Organic Chemistry Exam

1. (15 points) Three C4H10O isomers are shown below, along with their boiling points. bp: (CH3)3C–OH CH3–CH2–CH2–CH2–OH CH3–CH2–CH2–O–CH3 tert-butyl alcohol n-butyl alcohol methyl propyl ether 82 118 39 °C (a) Based on their boiling points, which substance has the strongest intermolecular interactions in the liquid state? (b) Which has the weakest? (c) Explain briefly why n-butyl alcohol has a much higher bp than methyl propyl ether. (d) Explain briefly the difference between the first two, i.e., why does tert-butyl alcohol have a lower bp than n-butyl alcohol. 2. (15 points) Early in the semester you did a modeling experiment using Hyperchem™. (a) In one part of the lab, you found that the energy of the optimized complex formed between formaldehyde and formamide (at right) was about 10 kJ/mol lower than the energy of the separated molecules. What kind of interaction exists between the molecules? H O H H N C H C H O (b) In the space below, sketch the relative energy as a function of the distance between the molecules. Show clearly what happens to the energy when they get closer and when they get farther apart than they are in the drawing above. (c) In another part of the modelling lab you found that H the strength of a similar interaction between two N "amide" groups, like the ones shown at right, was about 15 kJ/mol. H H O H N C H C H O There must be a structural reason that the oxygen of NH2–CH=O interacts more strongly than the oxygen of CH2=O with that NH hydrogen. What do you think that reason is? (hint: can you draw resonance structures with negative Os for these two molecules? If so, how do these contributions account for the difference?) 3. (10 points) Calculate the pH of each of the following aqueous solutions. (a) 2.72 x 10–4 M KOH (b) 3.68 x 10–11 M HBr 4. (25 points) The equilibrium constant for the gas-phase reaction below is 6.25 x 104 M–1. Cl2 + 2 NO ==== 2 NOCl (a) 0.810 moles of NOCl is added to a 1.00-L flask. What are the concentrations of Cl2, NO, and NOCl after equilibrium is established? (The "5% rule" is not appropriate here; please find the correct concentrations to three significant figures.) page 2 (b) 0.960 mol of Cl2 and 0.470 mol of NO are added to the equilibrium mixture from part a. Calculate the concentrations after equilibrium is reestablished. 5. (30 points) Chloroacetic acid, ClCH2–CO2H, has Ka = 1.38 x 10–3. (a) Calculate the pH of a solution made by dissolving 0.0117 moles of chloroacetic acid in 100 mL of water. (b) What is the extent of dissociation (in %) of the chloroacetic acid in part a? (c) When the solution from part a is combined with 100 mL of a solution of aq. HNO3 having a pH of 0.523, what is the pH of the solution that results? page 3 (d) What is the extent of dissociation (in %) of the chloroacetic acid in part c? (e) A new solution is prepared as described in part a, and 0.00822 moles of solid KOH is added. What is the pH of the resulting solution? (f) More solid KOH is then added, so that the total amount of KOH added is exactly 0.0117 mol (i.e. to reach the equivalence point). What is the pH of the solution? 6. (15 points) The pKas of acetic acid and its chloro-, dichloro-, and trichloro- derivatives are shown below. CH3–CO2H pKa: 4.76 ClCH2–CO2H 2.86 Cl2CH–CO2H 1.29 Cl3C–CO2H 0.65 (a) For the most acidic compound in this series, write the acid dissociation reaction. (b) We already know that resonance stabilizes the conjugate bases of all four acids by delocalizing the charge, and this is the reason that carboxylic acids like these are much more acidic than water or alcohols. How do you account for the effect of the chlorines on the acidities in the series above? page 4 7. (15 points) When 1 pound of each substance below is dissolved in a gallon of water, is the resulting solution acidic, basic, or neutral? (No need to calculate the exact pH, of course!) (a) CH3NH3Cl (b) CsCN (c) KClO4 (d) F2CHCH2CH2OH (e) CH3CH2CO2Na Last semester's first exam was later in the term than yours, so there was one more problem on material we haven't covered yet. So expect yours to include an additional problem or two. Detailed solutions to these problems will not be made available, but we will certainly be willing to go through the problems with you in discussion sections or office hours. page 5