[kissoftalons]

Hi, I found a site online that said most hydrocarbons were non-polar. Is there a set of rules to determine the polarity of a hydrocarbon? Are they the same ones for determining polarity of inorganic molecules? Sorry i haven't "attempted" the question, it was a question in a lab (to find the polarity of cyclohexane, cyclohexene and toluene  - and i searched online and all three are non-polar). Just wondering for the future if I had any way of determining this by myself. Thanks!

[lemonoman]

A general rule is that 'all hydrocarbons are non-polar' - this is because, by definition, hydrocarbons are only made of hydrogren and carbon...and the electronegativity difference between the two isn't very much.  So, carbon and hydrogen do a lot more sharing of electrons than a polar oxygen-hydrogen pair would.

This is a GENERAL rule though, and there are a few exceptions.  Alkenes and alkynes have double and triple bonds...and so there's a bit more electron density there than is found in most hydrocarbons.  And the same is true (maybe even moreso) for aromatic systems (like benzene, naphthalene, and you mentioned toluene)...because they have a ring of electron density on both sides of the molecule...[and by the way, that's not to say that alkenes, alkynes and aromatics ARE polar...but they have a tiny bit more separation in electron density than saturated hydrocrbons do]

Anyways the point is that polarity is caused by significant shifts in electron density.  hydrocarbons rarely have that shift.  Period.