Chem 270 Homework 4 Fall 2024 PDF

Chem 270 Homework 4 (46 pts) Fall 2024 1. (6 pts) Based on the structures of the following molecules: a. rank them from the most acidic (1) to least acidic (4) Structure Rank 1...

Chem 270 Homework 4 (46 pts) Fall 2024 1. (6 pts) Based on the structures of the following molecules: a. rank them from the most acidic (1) to least acidic (4) Structure Rank 1 4 OH 2 3 b. What properties of the acidic group did you use to rank these acids? Electronegativities: O, N, and C are on the same row of the periodic table – electronegativity goes O>N>C, so OH is more acidic than NH, which is more acidic than CH. Sizes: SH is more acidic than OH because S is larger than O. 2. (5 pts) Looking at the table below, a. Rank the following alcohols from the strongest (1) to the weakest (3) acid Structure Ka Rank 5.75 x 10-13 1 1.29 x 10-13 3 4.90 x 10-13 2 b. How do the structures explain the ranking? The greater the number of electron-withdrawing chlorine atoms equidistant from the OH group, the stronger the acid. Note that the larger the Ka, the stronger the acid. 3. (5 pts) Looking at the table below, a. Rank the carboxylic acids in order from strongest acid (1) to weakest acid (4) Structure pKa Rank 4.82 4 2.86 1 4.53 3 4.05 2 b. How do the locations of the chlorine atoms explain the rankings? The chlorines make the acid stronger by stabilizing the conjugate base. The stabilization occurs via an inductive effect that weakens as the chlorine gets further away from the anionic center. Chem 270 Homework 4 (46 pts) Fall 2024 4. (4 pts) Adding a nitro group to the benzene ring lowers the pKa of phenol. Placing the nitro group ortho or para to the OH has a greater effect than placing it meta. Adding a nitro group to the benzene ring also lowers the pKa of benzoic acid. In this case, placing the group ortho to the CO 2H group has a much greater impact than placing it meta or para. Why are these two patterns so different? Structure pKa Structure pKa The ortho and para nitro groups stabilize the phenolate anion via resonance. The meta nitro group cannot 10.0 4.21 stabilize the phenolate ion via resonance, so meta nitrophenol is a weaker acid than either ortho or para nitrophenol. 7.23 2.18 The nitro groups on benzoic acid stabilize the benzoate ion through an inductive effect (not a resonance effect). 8.36 3.47 Inductive effects weaken with distance, so the closest O nitro (ortho) gives the greatest effect and ortho 7.15 OH 3.41 nitrobenzoic acid is the strongest acid. O 2N 5. Consider the compound 3-aminopropan-1-ol a. (2 pts) indicate which group is the most basic by drawing this compound’s conjugate acid. b. (2 pts) indicate which group is the most acidic by drawing this compound’s conjugate base. 6. (8 pts) Draw the products of the following acid-base reactions and indicate whether the reactants or products are favored at equilibrium. Equilibrium favors weakest acid a. products favored b. products favored c. products favored d. products favored (note: the pKa of HCl in water is -5.9, while the pKa of CH3CH2OH2(+) is -2.5) Chem 270 Homework 4 (46 pts) Fall 2024 7. (10 pts) Match the following terms to their definitions A. conjugate acid B. basicity C. Brønsted acid D. pH E. equilibrium constant F. buffer G. pKa H. Brønsted base I. Lewis acid J. acid dissociation constant a. _F__ a weak acid used, along with its conjugate base, to maintain the pH of an aqueous system b. _D__ the negative log10 or the hydrogen ion concentration; used to describe the acidity of a solution c. _B__ the tendency of a substance to share its electrons with a proton d. __J_ a physical constant (= [H+][A-]/[HA]) that describes the degree to which an acid dissociates e. __H_ a substance that accepts a proton from an acid f. _E__ a numerical constant that reflects the ratio of the product concentrations to the reactant concentrations at equilibrium g. _C__ a substance that donates a proton to a base h. _A__ the substance formed when a Brønsted base accepts a proton i. _I__ a substance with an empty atomic orbital that accepts a pair of elections to form an adduct j. _G___ the negative log of the acid dissociation constant 8. (4 pts) You have been asked to prepare a buffer solution at pH 7.5 to use in a biochemical experiment. How will you decide which weak acid/conjugate base to use? You will pick a buffer that has a pKa as close to 7.5 as you can, as long as it is compatible with your biological system. If you have no clue about the compatibility of your system with the available buffers, you may have to try a few of them. HEPES (pKa 7.55), phosphate (pKa 7.2), and MOPS (pKa 7.15) would be good picks to start. I’m sure you could find others.