Module 6.1.1- Aromatic Compounds

• What is benzene

  simplest aromatic hydrocarbon. contain one or more rings with pi electrons delocalised all the way around them.
• What was Kekule's model of benzene?

  A six-membered ring with alternating double and single bonds

  
• How is the lack of reactivity of benzene used to disprove Kekule's model

  - benzene doesn't undergo electrophilic addition reactions- benzene doesn't decolourise BROMINE under normal conditions
• Name 2 other types of evidence to disprove Kekule's model

  - the carbon to carbon bonds are all the SAME length. (if the structure contained double bonds, the length of the carbon double bond would be shorter.)- Hydrogenation enthalpies(if benzene had Kekule's structure, it would have 3 times the enthalpy change of hydrogenation of cyclohexane but it doesn't and is lower than expected therefore it is more stable)
• Describe the delocalised model of benzene


  -benzene is a planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms & 6 hydrogen atoms.-each carbon atom has 1 electron in a p

  -orbital at right angles to the plane of the bonded C & H atoms.

  -adjacent p-orbital electrons overlap sideways, in both directions, above & below the plane of the carbon atoms to form a ring of electron density.

  -this overlapping of the p-orbitals creates a system of π bonds which spread over all the 6 C atoms in the ring structure.

  -the 6 electrons occupying this system of π bonds are delocalised.

  
• Name 3 common benzene compounds

  - benzoic acid- phenylamine- benzaldehyde
• Name this compound

  2-bromomethylbenzene

  Bromine is placed at the start since it is in alphabetical order

  
• The 3 steps in electrophilic substitution mechanism

  1) Generate the electrophile with catalyst

  2) Substitution

  3) Regenerate the catalyst
• What are the conditions/reactions for the Nitration of Benzene

  50 degrees Celsius (maintained by a water bath) with H2SO4 (conc) catalyst HNO3 (conc)
• If the temperature is greater than 50 degrees Celsius, what could happen?


  Lead to further substitution
• Mechanism for nitration of benzene

  Step 1) Generate electrophile

  HNO3 (conc) + H2SO4 (conc) -> NO2+ + HSO4- + H2O

  Step 2) See picture. NO2+ accepts a pair of electrons from benzene ring to form dative covalent bond.

  Step 3) H+ from step 2 produced is used and regenerates the catalyst

  H+ + HSO4- -> H2SO4

  
• Name common halogen carriers

  AlBr3, FeBr3

  AlCl3, FeCl3
• Bromination of Benzene


  Br2(l) + FeBr3/AlBr3 , both at room temperature

  Step 1) Generate the electrophile

  Br2 + AlBr3 -> Br+ + AlBr4-

  Step 2) See image. Br+ accepts a pair of electrons from the benzene ring to form a dative covalent bond

  Step 3) Regenerate the catalyst

  H+ + AlBr4- -> AlBr3 + HBr

  
• Balanced equation of the bromination of benzene

  
• Chlorination of Benzene


  Cl2(g) + AlCl3, room temperature(Mechanism similar to bromination of benzene)

  
• Alkylation of Benzene (Friedel Crafts reactions)

  Step 1) Generate the electrophile

  CH3Cl + AlCl3 -> CH3+ + AlCl4-

  Step 2) See picture

  Step 3) Regenerate the catalyst

  H+ + AlCl4- -> HCl + AlCl3

  
• Acylation of Benzene (Friedel Crafts reactions)

  Step 1) Generate the electrophile

  CH3COCl + AlCl3 -> CH3CO+ + AlCl4-

  Step 2) Similar mechanism

  Step 3) H+ + AlCl4- -> AlCl3 + HCl

  Balanced equation shown on picture

  
• Why does bromine react more readily with cyclohexene than with benzene

  - In benzene the pi electrons are delocalised

  - In cyclohexene, the pi bonds are localised so it has a higher electron density

  - benzene has a lower electron density and so benzene polarises bromine less
• Electrophilic addition (revision)

  Cyclohexene, as an example, induces dipole in non polar bromine molecule making bromine act like an electrophile

  
• Phenol (C6H5OH)


  OH group is bonded to the aromatic ring. (If its not then it is an alcohol)Weak Acid
• Phenol dissolved in water and acidity of compounds

  Acidity:

  1) Carboxylic Acid (Most)

  2) Phenol

  3) Alcohols (Least)

  
• Reactions of alcohols, phenols and carboxylic acids with a strong base (NaOH) and a weak base (Na2CO3)


  Ethanol + NaOH -> X

  Ethanol + Na2CO2 -> X

  Phenol + NaOH -> REACTS

  Phenol + Na2CO3 -> X

  Carboxylic Acid + NaOH -> REACTS

  Carboxylic Acid + Na2CO3 -> REACTS
• Reaction of phenol with sodium hydroxide


  Forms a salt (sodium phenoxide) and water in a neutralisation reaction

  
• Phenol + 3Br2State what you would observe

  2,4,6-tribromophenol + 3HBr2,4,6-tribromophenol

  - white precipitate

  bromine water decolourised

  
• Phenol + dilute HNO3


  2-nitrophenol and/or 4-nitrophenol + H2O
• Why does Phenol react more readily than benzene

  -The lone pair of electrons from the OH group (in oxygen p-orbital) in phenol is delocalised into the pi system of the benzene ring

  -So the electron density increases and the electrophile is more strongly attracted.

  -Therefore it undergoes electrophilic substitution more readily
• For some reactions, why are halogen carriers (AlCl3 etc.) not required?

  The electron density of the compound e.g. phenols is sufficient enough to polarise the non-polar molecules.
• Phenylamine + Br2 (Activation or Deactivation?)

  2,4,6-tribromophenylamine Reacts rapidly with phenyl amine with no halogen carrier. Amine group activates the ring as it makes the aromatic ring react more readily with electrophiles.

  
• Nitrobenzene + Br2 (Activation or Deactivation?)

  3-bromonitrobenzeneRequires halogen carrier and high temp, slow

  Nitroxide group deactivates the ring as it makes the aromatic ring react less readily with electrophiles
• Directing group of NH2


  Positions 2,4 (and/or 6)

  
• Directing group of NO2


  Positions 3 (and/or 5)

  
• Using directing effects in organic synthesis


  Consider the order in which reactions are carried out to ensure correct substitution pattern and required product is prepared.