chm 103 general chemistry - ltcc online · web viewin this course, we will find geometric isomerism...

CHM 103 GENERAL CHEMISTRYSPRING 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 Isomerism1. 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. CH3CH2CH2OHb. 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 Isomerism1. 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 carbon-carbon 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 Isomerism1. 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 activityi. Tartaric acid: dextrorotatory (rotates right)

ii. Racemic acid: nonec. 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 geometryb. 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.”

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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 is1. 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 agentc. 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 resinb. add the curing agent – allow to cure

3. Examples of thermosetsa. rubber: natural rubber cross linked with sulfur

(vulcanization)b. bakelitec. “epoxy” adhesivesd. “fiber glass” (the polymer matrix part)

C. Biological polymers (not in text)1. Synthesized in place by metabolic processes2. Limited reprocessibility

a. denaturing of proteins (example: cooking an egg)b. convert cellulose to celluloidc. convert cellulose to rayon

3. Examples of biological polymersa. proteinsb. DNAc. cellulose

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D. Thermoplastic polymers (these are discussed in the text)1. Synthesized to final chemical structure2. Generally capable of being melted and resolidified into a finished

article.3. Most can be used as “plastics”4. Some can be used as fibers5. Examples of thermoplastics

a. polyethyleneb. polystyrenec. nylond. polyestere. polycarbonate (“lexan”)

E. Addition polymers1. 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 Polymers1. 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 amino-carboxylic 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 properties1. 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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chm 103 general chemistry - ltcc online · web viewin this course, we will find geometric isomerism...

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