Problem Set 3
Chapter 4
Due: Monday, September 27, 2004
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 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. 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?
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 with the controversies
regarding theory in organic chemistry was Liebig, by 1840 he
turned his attention to the practical applications of
agricultural chemistry. Thus was born Liebig 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.
1. Study the Alkane Module in Organic
Reactions Go Online (ORGO).
2. Chlorination of 2-methylbutane in the presence
of light provides four constitutional monochloro isomers. Draw the
four isomers, name them (IUPAC), and what percentage of each is
formed? Radical bromination of this hydrocarbon gives only one one of
the four isomers. Which one is it and why?
3. Using data provided for the heat
of formation of alkanes, determine the
heat of formation of 2-methylhexane using two reference points.
4.What is the difference in the heat of combustion
of 2-methylpentane and 3-methylpentane? Which one liberates more
heat.?
5. Photochlorination of compound A (C6H12) affords a single monochlorination product B. Compound A is inert toward hydrogen in the presence of a catalyst.
a) What is the structures of A and B?b) Why is only one monochlorination product formed?
Write the propagation steps and the overall reaction.
c) How much heat is liberated in each step and in the overall reaction?d) Given the heat of formation of A and non-organic products of the reaction, calculate the heat of formation of B.
e) How does your value for B agree with the given answer?
6. How many dichlorocyclopentane constitutional
isomers are possible? How many stereoisomers of each? Do not include
mirror images. Draw the structures.
7. Two stereoisomeric, 1,4-disubstituted hydrocarbons, A and B, are cyclohexanes. Compound A has an energy difference of 3.4 kcal/mol between its two chair conformations while compound B has a value of 0 kcal/mol. Bromination of A or B with two molar equivalents of bromine gives the same mixture of dibromo stereoisomers, C and D. The difference in energy between the chair conformations of C is 2.2 kcal/mol.
a) What are the structures A-D?
Explain and illustrate.
b) What is the difference in energy between the
chair conformations of D? Explain and illustrate.
c) Why is the same mixture of C and D produced from A and B ?