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General Chemistry Principles, Patterns, and Applications, 2011

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9. ♦ L-Aspartic acid is an amino acid found in fossil bone. It can convert to a geometrically different form (Daspartic<br />

acid) at 20°C, with a half-life corresponding to the conversion of L → D of 14,000–20,000 yr. If the<br />

temperature of an archaeological site is constant, then the extent of the conversion can be used to date<br />

fossils. In one such case, archaeologists dated the arrival of humans on the North American continent to be<br />

20,000 yr ago, but the conversion of L-aspartic acid to D-aspartic acid in human fossils indicated that Paleo-<br />

Indians were living in California at least 48,000 yr ago. What would be the relative concentrations of the L-<br />

<strong>and</strong> D-forms that produced this result? Carbon-14 has a half-life of approximately 5730 yr. What percentage<br />

of the carbon-14 originally present would have been found in the bones?<br />

The<br />

technique described is frequently used in conjunction with radiocarbon dating. In cases<br />

where the results from the two techniques are in gross disagreement, what information<br />

can you get by comparing the two results?<br />

10. ♦ Peroxides are able to initiate the radical polymerization of alkenes. Polyethylene, for example, is a highmolecular-weight<br />

polymer used as a film in packaging, as kitchenware, <strong>and</strong> as tubing. It is produced by<br />

heating ethylene at high pressure in the presence of oxygen or peroxide. It is formed by the following radical<br />

process:<br />

RO×+CH 2 = CH 2- ® RO - CH2 - CH 2×<br />

RO―CH 2 ―CH 2· + CH 2 =CH 2 → RO―CH 2 ―CH 2 ―CH 2 ―CH 2·<br />

a. Label the steps that correspond to initiation <strong>and</strong> propagation.<br />

b. Show all available chain-terminating steps.<br />

c. The polymerization of styrene (C 6 H 5 CH=CH 2 ) occurs by a similar process to produce<br />

polystyrene, which is used as a packaging material. Draw the structure of the polymer that<br />

results from five propagation cycles.<br />

11. Lucite <strong>and</strong> Plexiglas are transparent polymers used as a glass substitute when a plastic material is preferred<br />

for safety. The compound used to synthesize Lucite <strong>and</strong> Plexiglas is methyl methacrylate, which is shown<br />

here. During the polymerization reaction, light produces a radical initiator from hydrogen peroxide (H 2 O 2 →<br />

Saylor URL: http://www.saylor.org/books<br />

Saylor.org<br />

1342

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