6.1.1 Aromatic compounds

What is benzene?

Aromatic compound with a very stable planar ring structure with delocalised electrons

Name the different formulas of benzene

Displayed formula, kekule model, delocalised electron model

What 3 pieces of evidence proved the kekule model wrong?

1. Benzene favours substitution reactions (when functional group replaces without breaking double bond) when the kekule model suggests it would break bond for addition reactions

2. All C-C bonds in benzene are the same length but if kekule was correct the C=C bond would be shorter

3. Enthalpy of hydrogenation of benzene is lower than the predicted kekule as more stable so proves there isn’t x3 C=C bonds but rather a more stable delocalised electron ring

What is the bond length of C-C and C=C according to the kekule model?

C-C is 154pm

C=C is 134 pm

What is the actual length of all bonds in benzene?

140pm

What was the expected enthalpy of hydrogenation from the kekule model?

3 × 120 = -360 as there are 3 C=C bonds in kekule

What is the actual enthalpy of hydrogenation of benzene?

-208 Kj mol^-1

Describe the actual structure of benzene:

p-orbitals of all 6 carbon atoms overlap and donate 1 electron each to form a delocalised ring of electrons ABOVE and BELOW the plane of carbon atoms

What is this called?

Chlorobenzene

What is this called?

Nitrobenzene

What is this?

Methylbenzene

What is this?

Chloromethylbenzene

What is this called?

1-Bromo 2-methyl benzene

What is this called?

Phenyl amine

What is this called?

Phenyl ethanone

What is this called?

Phenyl ethene

What is this?

Phenol

What is this?

2,4 dichlorophenol

What is the formula for a phenyl group?

C6H5

Why are electrophiles attracted to benzene / phenol?

hgih density electron ring which attracts electrophiles

Describe the steps of electrophilic substitution to a benzene ring?

1. Electrophile attracted

2. Electrons transfer to electrophile to form bond

3. Ring broken

4. Electrons in C-H bond is donated to ring

   1. Ring reform and H+ is released

What does adding a halogen require?

Requires a halogen carrier to make an electrophile as it’s not polar therefore an electrophile

Give examples of halogen carriers:

AlX3 and FeX3

Give the equation when Cl2 (halogen) is added to AlCl3 (halogen carrier):

Cl2 + AlCl3 → AlCl4- + Cl+

What will happen to the H+ that was released from the benzene?

React with AlX4 or FeX4 to produce the halogen carrier again as a catalyst and HX

What is nitration?

Adding NO2

What are the specific conditions for nitration?

warm benzene with HNO3 and concentrated H2SO4 catalyst at 55 degrees if one substitution

What is alkylation?

Adding an alkyl chain

What first has to be added before the alkyl or acyl can join to the benzene?

Halogen carrier has to be added as alkyl / acyl is not an electrophile to make the alkyl / acylelectrophile

What is acylation?

Adding an acylchloride to add an acyl group

What is this and why?

2-ethyl phenol as the OH has more priority than benzene