sping 02 midterm

sping 02 midterm - Seat # _ Last initial _ Practice Exam...

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Seat # ________ Last initial ________ Practice Exam KEY Chemistry and Biochemistry 153A Midterm Examination - KEY Question # Value Score 1 20 2 32 3 10 4 12 5 18 6 13 7 23 8 24 TOTAL 152 _____________ KEY _____________ Print your full name ( last name first) ______________________________ Student ID Number ______________________________ Print your TA's name ______________________________ Discussion section # [or time & day] I have read the instructions below. ______________________________ Signature INSTRUCTIONS READ EACH QUESTION CAREFULLY! SHOW YOUR CALCULATIONS! WRITE YOUR LAST NAME IN THE SPACE PROVIDED ON EVERY PAGE OF THE EXAM. NO CREDIT WILL BE GIVEN FOR ANYTHING WRITTEN ON THE BACK OF A PAGE OR ANYWHERE OTHER THAN THE SPACE PROVIDED FOR EACH ANSWER. ANSWERS MUST BE BRIEF AND TO THE POINT. WHERE A WORD LIMIT IS SPECIFIED, NO CREDIT WILL BE GIVEN FOR THE PART OF THE ANSWER IN EXCESS OF THE LIMIT. Numbers in parentheses represent the point values.
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Name _______ KEY _________________ 2 1.(20) a. (5) Although it is negatively charged, the deprotonated form of any carboxyl group is stabilized due to resonance ____. Thus the pK a ‘s of carboxyl groups are generally below neutral pH. Acetic acid is a weak carboxylic acid with an approximate pK a of 4.8 at 25 o C. The pK a of glycine’s carboxyl group is __ lower _____ than that of acetic acid due to the combined __ electrostatic _____ and inductive (or electron withdrawing) microenvironmental effects of the nearby _____ amino __________ group. b. (15 ) The four protonation states of phosphoric acid/phosphate are shown below along with the pK a for each deprotonation. P O OH OH HO P O O OH HO H + H + P O O O HO P O O O O pK a = 2 pK a = 6.4 pK a = 13.4 H + H + H + H + 1) (6) Using the concepts of microenvironmental effects discussed in class, briefly justify the reported change in pK a for each successive deprotonation of inorganic phosphate. [ 150 words] 1st deprotonation: Strong inductive effect of 3 oxygens (particularly the double-bond-O), withdraws electrons from hydroxyl, hyperpolarizes the O-H bond and promotes deprotonation. This lowers the pK a relative to acetic acid. (Note that the negative charge on the deprotonated phosphate is stabilized by resonance just in acetate). 2nd deprotonation: Inductive (electron withdrawing) effect is weaker than in 1st deprotonation due to the electron donating effect of the negatively charged oxygen. Increased negative charge density (electrostatic effect) makes product of 2nd deprotonation less stable than the 1st product and thus opposes the deprotonation raising the pK a . [Note, however, the increased resonance stabilization of this product.] 3rd deprotonation: Inductive (electron withdrawing) effect is even weaker than in 2nd deprotonation due to 2 negatively charged oxygens (electron donating). Increased negative charge density (electrostatic effect) makes product of 3nd deprotonation even less stable that the 2nd and thus opposes the deprotonation raising the pK a even further. [Note the
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sping 02 midterm - Seat # _ Last initial _ Practice Exam...

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