Aromatic Compounds & Benzene(finish from notes)

Question 1

Describe the structure of benzene

Answer 1

Benzene is a cyclic, planar molecule with the formula C6H6.

Each carbon is bonded to 2 other carbons and 1 hydrogen atom.

The final lone electron is in a p-orbital which sticks out above & below the planar ring.

Question 2

Describe the Kekule model of benzene

Answer 2

Kekule thought there were alternating single and double carbon bonds in benzene.

Question 3

Why are all C-C bond lengths the same in benzene?

Answer 3

Bond length of single bonds are longer than double bond.

But in benzene, all the C−C bonds are equal in length, and the length lies between the value for a single/double C bond.

This is due to the resonance effect in benzene.
The pi electron in benzene undergo delocalisation into the ring and make all the bond lengths identical.

Question 4

How is stability measured when comparing benzene to the theoretical cyclohexa-1,3,5-triene?

Answer 4

Stability is measured by comparing the enthalpy change of hydrogenation (adding hydrogen) in benzene and cyclohexene.

If we hydrogenate cyclohexene, it has an enthalpy change of -120 kJmol-1.
So cyclo-1,3,5-hexene (Kekule model of benzene) should have an enthalpy change of hydrogenation of -360kJmol-1.

However, benzene actually has an enthalpy change of hydrogenation of -208kJmol-1.
This lower value suggests benzene is more stable than theoretical cyclohexa-1,3,5-triene (due to its delocalised electron structure).

Question 5

What is an arene?

Answer 5

An arene is an aromatic compound. This means that they contain a benzene ring.

Question 6

What are the 2 ways arenes can be named?

Answer 6

Arenes can be named with :
- Benzene at the end e.g. bromobenzene
- Named as if phenyl (C6H5) is the functional group e.g. phenol (benzene ring with -OH group attached)

Question 7

How is bromine added across a double bond in alkenes?

Answer 7

• Alkenes undergo electrophilic addition.
• Adding bromine water to an alkene causes a colour change from brown -> colourless.
• Bromine is the electrophile and adds to the alkene forming a dibromoalkane (colourless).

Question 8

What type of reactions do arenes undergo when they react?

Answer 8

Electrophilic substitution

Question 9

Why don’t arenes undergo the same type of reactions as alkenes?

Answer 9

The electrons that would form double bonds delocalise into a ring, and this completely changes the type of reactions you can do with benzene.
Benzene is also more stable than alkenes, so they do not undergo electrophilic addition reactions (like the bromination of alkenes) as this would disrupt the stable ring of electrons.

Question 10

Why is benzene attractive to electrophiles?

Answer 10

Benzene has a high electron density due to its delocalised electron ring. This makes it very attractive to electrophiles.

Question 11

What happens during an electrophilic substitution reaction?

Answer 11

A hydrogen or functional group on the benzene ring is substituted for the electrophile.

Question 12

What are the 2 mechanisms you need to know for benzene?

Answer 12

• Friedel-Crafts Acylation
• Nitration reaction

Question 13

Why is benzene difficult to react?

Answer 13

It is so stable.

Question 14

What is Friedel-Crafts acylation, and why is it necessary?

Answer 14

Friedel-Crafts acylation is a reaction where an acyl group (RCO-) is added onto a benzene molecule.

After the acyl group is added, the benzene structure is weaker and it makes it easier to modify it further to make useful products e.g. pharmaceuticals.

Question 15

What must happen is order for an electrophile to be added to the benzene ring?

Answer 15

The electrophile must have a very strong positive charge.

Question 16

Why can’t acyl groups be added directly to benzene?

Answer 16

Acyl groups do not have a positive enough charge to add to the benzene ring. We can use a halogen carrier e.g. AlCl3 as a catalyst which will produce a much stronger electrophile with a stronger positive charge.