Question

Isoprene is the repeating unit in natural rubber. The structure of isoprene is

1. Give a systematic name for isoprene.
2. 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 around 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 around the carbon–carbon double bonds.

Short answer

1. Systematic name for isopreneare$2$-methyl, $1,3$–butadiene
2. In the polymer built by isoprene units, natural rubber, the configuration around the double bond is cis. While in the polymer gutta percha, the configuration around the double bond is trans.

Step-by-step solution

Systematic name for isoprene.

To be able to give the systematic name of isoprene, first one must number the longest chain and make sure that multiple bonds have the lowest numbers. Thus, numbering is done as follows:

Since the parent chain is four carbons, it starts with a parent name of but

Next double bonds are counted . In isoprene, there are two double bonds hence we use the suffix -diene. We use -di, in this case, to indicate that there are two double bonds attached in the first and third carbon hence we should write $1,3$-butadiene for the parent chain.

Lastly, an the additional functional groups are attached to the parent chain. One can see that one methyl group is attached to the second carbon. Hence, we get a suffix of $2$-methyl

Combining this all together, we name isoprene systematically as $1,3$– butadiene

Hence systematic name for isopreneare$2$-methyl, $1,3$– butadiene

To find When isoprene is polymerized

In the polymer built by isoprene units, natural rubber, the configuration around the double bond is cis. While in the polymer gutta percha, the configuration around the double bond is trans.

The structures of these polymers are shown in the figure below.

Hence In the polymer built by isoprene units, natural rubber, the configuration around the double bond is cis. While in the polymer gutta percha, the configuration around the double bond is trans.