Chem 225b - Comprehensive Organic Chemistry
Problem Set 3
Chapter 4, Chemical Reactions
Due: Monday, February 11, 2008
The prevailing theory of organic
structure in the early 19th century was Dualism or the
Electrochemical Theory, principally championed by
Berzelius.
Since inorganic sodium chloride could be considered as
Na+Cl-, then an alkyl halide such as
RCl could be thought of as R+Cl-. The
R group or "radical" of its day, was thought to be
immutable, the carbons and hydrogens behaving as though they
were an element. Liebig
(German) and Dumas (French), influential chemists of the
day, published a joint paper (1837), On
the Present State of Organic
Chemistry, extolling the
concept and claiming all that was left to do in organic
chemistry was to identify these immutable radicals (benzoyl,
ethyl, acetyl, etc.) As the story goes, a Parisian reception
at the Tuileries
was to change all of this. [The Tuilerie Gardens was
one of the images in Mussorgsky's "Pictures at an an
Exhibition", composed in 1874 for piano and later
orchestrated by Ravel. Select item 6 here.
For the history of this composition and background music, go
here.
(At
Yale, try this.]
The guests were discomforted by fumes from the burning
candles. Dumas was called in as a consultant. He found that
the waxes (fatty esters) had exchanged chlorine for
hydrogen, the culprit being the by-product hydrogen
chloride. [This story is likely apocryphal. It was
told by August
Hofmann at a eulogy for
Dumas (1884). If the event did occur, it is more likely that
the bleaching of candle wax involved addition of chlorine to
double bonds. Nonetheless, Dumas did investigate
substitution reactions.] Jean-Baptiste-André Dumas
(1800-1884) The concept of exchanging electropositive
hydrogen for electronegative chlorine was anathema to
dualism. Liebig was not enamored with substitution. Why
should he be? After all Liebig
and Wöhler
had done precisely this in 1832 during their work on the
benzoyl radical (C7H5O).
They
had converted benzaldehyde
[(C7H5O)H] into benzoyl
chloride [(C7H5O)Cl] by the
action of chlorine. So disenchanted was Liebig with the
controversies regarding theory in organic chemistry, by 1840
he turned his attention to the practical applications of
agricultural chemistry. Thus was born Liebig's beef extract.
Dumas's student, Laurent, not one to
shirk from controversy, was bold enough to call the process
substitution rather than exchange. Thus was Substitution
Theory born. Moreover, Dumas (1838) was able to substitute
three of the four hydrogens of acetic acid for chlorine to
form trichloroacetic acid, having similar properties to
acetic acid. The recognition of these similar properties led
to early Type Theory. In 1842, Melsen, a student of Dumas,
reversed Dumas's experiment by reducing trichloroacetic acid
to acetic acid by the action of zinc metal. The promulgation
of Substitution Theory gave the wry
wit of Wöhler, a.k.a., S. C.
H. Windler, an opportunity to shine. At the (beginning) turn
of the 20th century free radicals were detected and named
free radicals to distinguish them from the older radicals of
Radical Theory of the early 19th century. During the 19th
century chemists tried to isolate the older radicals to no
avail. When they (Kolbe and Frankland) thought they had
isolated methyl, they actually had the dimer of methyl,
ethane. The very process of substituting chlorine for
hydrogen is a free radical reaction.
Study the Alkane Module in Organic
Reactions Go Online (ORGO).
1. Determine the percent of each of the constitutional monochloro isomers expected from the chlorination of of 3-methylpentane. Show work.
2. Show the initiation and propagation steps for the free radical chlorination of cyclooctane.
a) Using Bond Dissociation Energies (BDEs), calculate the ΔHo for each relevant step and for the overall reaction. Show work.
b) Calculate the heat of formation (ΔHfo) of chlorocyclooctane. Show work.
3. Alexander
Borodin (1833-1887), the
accomplished Russian composer, did not quit his day job. He
was a Professor of Organic Chemistry at St. Petersburg
although he is more famous as a composer.
He unknowingly discovered a method for the decarboxylation
of carboxylic acids (via their silver salts) to afford an
alkyl halide with the liberation of CO2. The
reaction proceeds through a free radical chain mechanism.
Borodin did not isolate the alkyl halide. That
accomplishment rests with Herr und Frau Hunsdiecker (1942),
after whom the reaction is named. The silver carboxylate and
bromine form AgBr and A before the free radical chain
begins. What is A? Write the initiation and
propagation steps for this reaction. CH3CH2CO2Ag
+ Br2 -----> CH3CH2Br +
CO2 + AgBr
4. When cis-cyclooctene undergoes radical chain bromination, only one
monobromination product is isolated.
a) Draw the structures of the possible monobromination products. Using BDEs, explain which one is formed.
b) Write a radical chain mechanism for this reaction.
For this reaction to be successful,
bromine must be present in low concentration.
N-Bromosuccinimide (1, NBS) is
often used for this purpose (Wohl-Ziegler
reaction). Traces of HBr cause the formation of succinimide
2 and bromine as shown below.
c) Carbon tetrachloride is the classic solvent for this reaction.
Benzene may also be used as a solvent but not toluene
(methylbenzene). Explain why carbon tetrachloride and benzene are
acceptable solvents but not toluene.
d) From a preparative viewpoint, why is
this reaction not suitable for 1-methyl-1-cyclohexene
1.
5. After reading "Hess's Law of Constant Heat Summation" and/or "Heats of Combustion, Heats of Formation, and Bond Dissociation Energies (Powerpoint)" in the Thermochemistry Module, answer the following :
a) Draw a Standard State diagram that illustrates how to calculate the ΔHfo of a methyl radical. Make use of the BDEs and Heats of Formation Tables.
b) How does your answer in a) compare with the value in the BDEs table?
c) Using the value obtained in a), illustrate and calculate the C-C bond energy in ethane.
d) How does your answer in a compare with the
value in the BDEs
table?
6. A chemist desires samples of all of the monochlorination products
of propane and isobutane. Her only source of these gaseous
hydrocarbons is a gas cylinder containing both of these hydrocarbons.
She determines that the vapor density of the mixture is 1.99 g/L at 1
atm and 27 oC. Determine the composition of the mixture.
Using the values for the relative reactivity of primary (methyl),
secondary (methylene) and tertiary (methine) hydrogens with chlorine
--- 1:4.5:5.5, respectively ---, calculate the expected percentage of
each of the four monochloro constitutional (structural) isomers
formed during the chlorination of the mixture. Draw their structures
and name them.