Fall Workshop #4
Question #1
In round-robin fashion and using the molecule shown below,
a. Identify as many functional groups as you can (use the inside cover of your text as a guide)
b. Draw dipole arrows to show all polar covalent bonds.
c. Find all of the possible H-bond accepting groups.
d. Find all of the possible H-bond donating groups.
e. Draw the expected H-bonding interaction at SITE A between the carbonyl oxygen and water.
f. Draw the expected H-bonding interaction at SITE B between the hydroxyl group and acetone.
Question #2
Based upon intermolecular forces, explain each of the following observations:
a. Dimethyl ether is COMPLETELY soluble in water, but diethyl ether is onlySLIGHTLYsoluble in water, and di-tert-butyl ether is essentially INSOLUBLE in water.
b. Methanol (MW = 32 g/mol) is a liquid at room temperature, while propane (MW =44 g/mol) is a gas.
c. 2-Hexanone (bp = 127 ºC, MW = 100 g/mol) has a higher boiling point than 2,2-dimethyl-1-aminopropane (bp = 85.5 ºC, MW = 87 g/mol).
d. 1-Decanol is completely insoluble in water whereas ethanol is highly soluble in water?
Are any of the previous examples surprising to you? Which ones? What do you think is going on in these cases?
Question #3
Review as a group the 4 major types of intermolecular interactions (London dispersion forces, dipole-dipole, hydrogen-bonding, and ion-dipole).List the critical characteristics of each IM force. Then, apply your brainstorming list to the first system (flask of pure ethanol). Come to a consensus description of the important IM forces present in this system.
Next, each student should select one of the subsequent systems (b-i) and describe the type(s) of IM forces that will exist in each of the following examples.
Be sure to draw pictures to illustrate EACH of the possible intermolecular interactions.
(a) a flask of pure ethanol
(b) a flask of pure dichloromethane (a.k.a. methylene chloride)
(c) a flask of pure carbon tetrachloride
(d) a flask of pure cyclohexane
(e) a flask of pure acetone
(f)an aqueous solution of ethanol
(g) an aqueous solution of ethanoic acid
(h) an aqueous solution of NaCl
(i) a mixture of diethyl ether and ethanol
Match the following boiling points with each solvent in (a-e): 78 ºC, 56.5 ºC, 81 ºC, 76.5 ºC, 40 ºC. Use IM forces to explain the ordering of boiling points.
Question #4
Given the Newman projection shown below (eclipsed conformation) and the table of strain energy, answer the following questions. You will want to build this compound using your model kit.
(a) Calculate the potential energy for the given Newman projection.
(b) Which of the following best describes the interaction indicated by the BOX in the Newman projection above (circle your choice)?
1. torsional strain
2. steric strain
3. angle strain
4. both torsional and steric strain
5. both steric and angle strain
6. both torsional and angle strain
(c) Draw the Newman projection that would result from the 60° rotation of the BACK portion of the molecule (indicated by the arrow above) about the central C-C bond of the original Newman projection.
(d) Calculate the potential energy for this Newman projection.
(e) Which of the two conformations will most likely predominate at equilibrium? You can calculate an equilibrium constant using DE = -RT(lnKeq) Assume here that the sample exists at room temperature (27 ºC) and that R = 8.314 J mol-1 K-1. Show your work and explain how the values for DE and Keq agree.
Question #5
Using your model kit, build the following compound:
3-methylpentane
a) Locate the C2-C3 bond, and rotate so that the methyl group on C2 and the ethyl group on C3 are eclipsed (dihedral angle will be 0 degrees between these two groups)
b) Find all eclipsing and gauche interactions – be sure to determine whether each interaction is the result of steric strain, torsional strain, or both
c) Using the data table below, calculate the energy for each interaction from part (b)
d) Plot the energy data point on a graph of energy (y-axis) as a function of dihedral angle (x-axis) with the following x-axis points = 0, 60, 120, 180, 240, 300, and 360 degree [you can use the EXCEL program on the computer in S347 and display on the SmartBoard]
e) Rotate the front half of the molecule around C2-C3 by 60° to the right to generate a new conformation
f) Repeat steps (b) through (e) until you completed a full 360° rotation Evaluate your resultant graph looking for energy minima and maxima.
g) Correlate those with your Newman projections. Do they make sense?
h) Select any one (1) conformation and draw its Newman projection. Share renderings among the group.
Interaction / Energy (kcal/mol)CH3-CH3 gauche / 0.6
CH3-H eclipsing / 1.6
CH3-CH3 eclipsing / 2.6
CH3-CH3CH2 gauche / 0.9
CH3CH2-H eclipsing / 1.8
H-H eclipsing / 0.9
CH3-CH3CH2 eclipsing / 3.1