25: Benzene and its Compounds

CHAPTER SUMMARY

Benzene & Compounds

• Benzene: C6H6

• Planar (2D): Each carbon atom in a trigonal planar (120 degrees)

• Hexagon with a circle in the middle (electron ring consisting of 6 delocalised electrons, meaning that they are free to move above and below, like in metallic bonding and in C=C)

• Each carbon has 3 sigma bonds

• Each carbon is sp2 hybridised

• Special case: delocalised electrons in benzene can move around, while those in C=C have restricted movement

• Evidence that Kekule’s structure is not accurate: bond lengths do not match C-C and C=C bond length, and it requires harsher conditions to undergo electrophilic addition

• Alkane + Br2 (free radical substitution)

• Alkene + Br2 (electrophilic addition)

• Carbonyl + HCN (nucleophilic addition)

• Arenes + Br2 (electrophilic substitution)

• ES (halogen, NO2): Catalyst is needed for electrophilic substitution as benzene is stable: FeBr3, FeCl3, AlCl3, AlBr3. 2e- used for bonding with Br+ and e- ring disrupted, returned through heterolytic fission of C-H bond.

• Electron donating: 2, 4, 6

• Electron withdrawing: 3, 5

• FRS: boil the arene and add Cl2 under UV

• FCR: Introduction of a side chain by electrophilic attack using a halogenated compound, under heat.

• Oxidation of SC: Refluxed with alkaline potassium manganate(VII), and then acidified with dilute sulfuric acid. Produces benzoic acid (only) and water and/or CO2.

• EA (Hydrogenation): Addition of H2 with Pt/Ni catalyst, under heat and high pressure, producing cyclohexane.

Brief Overview

• Formula: C6H5OH

• Melting point: 43°C, relatively high due to H bonding

• Large non polar benzene ring disrupts H bonding, reducing its solubility

• Weak acid

• Acidity: Phenol > Water > Ethanol

  • Phenol: Electron pair of O atom overlaps with delocalised pi bonding system, reducing the (-) charge density of the ion. O- is less negatively-charged, hence it is less attracted to H+. The equilibrium lies more to the right and there will be more H+ (more acidic).

  • Ethanol: Alkyl groups are electron-donating. Therefore, the O atom will be more negatively-charged, and have greater attraction with H+. Consequently, the equilibrium will lie to the left and there will be less dissociation. Less H+ results in lower acidity.

Phenol Reactions

• C6H5OH + NaOH → Sodium phenoxide + water

• C6H5OH + Na → Sodium phenoxide + hydrogen gas (vigorous reaction)

Preparation

1. NaNO2 + HCl → HNO2 + NaCl (below 10°C)

2. Phenylamine + HNO2 +HCl → Benzenediazonium chloride + water

3. Benzenediazonium chloride (acts as electrophile) is heated with H2O, forming phenol + HCl + N2

Diazotisation

• Benzenediazonium chloride (from previous reactions) + phenol → azo dye (orange) + HCl

• With the presence of NaOH

• An example of electrophilic substitution

Electrophilic Substitution of Phenols

• Bromination: Milder conditions, decolourises bromine water, forms 2,4,6-tribromophenol (white ppt) and HBr, no catalyst needed

• Nitration: rtp with dilute HNO3 forms 2 and 4-nitrophenol, and 2,4,6-trinitrophenol if conc. HNO3 is used

Naphthol Reactions

• Similar reaction with Na and NaOH to that of phenol

• 1-Naphthol + Br2 → Br attaches to C2 and C4 since C6 is connected with the next ring

• 2-Naphthol + Br2 → No substitution in C4 since it is cluttered up with the other ring, so it substitutes the carbon atom between the other ring and the OH group