a ompound A ompound B rearrangment to the final product B. This rearrangment is called a tautomerization. Draw the complete Lewis structures of compounds A and B. (6 points) 3 (b) onsider the complete oxidation of acetylene in a torch. Provide a stoichiometric reaction for the complete oxidation. (4 points) == (c) Show how butanal, =, can be prepared from butyne via hydroboration. Draw the starting material and the products of all reaction steps. Give the reagents needed to accomplish each of the reaction steps. (6 points) B 2 B (d) 3-hexyne is treated with Li in liquid N 3. Draw the structure of the product, clearly indicate the stereochemistry of the product, name the product, and state whether the product is E or Z (4 pts.) trans- or E-3-hexene 6

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