Quiz4cor_E - less strerically congested. (Any plausible...

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CHM 2123, Quiz #4: Chemoselective Reductions of 3-Nitroacetophenone MARKING SCHEME Posted: October 1, 2009 Due: October 5, 2009 before 5pm 1. Why is it recommended to add the hydrochloric acid dropwise (instead of at once) to your solution of nitroacetophenone and tin? ( 1 point ) The reaction is very exothermic. Adding too much HCl at once could cause a violent, out of control reaction. 2. As with the reducing agents seen in the lab manual, hydrogen on Pd/C, which you have first seen as a hydrogenation reagent on alkenes, can also reduce other groups, including some pertaining to this week’s laboratory. If it were used on 3- nitroacetophenone, what would be the obtained product? Write a balanced equation for the reaction. ( 2 points ) NO 2 O 3 + NH 2 O H O H + 2 H H Pd/C 3. Which of the two ketones in the following molecule will be first reduced by sodium borohydride. Include a short reasoning for your answer. ( 1 Point ) O O The ketone on the left will be reduced preferentially. It is both more electrophilic and
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Unformatted text preview: less strerically congested. (Any plausible reasoning along those lines will do…) 4. Why is hydrochloric acid added at the end of the reaction with sodium borohydride? Why do bubbles appear in the solution? ( 2 points ) The aqueous hydrochloric acid solution is added to cause the hydrolysis of the borate ester to the alcohol and boric acid. It also quenches any unreacted sodium borohydride, producing hydrogen gas. 4. Give the mechanism of the hydrolysis of the B(OR) 4- intermediate, producing boric acid B(OH) 3 . ( 4 points ) OR B RO OR OR OR B RO OR OR H-HOR B RO OR OR H 2 O OH B RO OR OR H ± H OR B RO OR OH H etc. H OH 2-HOR-H 2 O The complete mecanism does not have to be shown. As long as the part shown here is there, with the detailed dissociation of the first OR, followed by association of the first water molecule to get back to a tetravalent boron species. The charges and their positions are important....
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This note was uploaded on 01/13/2010 for the course CHM chm2526 taught by Professor Louise during the Spring '09 term at University of Ottawa.

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Quiz4cor_E - less strerically congested. (Any plausible...

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