6.1.1. Benzenes

Question 1

What does aromatic mean?

Answer 1

contains a benzene ring

Question 2

Evidence against kekule’s structure of benzene

Answer 2

• benzene doesn’t react or discolour orange bromine solution at room temperature, therefore doesn’t have c=c bond
• x ray crystallography show all carbon carbon bonds are the same length
• the enthalpy of hydrogenation of benzene is lower than expected

Question 3

Shape and bonding within a benzene molecule

Answer 3

• 120, trigonal planar
• covalent sigma bonds - head on overlap of orbitals between 2 c atoms
• pi bond - p orbitals overlap sideways, form a delocalised pi bond which exists above and below the carbon ring

Question 4

Why is benzene reactive?

Answer 4

1. There is a relatively high e- density above and below due to the 6 delocalised pi electrons in the delocalised pi bond
2. Benzene will attracts positive electrophiles
3. Electrophiles are e- pair acceptors
4. Benzene will undergo electrophilic substitution, but not electrophilic addition because benzene is less reactive than benzenes

Question 5

Why is benzene less reactive with electrophiles than cyclohexane?

Answer 5

• In benzene the pi bond is delocalised and has a lower electron density than the localised pi bond in cyclohexane
• benzene induces weaker dipoles in the electrophile, and the electrophile is attracted to benzene less strongly
  evidence: alkenes will react with dilute Br2 solution but benzene will only react with PURE Br2 in the presence of a halogen carrier AlBr3 catalyst

Question 6

Chlorination of benzene

• type of reaction
• reagents
• conditions
• electrophile

Answer 6

• electrophilic substitution
• C6H6(l), Cl2(g)
• AlCl3 catalyst , room temperature
• Cl+
• C6H6 + Cl2 -> C6HClr + HCl

Question 7

Bromination of benzene

• type of reaction
• reagents
• conditions
• electrophile

Answer 7

• electrophilic substitution
• C6H6(l), PURE Br2
• AlBr3 catalyst , room temperature
• Br+
• C6H6 + Br2 -> C6H5Br + HBr

Question 8

Nitration of benzene

• type of reaction
• reagents
• conditions
• electrophile
• overall equation

Answer 8

• electrophilic substitution
• C6H6(l), conc HNO3
• conc H2SO4 catalyst , 55 c
• NO2+
• C6H6 + HNO3 -> C6H5NO2 + H2O

Question 9

Alkylation of benzene (aka Friedel-Crafts)

• type of reaction
• reagents
• conditions
• electrophile
• overall equation

Answer 9

• Any haloalkane, AlCl3 catalyst (removes the halogen atom)
• CH3+ or CH3CH2CH2CHBr+ (+ on C)
• C6H6 + CH3Cl -> C6H5CH3 + HCl

Question 10

Acylation of benzene

• type of reaction
• reagents
• conditions
• electrophile
• overall equation

Answer 10

• Any acyl chloride, AlCl3 chloride

- CH3C+O