Assignment_5_Answers

Assignment_5_Answers - CHM 1321 Assignment#5 ANSWERS 1 Use...

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CHM 1321 Assignment #5 - ANSWERS 1. Use arrow notation to show the mechanisms of the following reactions. Use your mechanism to predict the product of the reaction. Identify the nucleophile, its nucleophilic atom, the α carbon of the electrophile and the leaving group. Cl Br Cl CH 3 CH 2 O NaCN CH 3 CH 2 ONa Na CH 3 CH 2 OH Cl NaI Br CN Cl I I CN O b) + acetone c) + acetone a) + + + NaCl nucleophile (oxygen is nucleophilic atom) Cl = leaving group α carbon + + Br nucleophile (carbon is nucleophilic atom) Br = leaving group α carbon + + Cl Cl = leaving group α carbon nucleophile Na Na omitted: it is a spectator ion
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Assignment 5 Answers 2 OH Br (CH 3 ) 3 CO O Br O d) acetone + Hint: (CH 3 ) 3 CO is a hindered base nucleophilic atom = O +B r Br = leaving group carbon O Br (CH 3 ) 3 CO + H Note: This mechanism could also be drawn such that the S N 2 reaction happens first, then deprotonation by the strong base. The first mechanism is better for 2 reasons: 1) (CH 3 )CO - is a stronger base than RO - . This is because the alkyl groups on (CH 3 )CO - are electron donating. This increases the negative charge on the oxygen and increases the base strength of (CH 3 )CO - making initial deprotonation more likely 2) RO - is a better nucleophile than ROH because it can more easily donate electron density. For this reason, the S N 2react ionw ithRO - will proceed faster than that starting from ROH O Br O carbon H H (CH 3 ) 3 CO Alternative mechanism, incorrect HO OH Br H 3 C HO OH NaOCH 3 H 2 SO 4 CH 3 OH Br H 3 C OCH 3 OCH 3 H 3 C HO OH 2 O O H e) f) +H 2 O r nucleophile (oxygen is nucleophilic atom) H 2 O = leaving group carbon nucleophile (oxygen is nucleophilic atom) carbon Br = leaving group Note: H 2 SO 4 is simply a source of H + 1 2 3 4 5 5 4 3 2 1 note the inversion of configuration This notation indicates proton removal by a base present in the solution. An example of the base must be shown. B B=HSO 4 - ,H 2 O, etc
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Assignment 5 Answers 3 HO NH 2 HO NH 2 Br Br HO N H HO N HH g) + weak base weak base: nucleophile (nitrogen is nucleophilic atom) Br = leaving group carbon Note: In this reaction, 2 nucleophiles are present. N is chosen as the nucleophile since is lies furthest left in the periodic table. It will be less electronegative than O and is therefore better able to donate electrons Because the base is weak, it cannot deprotonate RNH 2 , therefore the H + is removed after the S N 2 displacement takes place 2. A solution of 0.25 M cyclohexylbromide and 0.36 M CH 3 NH 2 is created. Measurements show that the reaction produces 3.6 X 10 -7 mol L -1 of product each second. a) Calculate the rate constant for the reaction. Br + NH 2 N reaction involves a good nucleophile and secondary electrophile. Therefore it will have an S N 2 mechanism =k Br NH 2 3.6 X 10 -7 mol l -1 s -1 =k(0 .25mo l l -1 )(0.36 mol l -1 ) k=4.00X10 -6 lmo l -1 s -1 H H H N NH 2 R b) If the concentration of CH 3 NH 2 is changed to 0.15 M, what is the initial rate of the reaction?
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This note was uploaded on 01/25/2011 for the course CHM 1321 taught by Professor Flynn during the Spring '08 term at University of Ottawa.

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Assignment_5_Answers - CHM 1321 Assignment#5 ANSWERS 1 Use...

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