Exam 3 Solution Key

Exam 3 Solution Key - Chem 10172 Spring 2008(1 EXAM 3 KEY 1...

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Chem 10172, Spring 2008 EXAM 3 KEY March 20, 2008 1 (1) In each of the groups of three molecules below, circle the most acidic compound (4 pts. each). (a) NH 2 SH OH (b) N H H H H 3 C N H 2 C H H C H 3 C N H (c) O OH O OH Cl O O OH (d) O O O O O O O O O O
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Chem 10172, Spring 2008 EXAM 3 KEY March 20, 2008 2 (2) The amino acid arginine is sketched below in its fully protonated form. Arginine has three acidic groups; their pK a ’s are given on the sketch. N N N O O H N H H H H H H H H Arginine pK a = 2.2 pK a = 9.0 pK a = 12.5 (a) (6 pts.) The carboxylic acid in arginine differs significantly in its acidity from a typical carboxylic acid such as acetic acid (CH 3 CO 2 H). Circle the statement that correctly describes the difference, and briefly rationalize it in terms of the structure and bonding in the two compounds. Arginine is more acidic than CH 3 CO 2 H Arginine is less acidic than CH 3 CO 2 H Rationalization : The inductive effect of the nearby -NH 3 + group (as well as its positive charge) makes the carboxylic acid more acidic in arginine. (b) (6 pts.) The guanidino group in arginine (pK a = 12.5) and the amino group in arginine (pK a = 9.0) both contain hydrogens bonded to positively charged nitrogen, yet the guanidino group is significantly less acidic than the amino group. Rationalize this
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Exam 3 Solution Key - Chem 10172 Spring 2008(1 EXAM 3 KEY 1...

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