6A Aromatic Compounds

Name 3 pieces of evidence which disprove Kekulé’s structure of benzene

• The standard enthalpy of hydrogenation (adding hydrogen) of benzene is more exothermic than it ‘should’ be

• Benzene doesn’t react with bromine water, unlike other alkenes, this must be due to benzene not containing an electron dense enough area to polarise bromine molecules

• Studies using scanning electron microscopy revealed benzene has a regular hexagonal shape, but if it contain 3 double bonds, they should be shorter in size, causing an irregular shape

Draw Kekulé’s model and the delocalised style of benzene

Describe the actual structure of Benzene

• Cyclic hydrocarbon with 6 Cs and 6 Hs

• Carbons are arranged in a planar hexagonal ring with 120 degree bond angles

• Each C has 4 bonding electrons, the 4th electron from each C from the 2p orbital is extended above and below the plane of the molecule

• The electrons in 2p orbitals form a delocalised ring of electron density above and below the plane of the molecule

How do you name aromatic compounds

• when the benzene ring is the main molecule its the root - __benzene__

• If benzene is a substituent of a larger molecule then you use the prefix phenyl-

• Benzene rings with 1 OH group attached are called phenols

State the reagents and conditions required to carry out these reactions of benzene:

1. Nitration

2. Halogenation (Br or Cl)

3. Alkylation

4. Acylation

1. Reagents: conc nitric acid Catalysts: low temp, conc sulphuric acid

2. Reagents: Halogen gas Catalysts: AlCl3 or FeBr3 (Halogen carrier catalyst)

3. Reagents: RCl Catalysts: AlCl3

4. Reagents: RCOCl Catalysts: AlCl3

Draw the mechanism of Electrophilic substitution using benzene and chlorine

Write out the stages for formation of the electrophile and catalyst regeneration using Br2 and FeBr3

Formation of the electrophile: FeBr3 + Br2 → FeBr4- + Br+

Regeneration of the catalyst: FeBr4-+ H+ → FeBr3 + HBr

Whats the standard state / look of phenol?

Pink solid

What observations can be made when phenol reacts with water or sodium hydroxide

• Water: Phenol will slightly dissolve, more so in warm water

• Sodium Hydroxide: Phenol dissolves

What observations occur when phenol is added to bromine water

Bromine water decolourises and a white precipitate is formed

What type of acid is phenol

A weak acid, so it only partially dissociates in aqueous solution

Why are phenols acidic, despite other alcohols having almost no acidic properties

• in phenol the electrons in the OH bond are drawn to the aromatic ring

• This weakens the bond between O and H, making it easier for H+ to be released

• Once the phenoxide ion is formed it’s more stable as the electrons are dispersed across the pi cloud

Why does phenol undergo multiple substitutions/not require a catalyst unlike benzene when reacting with bromine

• In phenol one of the lone pairs of electrons on the oxygen of the OH group is in a p-orbital which can overlap with the ring of delocalised electrons

• This lone pair then delocalises into the ring

• More electrons being in the benzene ring increases its electron density

• Making it better able at polarising bromine molecules than normal benzene

What are activating, deactivating, and neutral groups

• Activating groups

  • increase the electron density of the benzene ring, making it more susceptible to Electrophilic substitution

  • Direct multiple substitutions to the ortho and para positions

  • E.g. OH,NH2

• Deactivating groups

  • decrease the electron density of the benzene ring, making it less susceptible to Electrophilic substitution

  • direct single substitutions to the meta positions

  • E.g. NO2, COOH

• Neutral groups

  • Don’t have a significant effect on electron density

  • Direct single substitutions to ortho or para positions

  • E.g. halogens

Compare the reaction with bromine for cyclohexene and benzene

• Type of attacking species - electrophile

• Type of reaction / mechanism undergone - Electrophilic addition v Electrophilic substitution

• Conditions: RTP v halogen carrier catalyst

• Reactivity: cyclohexene is more reactive because it has a higher electron density around the pi bond, meaning its strong enough to polarise promine

• Organic product - (picture)

Compare the reaction with bromine for benzene and phenol

• Type of attacking species: electrophile

• Conditions: halogen carrier catalyst v RTP

• Type of reaction / mechanism: Electrophilic substitution

• Reactivity: Phenol is more reactive because the OH group donates a pair of pi electrons to the benzene ring, increasing its electron density