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QUESTION

# Why are aromatic rings hydrophobic?

An aromatic ring is composed of relatively non-polar C-C and C-H bonds, which are not effectively solvated by water.

The electronegativity difference between C and H is rather small, and that between C and C is zero. There is thus no charge separation on the benzene ring, no polarity, which would lend itself to solvation by the polar solvent water. An interesting exercise might be to compare the solubility of phenol, C_6H_5OH, with that of ethanol and methanol, which are both infinitely miscible with water.

This site tells me that 8*g of phenol will dissolve in 100*mL of water. For another comparison we could consider the solubility of the carbocyclic alcohol, cyclohexanol, C_6H_11OH. This site reports a solubility of 3.6*g per 100*mL. How to account for this? Well, in cyclohexanol there are more C-H bonds than in C_6H_6.