Physical properties of Methylbenzene
In benzene, the only attractions between neighbouring molecules are van der Waals dispersion forces. There is no permanent dipole on the molecule.
Benzene boils at 80°C – rather higher than other hydrocarbons of similar molecular size (pentane and hexane, for example). This is presumably due to the ease with which temporary dipoles can be set up involving the delocalised electrons.
Methylbenzene boils at 111°C. It is a bigger molecule and so the van der Waals dispersion forces will be bigger.
Methylbenzene also has a small permanent dipole, so 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).
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 make best use of their intermolecular forces. Benzene is a tidy, symmetrical molecule and packs very efficiently. The methyl group sticking out in methylbenzene tends to disrupt the closeness of the packing. If the molecules aren’t as closely packed, the intermolecular forces don’t work as well and so the melting point falls.
Solubility in water
The arenes are insoluble in water.
Benzene is quite large compared with a water molecule. In order for benzene to dissolve it would have to break lots of existing hydrogen bonds between water molecules. You also have to break the quite strong van der Waals dispersion forces between benzene molecules. Both of these cost energy.
The only new forces between the benzene and the water would be van der Waals dispersion forces. These aren’t as strong as hydrogen bonds (or the original dispersion forces in the benzene), and so you wouldn’t get much energy 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.
You have to consider the reactivity of something like methylbenzene in two distinct bits:
For example, if you explore other pages in this section, you will find that alkyl groups attached to a benzene ring are oxidised by alkaline potassium manganate(VII) solution. This doesn’t happen in the absence of the benzene ring.
The tendency of the CH3 group to “push” electrons away from itself also has an effect on the ring, making methylbenzene react more quickly than benzene itself. You will find this explored in other pages in this section as well.