What is the monomeric chemical unit of natural rubber, and what is its geometric stereochemical configuration?
Answer and explanation
Correct answer: D. Isoprene (2-methylbuta-1,3-diene) with cis-1,4 configuration
- Step 1: Natural rubber is harvested as latex from the Hevea brasiliensis tree and is a polymer of isoprene (2-methylbuta-1,3-diene).
- Step 2: All double bonds along the polyisoprene backbone are oriented with cis stereochemistry.
- Step 3: Natural rubber is cis-1,4-polyisoprene; the cis geometry prevents regular crystal packing, creating coiled chains responsible for its high natural elasticity (in contrast to gutta-percha, which is the crystalline trans-isomer).
Answer
Isoprene (2-methylbuta-1,3-diene), having cis-1,4 configuration.
Why not the others: trans-1,4 configuration is gutta-percha, a hard and non-elastic natural resin; Chloroprene is the monomer for synthetic neoprene rubber; Buta-1,3-diene produces polybutadiene synthetic rubber.
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