A - Aromatic Chemistry Flashcards
What formula does benzene have?
C6H6.
Describe the structure and bonding of benzene.
It has a planar cyclic structure (six carbon atoms joined together in a flat ring).
Each carbon atom forms single covalent bonds to the carbons either side of it, and to one hydrogen atom, which stick out on the flat plane.
The final unpaired electron on each carbon atom is located in a p-orbital that sticks out above and below the plane of the ring. The p-orbitals on each carbon atom combine to form a ring of delocalised electrons.
What does delocalised mean?
That electrons don’t belong to a specific atom or ion, they are shared between all of them.
How is benzene drawn?
A circle inside of a hexagon. The circle represents the ring of delocalised electrons.
Describe the bonding length in benzene.
All the carbon-carbon bonds in the ring are the same, so they are the same length - 140pm. This is an intermediate length between that of a single C-C bond (154pm) and of a C=C (135pm).
What bonds would the theoretical compound, cyclohexane-1,3,5-triene contain?
The ring would be made up of alternating single and double bonds.
Why is benzene more stable than the theoretical compound, cyclohexane-1,3,5-tríeme?
Because of the ring of delocalised electrons.
Using thermochemical evidence from enthalpies of hydrogenation, explain why benzene is more stable than the theoretical compound, cyclohexane-1,3,5-triene.
Cyclohexene has one double bond. When it’s hydrogenated, the enthalpy change is -120 KJ mol-1. If benzene had three double bonds, you’d expect it to have an enthalpy of hydrogenation of -360 KJ mol-1.
However, the experimental enthalpy of hydrogenation of benzene is -208 KJ mol-1. So it is far less exothermic than expected. Therefore, more energy is required to break bonds in benzene compared to that of the theoretical compound, indicating benzene is more stable.
What are aromatic compounds? What’s another name for these compounds?
Compounds containing a benzene ring.
Arenes.
How are arenes named?
There’s two ways:
- Some as substituted benzene rings (e.g - chlorobenzene, nitrobenzene)
- Some as compounds with a phenol group (C6H5) attached (e.g - phenol, phenylamine)
What type of reaction do arenes undergo?
Electrophilic substitution reactions.
Why do arenes undergo substitution reactions in preference to addition reactions?
The benzene ring is a region of high electron density, so it attracts electrophiles.
As the benzene ring is so stable, it doesn’t undergo electrophilic addition reactions as these would destroy the delocalised ring of electrons.
Instead, they undergo electrophilic substitution reactions where one of the hydrogen atoms (or another functional group on the ring) is substituted for the electrophile.
Name some useful chemicals which contain benzene rings.
Dyes, pharmaceuticals etc
Why is it difficult to make chemicals that contain benzene?
Because benzene is very stable which means it’s fairly unreactive.
What are Friedel-Crafts acylation reactions used for?
Used to add an acyl group (RCO-) to the benzene ring. Once an acyl group has been added, the side chains can be modified using further reactions to make useful products.
