Aromatic compounds and Benzene
Arenes or aromatic compounds contain a benzene ring
They are named in two ways:
Some are named as substitute benzene rings:
Some are named as compounds with a phenyl group (C6H5) attached:
Benzene has the formula C6H6. It has a cyclic structure as its six carbon atoms are joined together in a ring. Represented as the Kekule Model or the Delocalised Model.
Ring of C atoms with alternating single and double bonds between them, adapted later to say that the benzene mlecule was constantly flipping between two ismoers by switching the single and double bonds.
With this you would expect there to be always be three bonds that are single (length 147pm) and three double (135pm). But x-rays show they are all the same length (140 pm), so the model cannot be correct, but it still good for drawing.
Each C donates an e- from its p-orbital. Electrons combine to form a ring of delocalised electrons. All the bonds in the ring are the same, so the same length.
Electrons said to be delocalised because they don't belong to a specific carbon, represented as a circle.
Enthalpy change as evidence...
Cyclohexane has one double bond. When hydrogenated, enthalpy change is -120Kjmol-1.
If it has 3 double bonds (in Kekule), expected enthalpy would be -360 Kjmol-1
But the experimental enthalpy of the hydrogenation of benzene is -208 Kjmol-1, far less exothermic than expected.
Energy is put in to break bonds and released when bonds are made. So more energy must have been put in to break the bonds than would be need to break the bonds in the Kekule structure.
The difference indicated that benzene is more stable that the Kekule structure would be. This is thought to be due to the delocalised ring of electrons.
Arenes undergo Electrophilic Substitution reactions
Benzene ring is a region of high electron density, so it attracts electrophiles. As benzene is so stable, it undergoes electrophilic substitution reactions, which preserve the delocalised ring.
Used to add an acyl group (-C(=O)-R) to the benzene ring to allow side chains to be modified into useful products (as Benzene is so stable it is fairly unreactive).
Electrophile has to have a strong positive charge to be able to attack the stable benzene ring- most aren't polarised enough. Some can be made into stronger electrophiles using a catalyst called a halogen carrier.
Friedel- crafts uses an acyl chloride as a electrophile and a halogen carrier such as AlCl3.
How AlCl3 makes the acyl chloride electrophile stronger:
1. AlCl3 accepts a lone pair of electrons from the acyl chloride
2. Lone pair of electrons is pulled away. Polarisation in the acyl chloride increases and forms a carbocation.
3. Much stronger electrophile, strong enough charge to react with benzene:
How electrophile is substituted into benzene ring:
Step one: Electrons in benzene ring are attracted to the positively charged carbocation. Two e-s from benzene bond with the carbocation. This partially breaks delocalised ring and gives it a positive charge.
Step Two: Negatively charged AlCl4- ion is attracted to positively charged ring. One chloride ion breaks away and bonds with the hydrogen ion. This removes the hydrogen from the ring, forming HCl. Catalyst reforms.
Reactants need to be heated under reflux in a non-aqueous environment for the reaction to occur: