Download Chapter 6: Molecular Modeling Problems

Molecular Modeling Problems
Chapter 6: Properties of Haloalkanes
1. Charge Distributions in Transition States. Organic chemists have
developed a powerful nomenclature to designate chemical reactions. Here,
curved or “curly” arrows are used to indicate transfer of a pair or electrons (a
bond or a lone pair) from one place in a molecule to another. A particularly
simple example is found for the SN2 reaction of cyanide anion and methyl
iodide leading to acetonitrile and iodide anion. One arrow indicates that the
lone pair on cyanide will be used to make a new carbon-carbon bond, while
the second arrow indicates that the carbon-iodine bond will break with the
pair of electrons going to iodine.
H
H
N
C–
+
C
H
H
N
C
H
+ I–
C
H
H
Curly arrows do not say anything either about the geometry or charge
distribution in the transition state, or about changes in geometry and charge
distribution that occur in moving from the reactants to the transition state
and onto the product. All that they do is identity the product. On the other
hand, quantum chemical calculations are able to provide insight into both the
geometry and charge distribution of the transition-state. Comparison with
the corresponding information for the reactants, reveal any changes that
occur during the course of a chemical reaction.
An electrostatic potential map for “one frame” in the SN2 reaction cyanide
anion and methyl iodide appears on screen. A plot of energy vs. CI distance
is also provided. Step through the frames by clicking on
at the bottom
left of the screen, or animate the sequence by clicking on . Does “cyanide”
or “iodide” appear to hold the bulk of the negative charge at the transition
state? Is cyanide or iodide the better leaving group? Elaborate. Is your
conclusion consistent with the fact that the reaction as written is exothermic?
Explain.
2. Carbon-Fluorine Bond Lengths in Fluoromethanes. Do the lengths of
bonds between sp3 carbon and attached atoms or functional groups vary
significantly with the number of groups? Experimental carbon-carbon bond
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore
Molecular Modeling Problems
lengths for ethane (1.531Ǻ), propane (1.526Ǻ), 2-methylpropane (1.541Ǻ)
and 3,3-dimethylpropane (1.540Ǻ) show only slight variation. Is this always
the case? Obtain carbon-fluorine bond lengths in fluoromethanes, CFnH4-n
(n=1-4). Do they increase or decrease significantly with increasing number
of fluorines, or are they relatively constant? If you find that bond lengths
change significantly, offer an explanation as to why.
Repeat your calculations and analysis for the chloromethanes, CCl nH4-n
(n=1-4). Point out any significant differences with the results for
fluoromethanes. For which, if either, set of compounds is the change in bond
lengths with increasing substitution most similar to what is observed for
alkanes?
3. Bond Stengths in Fluoromethanes. Carbon-fluorine bond distances
change significantly with increased number of fluorine atoms attached to
carbon. Are these changes are mirrored by changes in carbon-fluorine bond
strengths? One measure is provided by comparing the energies of the three
reactions below.
CF4 + CH4
CF3H + CH4
CH2F2 + CH4
CF3H + CH3F
CH2F2 + CH3F
2CH3F
Calculate energies for all reactants and products for (alternatively use heats
of formation from the database accessible from SpartanModel), and
evaluate reaction energies. Is replacement of a CF bond in carbon
tetrafluoride by a CH bond more or less difficult than replacement in
fluoroform (CF3H)? Is replacement of a CF bond in fluoroform more or less
difficult than replacement in methylene fluoride (CF2H2)? Is there a
relationship between CF bond lengths and bond energies? Elaborate.
Repeat your calculations and analysis for the chloromethanes, CClnH4-n
(n=1-4). Point out any significant differences with the results for
fluoromethanes. Are any differences in bond energy changes between
fluoromethanes and chloromethanes reflected in differences in bond length
changes?
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore