Benzene (Structure and Reactions)

What is benzenes molecular formula

C6H6

What did Kekule's model of benzene suggest?

3 alternating double bonds

What are the 3 reasons why Kekule's model was proved wrong

- Bond lengths were expected to be irregular however, were all the same length
-Enthalpy of hydrogenation was less exothermic than expected
- Did not readily undergo halogenation

Explain why bond lengths was an issue for Kekule?

C=C and C-C are expected to have different lengths therefore the structure should have been irregular but it wasn't

Explain why the enthalpy of hydrogenation of benzene was an issue for Kekule?

Experimental results suggested that the enthalpy for hydrogenation of a cycloalkene bond was -120 therefore, following this structure it should have been -360 but it was only -208

Explain why benzenes resistance to halogenation was an issue for Kekule

Unlike alkenes, benzene does not easily undergo electrophilic addition of Br2/Cl2, it requires a halogen carrier to undergo nucleophilic substitution

How does the delocalised structure of Benzene form

Due to Sp3 hybridisation, there is an overlap of p-orbitals each containing one electron. This creates pi bonds and the electrons are delocalised therefore, could be found above or below the ring

What does the delocalised structure mean for the electron density of Benzene compared to an alkene

Lower electron density

What shape molecule and bond angle is Benzene

Planar and 120 degrees

What is the most common type of reaction of Benzene

Electrophilic substitutions

What does nitration of benzene result in

Nitrobenzene

What are the reagents for Nitration of Benzene

HNO3 and H2SO4 as a catalyst

Give the equation for the formation of the electrophile in nitration of benzene

HNO3 +H2SO4 --> NO2+ + HSO4- + H2O

What does halogenation of Benzene result in

Halobenzene

What are the reagents for halogenation of Benzene

Br2/Cl2 and FeBr3/AlBr3/FeCl3/AlCl3 as a catalyst

Give the equations for the formation of the electrophiles in halogenation of Benzene (FeCl3/FeBr3)

FeBr3 + Br2 --> FeBr4- + Br+
FeCl3 + Cl2 --> FeCl4- + Cl+

What does friedel crafts alkylation result in

alkylbenzene

What are the reagents for friedel crafts alkylation

Chloroalkane and anhydrous AlCl3 as a catalyst and heat under reflux

What does friedel crafts acylation result in

phenyl ketone

What are the reagents of friedel crafts acylation

Acyl chloride and anhydrous AlCl3 as a catalyst and heat under reflux

What is a phenol

Benzene ring with an alcohol group directly attached

Why are phenols more reactive than benzene

Due to the lone pair of electrons on the oxygen atom being delocalised with the electrons within the benzene ring

What is the result of a reaction between a phenol and bromine

2,4,6 - tribromophenol

Why is the reaction with bromine easier with phenols than benzene

Due to the higher electron density which means Br2 is more polarised

What is the observation of the reaction of bromine and phenol

a white solid

What are some uses of phenols

Antiseptics and disinfectants

What is the result of a reaction between phenol and nitric acid

2 - nitrophenol or 4-nitrophenol

How does the reaction of nitric acid and phenol differ to the nitration of benzene

Does not require H2SO4 as a catalyst

What are the two types of groups which could have an effect on position of substitution

Electron withdrawing or donating

Where will electron donating groups force further substitution

position 2 and 4

What are examples of electron donating groups

-OH or -NH2

Where will electron withdrawing groups force further substitution

position 3

What are examples of electron withdrawing groups

-NO2