Boiling points

In benzene, the only attractions between the neighbouing molecules are the van der Waals dispersion forces. There is no permanent dipole on the molecule. Benzene boils at 80°C, which is higher than other hydrocarbons of similar molecular size (pentane and hexane, for example). The higher boiling point is presumably due to the ease with which temporary dipoles can be set up involving the delocalized electrons.

Methylbenzene boils at 111°C. Methylbenzene is a larger molecule, thus, the van der Waals dispersion forces will be increased. Methylbenzene also has a small permanent dipole; thus, there will be dipole-dipole attractions as well as dispersion forces. The dipole is due to the CH₃ group's tendency to "push" electrons away from itself. This also affects the reactivity of methylbenzene (see below).

Melting points

You might have expected that methylbenzene's melting point would be higher than benzene's as well, but it isn't - it is much lower! Benzene melts at 5.5°C; methylbenzene at -95°C.

Molecules must pack efficiently in the solid if they are to optimize their intermolecular forces. Benzene is a tidy, symmetrical molecule and packs very efficiently. The methyl group that protrudes from the methylbenzene structure tends to disrupt the closeness of the packing. If the molecules are not as closely packed, the intermolecular forces don't work as well, causing the melting point to decrease.

Solubility in water

The arenes are insoluble in water. Benzene is quite large compared with a water molecule. For benzene to dissolve, it would have to break a significant number of the existing hydrogen bonds between the water molecules. In addition, the quite strong van der Waals dispersion forces between the benzene molecules would require breaking; both of these processes require energy. The only new forces between the benzene and the water would be van der Waals dispersion forces. These forces are not as strong as hydrogen bonds (or the original dispersion forces in the benzene), therefore, only a limited amount of energy is released when they form. It simply isn't energetically profitable for benzene to dissolve in water. It would, of course, be even worse for larger arene molecules.