CM 241 Organic Chemistry Fall 1999

Transcription

1 M 241 rganic hemistry Fall UR EXAM III KEY Date: 12/14/99 The marks for each question are given in italics. Write all answers in booklets provided. Question 1. ( ) Give names for the following structures. a) I 3-iodopropene b) l l F F F F 1,2-dichloro-1,1,2,2-tetrafluoroethane Give structures for the following names. c) cis-1-bromo-2-chloro-1-fluoroethene Br l F d) 3-bromocyclohexene Br 1

2 Question 2. (4+4+2) onsider the reaction below to answer the following questions. l 2 light a) Draw all the monochlorination products of methylcyclopentane (ignore stereoisomers). 2 l l l l b) Label the stereocenters in the monochlorination products of methylcyclopentane with an asterisk. 2 l * * * * l l c) Is the monochlorination of methylcyclopentane an oxidation or reduction process? A - bond is broken and a -l bond is formed, so the process is an organic oxidation process. Question 3. (2+4+2) onsider the pair of reactions below to answer the following questions. l (A) (B) Br Br - S Br 2 2 S + Br - a) Which reaction would be predicted to be faster (A or B)? Reaction B is faster. b) Explain your answer. The S - is a better nucleophile than - (nucleophilicity increases going down a column of the Periodic table sulfur is lower than oxygen within the same column). 2

3 c) The mechanism for these reactions is (choose one): (i) S N 1 (ii) S N 2 (iii) E1 (iv) E2 The reaction is an S N 2 reaction answer is (ii). Question 4. ( ) The reaction of bromomethane ( Br) with sodium hydroxide (Na) in water forms methanol ( ). If sodium iodide (NaI) is added to the reaction mixture, the rate of methanol formation is dramatically increased (i.e. sodium iodide acts as a catalyst). a) The mechanism for these reactions is (choose one): (ii) (i) S N 1 (ii) S N 2 (iii) E1 (iv) E2 b) Write a reaction pathway that accounts for the effect of added NaI (i.e. something like A + B + D E + F). Br + Ī I + Br - + Ī c) Draw a reaction energy diagram showing the two different reaction pathways (i.e. catalyzed and uncatalyzed). Indicate each species formed at all energy minima in the diagram. Energy I Br Reaction oordinate d) Explain why adding NaI increases the reaction rate. Iodide is a better nucleophile than hydroxide, so we expect reaction between Br and I - to proceed faster than between Br and -. I is more reactive than Br because iodide is a 3

4 better leaving group than bromide so when I forms it reacts with hydroxide faster than Br to form. The overall result is faster formation of from Br when NaI is added. e) Would you expect the same catalytic activity on this reaction if you added Nal (sodium chloride) instead? Explain your answer. No. l - is a poorer nucleophile and poorer leaving group than bromide. Question 5. ( ) Draw the structure of the major organic product(s) for each of the following reactions. Indicate the stereochemistry for each reaction when appropriate. a) (hint: there are 2 products) l + b) 3 Br Na ethanol 2 c) l ( 3 ) 3 l ( 3 ) l ( ) 3 d) I NaS S 4

5 Question 6. (4+2) onsider the reaction below to answer the following questions. 3 l 2 ethanol % 60% a) Write a complete stepwise mechanism for this reaction. learly show the formation of both products, showing all electron flow with arrows and draw all intermediate structures. Attack from top 3 l 2 Attack from bottom - + l Top Bottom 3 l - l - 60% 40% l + l 5

6 b) This reaction obeys a rate law of the form (choose one): (iii) (i) rate = k [Rl] [ 2 ] (ii) rate = k [Rl] [ 2 ] [ 2 ] (iii) rate = k [Rl] (iv) rate = k [Rl][ 2 ] Question 7. ( ) The compound whose structure is given below is known as RSR 13. It is used to treat angina and to increase oxygenation of tumors to improve effectiveness of radiation therapy for patients with brain or lung cancer. (v) 2 N (iv) (iii) (ii) (i) a) Redraw and then circle the functional groups in RSR N b) Predict the hybridization of the indicated atoms (i iv only) in RSR 13. (i) sp 3 (ii) sp 3 (iii) sp 2 (iv) sp 2 6

7 c) Indicate the direction of the dipole moment between the atoms labeled (i) and (ii). What is the name of the group that the atom labeled (ii) forms? 2 N (ii) (i) The name of the group that is labeled (ii) is ether. d) Would the nitrogen atom act better a Bronsted-Lowry acid or as a base? It would be a stronger base since the N atom has a pair of non-bonding electrons available. e) alculate the formal charge on the oxygen atom labeled (v). F = formal charge = Z (S/2) U = 6 (2/2) 6 = -1 Question 8. ( ) Predict the products of each reaction below. Indicate regiochemistry and stereochemistry when relevant. a) l 2 l l b) 3 Br 2 / 2 3 Br 7

8 For the following reactions (parts c and d), write down what other reactant would need to be added in order to produce the product shown. hoose one from the following list of reagents (parts (i) to (v). The 1. refers to the first step, and the 2. refers to the second step in the reaction if needed): (i) 1. 2, Lindlar catalyst (ii) 1. NaN 2, N Br (iii) 1. Li/N 3 (iv) 1. B 3, TF (v) 1. l 2. NaN 2, N 3 c) 3 3 (iii) Li/N 3 d) 2 (ii) 1. NaN 2, N Br Question 9. ( ) The following structure is a compound called streptimidone. N 3 * * a) Assign R or S configurations to each stereocenter indicated in streptimidone. S N * * 3 R b) ow many stereoisomers of streptimidone are possible? 2 2 = 4 possible stereoisomers 8

9 c) Will streptimidone have a meso stereoisomer? Explain your answer. No. Meso compounds contain stereocenters, but posses a plane of symmetry. Because the substituents on the two stereocenters are different in streptimidone it is impossible for any stereoisomer to have any symmetry. d) Shown below are Newman projection formulas for (R,R)-, (S,S)- and (R,S)-2,3-dichlorobutane. l l l l l l A B (i) Which is which? A is (R,S)-2,3-dichlorobutane, B is (S,S)-2,3-dichlorobutane and is (R,R)-2,3- dichlorobutane. (ii) Which formula is a meso compound (A, B or )? A Question 10. ( ) Tartaric acid [ 2 ()() 2 ] was an important compound in the history of stereochemistry. Two naturally occurring forms of tartaric acid are optically active. ne form has a melting point of 206 o, the other a melting point of 140 o. The inactive tartaric acid with a melting point of 206 o can be separated into two optically active forms of tartaric acid with the same melting point (170 o ). ne optically active tartaric acid has [α] D 25 = +12 o, and the other [α] D 25 = -12 o. All attempts to separate the other inactive tartaric acid (melting point of 140 o ) into optically active compounds fail. a) Write the three-dimensional structure of the tartaric acid with melting point of 140 o. Since the compound with a melting point of 140 o is not resolvable, it must be the meso compound. 2 2 b) What are possible structures of the optically active tartaric acid with melting points of 170 o? 9

10 c) an you be sure which tartaric acid in part b) of this question has a positive rotation and which has a negative rotation? No. d) What is the nature of the form of tartaric acid with a melting point of 206 o. It is a racemic mixture. 10