chapter 25

• What is benzene?


  A colourless, sweet smelling, highly flammable liquid. It is found naturally in crude oil, is a component of petrol, and is also found in cigarette smoke.

  It is classified as a carcinogen (it can cause cancer).
• What does a benzene molecule consist of?

  A hexagonal ring of six carbon atoms. It is classed as an aromatic hydrocarbon orarene.

  
• What does aromatic mean?

  Contains a benzene ring
• What is the Kekulé structure?

  The structure of benzene suggested by German chemist Friedrich August Kekulé.

  
• What three pieces of evidence appear to disprove Kekulé's model?

  1- Benzene is unreactive- it doesn't undergo electrophilic addition reactions, nor does it decolourise bromine under normal conditions. This has led scientists to suggest it cannot have any C=C bonds.

  2- The lengths of the carbon-carbon bonds in benzene are 0.139nm in length, which is between the length of a single bond and a double bond.

  3- The Kekulé structure is expected to have an enthalpy change of hydrogenation 3x that of cyclohexene(-120kJmol⁻¹), but the actual enthalpy change is only -208kJmol⁻¹, meaning the actual structure of benzene is more stable than the Kekulé model.
• What is the delocalised model of benzene?

  -Benzene is a planar, cyclic, hexagonal hydrocarbon containing six C atoms and six H atoms.

  -Each C atom uses 3 out of 4 electrons in bonding to two C atoms and one H atom.

  -The other electron is in a p-orbital, with adjacent electrons overlapping sideways above and below the plane of C atoms.

  -The six electrons occupying this system of π-bonds are said to be delocalised.
• [diagram-delocalised model of benzene]

  
• What is a monosubstituted compound?

  Aromatic compounds with one substituent group.

  In aromatic compounds, the benzene ring is often considered to be the parent chain, so the side groups are considered prefixes to benzene (e.g. ethylbenzene, chlorobenzene)
• [diagram- naming monosubstituted aromatic compounds]

  
• In what cases is the benzene ring a substituent group instead of the parent chain?

  
• What are some common aromatic compounds you need to remember?

  
• Benzene undergoes electrophilic substition reactions. What is electrophilic substitution?

  Where an H on benzene is replaced by another atom/group of atoms.

  
• How does benzene react with nitric acid?

  It reacts slowly with nitric acid to form nitrobenzene in an electrophilic substitution, with a catalyst of sulfuric acid and heated to 50°C using a water bath.

  
• What happens when benzene is nitrated above 50°C?

  Further substitution reactions may occur, leading to the production of dinitrobenzene.

  
• What can nitrobenzene be used for?

  It is an important starting material in the preparation of dyes, pharmaceuticals, and pesticides. It can be used as a starting material in the preparation of paracetamol.

  
• What is the mechanism for the nitration of benzene?

  1- The electrophile, NO₂⁺, is produced by the reaction of the conc. nitric acid with conc. sulfuric acid

  2- The electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate is unstable and breaks down to form nitrobenzene (with a stable benzene ring) and an H⁺ ion.

  3- The H⁺ ion reacts with the HSO₄⁻ ion from step 1 to regenerate the catalyst.
• [diagram- nitration of benzene mechanism]

  
• How does benzene react with halogens?

  Halogens don't react with benzene unless a catalyst called a halogen carrier is present. These can be generatedin situfrom the metal and the halogen.
• What are common halogen carriers?

  AlCl₃, FeCl₃, AlBr₃, and FeBr₃
• How does benzene react with bromine in the presence of a halogen carrier?

  It reacts with bromine to form bromobenzene in an electrophilic substitution reaction. This happens at room temperature and pressure, and in the presence of a halogen carrier.Other halogens react with benzene in the same way.

  
• What is the mechanism for the bromination of benzene?

  1- The electrophile, the bromonium ion Br⁺, is generated when the halogen carrier catalyst reacts with bromine.

  2- The Br⁺ ion accepts a pair of electrons from the benzene ring to form a dative covalent bond. The organic intermediate breaks down to form bromobenzene and an H⁺ ion.

  3- The H⁺ ion reacts with the negative halogen carrier ion from step 1 to regenerate the halogen carrier.
• [diagram-mechanism for bromination of benzene]

  
• What is the alkylation of benzene?

  The substitution of an H atom in the benzene ring by an alkyl group. It increases the number of carbon atoms in a compound by forming carbon-carbon bonds.

  It is sometimes called a Friedel-Crafts alkylation after the two chemists who first carried out the reaction.
• How is the alkylation of benzene carried out?

  Benzene is reacted with a haloalkane in the presence of AlCl₃, which acts as a halogen carrier catalyst, generating the electrophile in the electrophilic substitution reaction.

  
• What is the acylation of benzene?

  When benzene reacts with an acyl chloride in the presence of an AlCl₃ catalyst to produce an aromatic ketone. The reaction forms C-C bonds and is useful in organic synthesis.
• How does benzene react with ethanoyl chloride?

  It reacts with a catalyst of AlCl₃ to produce phenylethanone and hydrochloric acid.

  
• What is phenylethanone used for?


  It is used in the perfume industry as an ingredient in fragrances.
• How does bromine add across the double bond in cyclohexene?

  1- The π bond in the alkene represents an area of high electron density.

  2- The localised electrons in the π-bond induce a dipole in the non-polar bromine molecule

  3- The slightly positive bromine atom enables the bromine molecule to act like an electrophile.

  4- Electrophilic addition (see diagram)

  
• Why doesn't benzene react with bromine without a halogen carrier?

  Benzene has delocalised π electrons spread above and below the planar ring. This means the electron density in benzene is less than in a C=C bond.

  The bromine is therefore not polarised.
• What are phenols?


  A type of organic chemical containing a hydroxyl group directly bonded to an aromatic ring. The simplest member of the group is C₆H₅OH.

  If the -OH group were bonded to a carbon side chain rather than the aromatic ring, the compound would be classified as an alcohol rather than a phenol.

  
• [diagram- phenols v alcohol]

  
• Why are phenols less soluble than alcohols?

  The presence of the non-polar benzene ring.
• What happens when phenol dissolves in water?

  It partially dissociates, forming the phenoxide ion and a proton.

  This means phenol is classified as a weak acid.

  
• Compared to alcohols and carboxylic acids, how acidic is phenol?

  It is more acidic than alcohols but less acidic than carboxylic acids.

  -Ethanol doesn't react with NaOH (a strong base) or NaCO₃ (a weak base)

  -Phenols react with NaOH but not NaCO₃

  -Carboxylic acids react with NaOH and are strong enough to react with NaCO₃
• How can you distinguish between a phenol and a carboxylic acid?

  React it with sodium carbonate. A carboxylic acid will react to produce carbon dioxide, which is evolved as a gas.
• How does phenol react with sodium hydroxide?

  Phenol reacts with NaOH to form a salt, sodium phenoxide, and water, in a neutralisation reaction.

  
• How do phenol's reactions differ from those of benzene?

  They take place under milder conditions and more readily than benzene's reactions.
• How does phenol react with bromine?

  It reacts with an aqueous solution of bromine at room temperature to form a white precipitate of 2,4,6-tribromophenol.

  The reaction decolourises the bromine water.

  
• How is phenol nitrated?

  It reacts readily with dilute nitric acid at room temperature to form a mixture of 2-nitrophenol and 4-nitrophenol.

  
• Why is phenol more reactive than benzene?

  A lone pair of electrons from the oxygen p-orbital of the -OH group is donated into the π-system of phenol. This increases the electron density of the benzene ring in phenol, which attracts electrophiles more strongly than with benzene.

  This means the aromatic ring is more susceptible to attack from electrophiles-e.g. with bromine, the electron density is sufficient to polarise bromine molecules and no halogen carrier catalyst is required.