Download CHM 253L Organic Chemistry Laboratory I

CHM 253L Organic Chemistry Laboratory I
Laboratory Follow-Up Assignment Exp. #3
Due: Wednesday, October 3, 2007 by 5:00 pm
This experiment will be written up with in the style of an iterative Results and Discussion
Section. Adherence to the expectations of good scientific writing will be expected.
Data Section: (R = result, D = discussion)
Part 1
R #1 Create a table of the bond angle and bond length parameters for methane, ethane, ethylene,
acetylene, 1-butanol, and 2-butanone.
D #1 How well do the calculated values compare with the values that you looked up in
the Jones text or via any other source (please indicate your source!!)? Discuss any
deviations between the two data sets.
R #2 Create a graph in EXCEL plot depicting the variation of single point energy values for 2bromopentane as a function of dihedral angle. Be sure to label energy minima and maxima.
D #2 Evaluate the graph of energy vs. dihedral angle. Be sure to include Newman
projections for each dihedral angle and to discuss contributions of torsional and steric
strain relating to your results and drawings.
R #3 Print/label the pictures of the bonding and antibonding molecular orbitals for the sigma
bond in H2. Likewise, Print/label the pictures of the bonding and antibonding molecular orbitals
for the pi bond in ethylene. Label each orbital as BONDING or ANTIBONDING. Indicate any
nodes. Report the energy values and the E between the bonding and antibonding MOs for both
H2 and ethylene.
D #3 Compare and contrast the orbital pictures and energy values for both the H2 sigma
bond and the ethylene pi bond in relation to the expectations as shown in Figures 1.37
and 3.12 of the Jones text.
R #4 Arrange the 4 alcohols (from prelaboratory question #2) in order of increasing electron
density surfaces. Also, record the dipole moment of each alcohol (show direction and magnitude
-- you may have to draw this in).
D #4 Compare the shapes and charge distribution of the 4 alcohols being sure to
comment on how well these factors correlate to the boiling point trend (that you looked
up in your prelab exercise). Does there seem to be a clear-cut trend? Provide any
alternative explanations that may better describe the observed trend in boiling point.
You will want to think about potential intermolecular forces here!
R #5 Record the energies for each of the alkene isomers (from prelaboratory question #4) and
arrange them in order of increasing stability.
D #5 Compare your results with the answer that you proposed for question #4 of the
prelab exercises. Explain why the alkenes are ordered as observed (with respect to
stability). You may want to consult Section 3.6 of your Jones text – What model serves
to explain the ordering here?
R #6 Create a table that summarizes the dipole moment values and hydrogen bonding capabilities
for the compounds selected in prelaboratory question #3.
D #6a Compare and contrast the polarity (in terms of measured dipole moment) for the
compounds in question #3 of the prelab exercises. Given this information, rank them in
order of increasing solubility in water (a polar solvent). What evidence have you
uncovered in your molecular models that suggests polarity?
D #6b If we also consider that hydrogen bonding is an important intermolecular force in
determining solubility, determine the solubility of each compound from question #3 of
the prelab exercises in water (insoluble, partially soluble, soluble). Be sure to indicate
whether the compounds are likely to accept a hydrogen bond from the solvent (water),
donate a hydrogen bond to the solvent, or both. What evidence have you uncovered in
your molecular models that suggests hydrogen bonding capability?
D #6c Compare your answers to D #6a and D #6b with literature solubility data in
water for each compound (cite sources). In light of the literature values, comment on the
variable contributions of polarity AND hydrogen bonding to the known solubility data.
R #7 Record the atomic charges and results of electrostatic potential maps that you collected for
the various ions.
D #7 Describe how the modeled ions demonstrate the hybrid structure or weighted
average of the resonance structures that you drew as part of the prelaboratory exercises.
R #8 Record the energies for each cyclic hydrocarbon (cyclopropane through cyclohexane) as
Hf per CH2. See Table 5.1 on page 200 of the Jones text.
D #8 How well do your calculated (experimental) heats of formation per CH2 for the
series of cyclic hydrocarbons cyclic hydrocarbon (cyclopropane through cyclohexane)
agree with the data in Table 5.1 on page 200 of the Jones text? Explain.
Part 2
R #9 For each series of acids (1-5), create a table of largest value for electrostatic potential
around H vs. pKa.
D #9a For each series of acids (1-5), rank the compounds from MOST to LEAST acidic
and explain their ordering. In the process of your explanation, be sure to mention which
effects (formal charge, electronegativity, inductive, resonance, hybridization)
contribute most significantly to the rank ordering of acids in each series.
D #9b If we were to repeat the experiment, but now looking at the structures after
removal of the acidic hydrogen, what do you think you would observe in each series for
the relative negative electrostatic potential resulting from deprotonation of each acidic
hydrogen vs. pKa? That is, what do you think you’d observe for the relative ranking of
each acid’s corresponding conjugate base anion? Give a sketch of the approximate graph
of negative electrostatic potential of the conjugate base anion vs. pKa for Series 2 and 4
and COMMENT.
R #10 Write a chemical equation for each of the acid dissociations in Part 2: Energy of
Dissociation. Then, calculate the Hrxn for each of the dissociations. You’ll need to know that
the energy for H+ = 0 kcal/mol. From this value of Hrxn, calculate the Keq of dissociation.
Finally, use this information to create a table to summarize the data.
D #10 Do your numbers make sense relative to the 5 series of acids that you did above?
Are the results also consistent with Figure 5b on page 69 of the purple book? Explain.
Extending Your Learning Questions
1. Given what you learned from this experiment, circle the most acidic hydrogen atom in each
compound and be sure to report an estimated pKa value. Arrange the following compounds in
order of INCREASING positive electrostatic potential of the acidic hydrogen. Also, provide
a BRIEF explanation why you ranked them as you did according to the various stabilizing
effects – formal charge, electronegativity, inductive effect, and resonance.
O
O
SH
OH
Cl
Cl
A
F
F
E
C
O
OH
Br
B
D
HO
F
G
H2N
Br
F
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2. Rank the compounds in the previous question (#1) in order of LARGEST to SMALLEST H
of acid dissociation and sketch approximate energy diagrams for each compound (like Figure
2b on page 58 of the purple book).
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