Properties of Arenes
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Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to the pleasant smells given off by arenes, but now implies a particular type of delocalized bonding (see below). The arenes you are likely to encounter at this level are based on benzene rings. The simplest of these arenes is benzene itself, C6H6. The next simplest arene is methylbenzene (common name: toluene), which has one of the hydrogen atoms attached to the ring replaced by a methyl group - C6H5CH3.
The structure of Benzene
The structure of benzene is covered in detail on two pages in the organic bonding section of this site. It is important to understand the structure of benzene thoroughly to understand benzene and methylbenzene chemistry. Unless you have read these pages recently, you should spend some time on them now before you go any further on this page.
This diagram shows one of the molecular orbitals containing two of the delocalized electrons, which may be found anywhere within the two "doughnuts". The other molecular orbitals are almost never drawn.
- Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms, each with a hydrogen atom attached.
- The six carbon atoms form a perfectly regular hexagon. All of the carbon-carbon bonds have exactly the same lengths - somewhere between single and double bonds.
- There are delocalized electrons above and below the plane of the ring.
- The presence of the delocalized electrons makes benzene particularly stable.
- Benzene resists addition reactions because those reactions would involve breaking the delocalization and losing that stability.
- Benzene is represented by this symbol, where the circle represents the delocalized electrons, and each corner of the hexagon has a carbon atom with a hydrogen attached.
The structure of methylbenzene (toluene)
Methylbenzene has a methyl group attached to the benzene ring replacing one of the hydrogen atoms.
Attached groups are often drawn at the top of the ring, but you may occasionally find them drawn in other places with the ring rotated.
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).
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.
Benzene is resistant to addition reactions. Adding something new to the ring would require that some of the delocalized electrons form bonds with the substituent being added, resulting in a major loss of stability because the delocalization is broken. Instead, benzene primarily undergoes substitution reactions - replacing one or more of the hydrogen atoms with a new substituent, preserving the delocalized electrons as they were.
The reactivity of a compound like methylbenzene must be considered in two distinct parts:
For example, if you explore other pages in this section, you will find that alkyl groups attached to a benzene ring are oxidized by alkaline potassium manganate(VII) solution. This oxidation does not occur 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.
Jim Clark (Chemguide.co.uk)