Download Chapter 8: Molecular Modeling Problems

Molecular Modeling Problems
Chapter 8: Alcohols
1. Boiling Points of Alcohols and Ethers. To gain insight why the boiling
point of ethanol (79oC) is much higher than that of dimethyl ether (-23oC),
build clusters of 15-20 molecules for both molecules. Use molecular
mechanics.
The cluster geometries that you have obtained are assumed to be
representative of the enormous number of possible structures. A more
quantitative approach to calculate the properties of clusters (and
ultimately liquids) is to average over all possible geometries. Obtaining
the geometry for even a single cluster structure is difficult and beyond the
practical range of the quantum chemical model available in
SpartanModel. Molecular mechanics presents the only viable alternative.
What is the maximum number of hydrogen bonds involving a single
molecule of ethanol? What is the maximum number of hydrogen bonds
involving a single molecule of dimethyl ether? Provide a plausible
explanation for the difference in boiling points between the two molecules.
2. Sulfur Analogues of Alcohols and Ethers. The dipole moment for a
molecule may be thought of as a vector sum of the dipole moments of its
component bonds. Each bond dipole is proportional to the product of the
difference in charge on the two atoms and the distance between them.
Oxygen is more electronegative than sulfur, which suggests that OH and OC
bond dipoles will be larger than the corresponding SH and CS bond dipoles.
However, an OH bond is shorter than a SH bond (and a CO bond shorter that
a CS bond), which leads to the opposite conclusion. Are dipole moments of
thiols and sulfides typically larger or smaller than those of the corresponding
alcohols and ethers?
Obtain equilibrium geometries and atomic charges for methanol and
methanethiol and for dimethyl ether and dimethyl sulfide. Is the OH bond
distance in the alcohol shorter than the SH distance in the thiol? Is the
average of the CO bond distances in the alcohol and ether shorter than the
average of the CS bond distances in the sulfide? Is the charge separation in
an OH bond larger than that in an SH bond? Is the average charge separation
in a CO bond larger than that in a CS bond (count the charges on the three
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore
Molecular Modeling Problems
hydrogen atoms)? Are the dipole moments in the oxygen compounds larger,
smaller or about the same as those in their sulfur analogs?
3. 18-Crown-6. A crown ether is a ring made up of sp3 carbon and oxygen
atoms, for example, 18-crown-6 is made up of six repeating –OCH2CH2units. 18-crown-6 and molecules like it provide an electron-rich environment
and may be able to “tie up” the cation in a salt, thereby releasing the anion to
“do chemistry”. Strong binding requires not only that the “host” (the crown
ether) presents a suitably hospitable (negative) environment for the “guest”
(the positive ion), but also that the guest fits comfortably inside the host.
18-crown-6 and a few of its complexes with alkakli metal cations appear on
screen. Measure the distance between oxygen atoms at opposite sides of the
ring in “free” 18-crown-6 (top left of the screen). Compare this to the
corresponding distances in 18-crown-6 complexes of lithium (top right),
sodium (bottom left) and potassium (bottom right) cations. (The complexes
are initially shown as space-filling models in order to give you an idea of fit.
You can display them as ball-and spoke models to better see the relationship
between the host and the different guests and to make distance
measurements.) Which alkai metal ion requires the smallest change to the
structure of free 18-crown-6 in order to be accommodated? Which cation
requires the greatest change?
To accompany Organic Chemistry, Sixth Edition by Vollhardt & Schore