[WC Fan]

Hi,
Im just totally stuck in this question. I googled in the internet for answers for almost a day and i didnt find the answer that im looking for,
the question is simply:
Explain in terms of bonding and structure of rubber to its properties below.
a) water repellent
b) resistent to alkalis and weak acids
c)electrical insulator

[RandoFlyer]

I would imagine it is hydrophobic because it is largely non-polar, due to the fact that it is a very long polymer of a fairly non-polar monomer.  It is an electrical insulator because it has no free electrons to conduct with, all of the electrons are in a covalent bond.  Im not 100% about the weak acid/base thing so I'll leave that for someone else.

[tamim83]

a.) Its water repellant because its structure has no polar bonds or bonds capable of participating in hydrogen bonding.  This makes the polymer hydrophobic and the bulk material water repellant.

b.) Alkalis (bases) generally donate electron pairs (Lewis Bases).  Since the rubber structure contains electron rich pi bonds and saturated carbon centers, it is not likely that a base would react with it.  Also, even if a base attacked one of the sp² carbons, there is no way to stabilize the negative charge.  Pi bonds are reactive towards strong acids, but much less so in the case of weak acids, which are poor H⁺ donors.  

c.) Although the polyisoprene structure has pi bonds, they are not conjugated so there will be very little conduction along the carbon chain itself.  Also, bulk rubber has a disordered structure (I am guessing) so there is probably little or no pi stacking so there will be no conduction between polymer chains.  Hence, rubber is an electrical insulator.