CHM 103 GENERAL CHEMISTRY
SPRING QUARTER 2004
Lesson Plan for 6/15/2004
I. Quiz at 12:00.
II. Precision vs. Accuracy and the Calibration of pH Meters.
A. Precision: repeatability and significant figures.
B. Accuracy: How close is the measured value to the actual value?
C. If the pH meter has a precision of ±0.05 pH units, how precisely need we
prepare our pH standard buffers?
III. Isomerism in Organic Compounds
A. Structural Isomerism
1. We can distinguish three different structural isomers for the
alkenes with the molecular formula C4H8:
a. CH3CH=CHCH3
b. CH2=C(CH3)2
c. CH2=CHCH2CH3
2. We can also distinguish three different structural isomers for the
substances with the molecular formula C3H8O. Two are alcohols,
one is an ether:
a. CH3CH2CH2OH
b. CH3CH(OH)CH3
c. CH3OCH2CH3
3. Structural isomers are isomers whose differences can be seen
when the formulas are written as above. The above two sets of
structural isomers are pictured in your text at the top of p. 589:
4. Class Exercise: Construct all the structural isomers of hexane,
C6H14.
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B. Geometric Isomerism
1. In this course, we will find geometric isomerism whenever we
have double bonded carbons, and each of the double bonded
carbons has two different kinds of substituents attached to it.
2. An example is the alkene, CH3CH=CHCH3, pictured above in
IIIA1a. This structure has two geometric isomers:
3. Geometric isomerism is caused by the rigidity of the carboncarbon double bond. There is no rotation that will change the cis
isomer shown above, left, into its trans isomer, shown on the
right.
4. Cis-trans isomerism can be important in human metabolism and
nutrition. In nature, unsaturated fats invariably have cis
geometries at their double bonds, and the human body “knows”
how to assimilate them. Partial hydrogenation of polyunsaturated
fats produces a mixture containing so-called trans-fats which do
not occur in nature and are therefore difficult for the metabolism
to handle.
C. Optical Isomerism
1. Early 19th century observations: solutions of some, naturally
occurring organic chemicals would rotate the plane of polarized
light. Such materials were called optically active:
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2. At the time, there were no synthetic organics with optical
activity, even chemicals that were otherwise identical to optically
active natural products.
3. Tartaric acid and racemic acid (Pasteur, 1848)
a. Molecular formulas: both C4H6O6
b. Optical activity
i.
Tartaric acid: dextrorotatory (rotates right)
ii. Racemic acid: none
c. Crystal geometry
i.
Tartaric acid: hemihedral (all of same handedness)
ii. Racemic acid: hemihedral (mix of left and right
handed)
d. Pasteur sorted the racemic acid crystals by their handedness
into two fractions:
i.
One was dextrorotatory and otherwise identical to
tartaric acid.
ii. The other was an unknown form of tartaric acid that
was levorotatory (rotated left).
iii. If he mixed them back together, he lost the optical
activity.
4. Structural Basis of Optical Isomerism (van’t Hoff and Le Bel,
1874)
a. Carbon has tetrahedral geometry
b. If substituents are all different, the tetrahedral carbon is
asymmetric.
c. A tetrahedron with 4 different vertices is asymmetric and
has two mirror image “isomers.”
d. Examples of compounds with asymmetric carbon
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e. Three different representations of lactic acid
f. A representation of CHClBrI from your text:
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g. Important definitions:
i.
Chiral molecule: a structure with two mirror image
forms.
ii. Enantiomers: the two mirror image forms of a
chiral molecule.
iii. Optical isomers: same as enantiomers.
iv.
Chiral center: carbon atom with four different
groups bonded to it.
v.
Racemic mixture: a 50:50 mixture of optical
isomers. Has no optical activity.
IV. Synthetic Organic Polymers
A. What is a polymer? In general, a polymer is
1. A molecule with a high molecular weight.
2. A long chain molecule.
3. A molecule built from small repeating units called monomers.
4. Often called a macromolecule.
B. Thermoset polymers (not in text)
1. Components of a thermoset
a. resin – a cross-linkable polymer that is liquid at room
temperature
b. a curing or cross linking agent
c. What you get is what you get. There is no way to melt and
refashion a thermoset after it has been cured.
2. Making things out of thermosets
a. mold the resin
b. add the curing agent – allow to cure
3. Examples of thermosets
a. rubber: natural rubber cross linked with sulfur
(vulcanization)
b. bakelite
c. “epoxy” adhesives
d. “fiber glass” (the polymer matrix part)
C. Biological polymers (not in text)
1. Synthesized in place by metabolic processes
2. Limited reprocessibility
a. denaturing of proteins (example: cooking an egg)
b. convert cellulose to celluloid
c. convert cellulose to rayon
3. Examples of biological polymers
a. proteins
b. DNA
c. cellulose
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D. Thermoplastic polymers (these are discussed in the text)
1. Synthesized to final chemical structure
2. Generally capable of being melted and resolidified into a finished
article.
3. Most can be used as “plastics”
4. Some can be used as fibers
5. Examples of thermoplastics
a. polyethylene
b. polystyrene
c. nylon
d. polyester
e. polycarbonate (“lexan”)
E. Addition polymers
1. Made by adding monomers to one another with the entire
monomer being incorporated into the polymer, for example, the
synthesis of polyethylene from ethylene:
2. Other common addition polymers
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F. Condensation Polymers
1. Made by reacting monomers with functional groups at both ends.
a. Two functional groups on different monomer molecules
react.
b. Form a functional linkage.
c. Eliminate (usually) water.
d. The resulting polymer is a chain of monomer units joined
by the functional linkages.
2. Polyesters, a major class of condensation polymers.
a. Formed by the reaction of a dihydroxy alcohol with a
dicarboxylic acid to produce an ester linkage and eliminate
a water molecule:
b. The reaction continues with carboxyl ends reacting with
dihydroxy alcohol monomer or with hydroxyl ends on other
growing polymer chains (and with hydroxyl ends reacting
with dicarboxylic acid monomer or with carboxyl ends) to
produce:
c. A particular example of a polyester (and one of the most
commercially important ones) is polyethylene terephthalate
(PET), whose monomers are:
These react to form first, the monoester (a), and eventually
the polymer (b):
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d. PET is a very versatile polymer. You may recognize the
following duPont brand names and products:
i.
Dacron (fiber)
ii. Mylar (film)
iii. Rynite (engineering plastic)
At AlliedSignal, we were the “other guys.” Our brands and
products were:
iv.
DSP (tire cord fiber)
v.
Petra (engineering plastic)
3. Polyamides (also known as nylons) comprise another very
important family of condensation polymers.
a. One class of polyamides is made by reacting a diamine
with a dicarboxylic acid:
b. The monomers adipic acid, HOOC—(CH2)4—COOH, and
hexamethylenediamine, H2N— (CH2)6—NH2, can be
polymerized to form the polyamide, nylon 6,6:
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c. Nylon 6,6 is a very versatile polymer. You may recognize
the following duPont brand names and products:
i.
Antron (carpet fiber)
ii. Zytel (engineering plastic)
d. It is also possible to make polyamides out of aminocarboxylic acids. The most industrially important of these
polyamides is nylon 6, which you can think of as being
made from aminocaproic acid, NH2—(CH2)5—COOH.
At AlliedSignal, we “other guys” had the following nylon 6
products:
i.
Anso (carpet fiber)
ii. Capron (engineering plastic)
G. Polymer properties
1. Polymer properties are dependent on composition and structure.
Take polyethylene, for example, which comes in two major
types,
a. branched polyethylene:
b. and linear polyethylene:
c. Figure 22.12 in your text illustrates the dramatic difference
in properties between branched (low density polyethylene:
LDPE) and linear (high density polyethylene: HDPE):
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d. The compact structure of HDPE gives the material a high
degree of crystallinity, which imparts stiffness. In LDPE,
the butyl side chains interfere with crystallization and the
result is a more rubbery, pliable structure.
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