Problem 74 - Chemistry An Atoms First Approach

Question

Isoprene is the repeating unit in natural rubber. The structure of isoprene is a. Give a systematic name for isoprene. b. When isoprene is polymerized, two polymers of the form are possible. In natural rubber, the cis configuration is found. The polymer with the trans configuration about the double bond is called gutta percha and was once used in the manufacture of golf balls. Draw the structure of natural rubber and gutta percha showing three repeating units and the configuration about the carbon-carbon double bonds.

Step-by-Step Solution

Answer

a. The systematic name for isoprene is 2-methyl-1,3-butadiene. b. Natural Rubber (cis configuration): CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2 Gutta Percha (trans configuration): CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2

Step 1: a. Systematic name for isoprene

Isoprene's structure consists of a five-carbon chain (a pentene) with two double bonds and a methyl group at one end: CH2=C(CH3)CH=CH2 We can count the carbon atoms from the double-bonded carbon with the methyl group (C1), and following the chain to the end (C5). The systematic name for this compound follows the IUPAC nomenclature, recognizing the presence of two double bonds and a methyl group. Therefore, the systematic name for isoprene is 2-methyl-1,3-butadiene.

Step 2: b. Drawing natural rubber and gutta percha structures

Natural rubber and gutta percha are both polymers of isoprene and differ in the configuration of the double bond in the repeating unit. In this step, we will draw their structure showing three repeating units and the configuration of the carbon-carbon double bonds. - Natural Rubber: The cis configuration is found in these polymers. In this configuration, the substituents adjacent to the double bond are on the same side. Structure of Natural Rubber (cis configuration): CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2- CH2=C(CH3)-CH=CH2 - Gutta Percha: The trans configuration is found in these polymers. In this configuration, the substituents adjacent to the double bond are on opposite sides. Structure of Gutta Percha (trans configuration): CH2=C(CH3)-CH=CH2-CH2=C(CH3)-CH=CH2- CH2=C(CH3)-CH=CH2