Aromatics

Question 1

Reasons Kekule’s structure was disproved

Answer 1

1. All six C-C bonds were the same length - C-C>C=C
2. The ring is thermodynamically more stable than expected
3. Doesn’t undergo electrophilic addition (so no C=C bond)

Question 2

Why is benzene more stable than expected?

Answer 2

• the extra p electron on each C is in parallel orbitals at right angles to the plane of the ring
• the 6 p electrons are delocalised over the ring

Question 3

Why can benzene rings not undergo addition reactions?

Answer 3

The 6p electrons are delocalised in the pi bonded ring, making the ring less electron dense

Question 4

Naming aromatic compounds - when to use benzene/phenol

Answer 4

When benzene is the main functional group, use the suffix ‘benzene’
* alkyl
* halogens
* nitro

when benzene ring is a branch, use the suffix ‘phenol’
* amine (NH2)
* carbonyls (ketones/aldehydes)
* hydroxy (OH)

Question 5

Reactions of benzene - electrophilic sub: nitration (reagents&cond + formation of electrophile)

~~MECHANISM~~

Answer 5

Nitration:
reagents: conc nitric acid (HNO3) & conc sulfuric acid (H2SO4) (together forming ‘fuming’ nitric acid)

cond: heat under reflux <55deg

formation of electrophile:
HNO3 + H2SO4 –> HSO4- + H2NO3+

~~MECHANISM~~

Question 6

Reactions of benzene - electrophilic sub: bromination (reagents&cond + formation of electrophile)

~~MECHANISM~~

Answer 6

Bromination:
reagents: FeBr3 & Br2
cond: Heat under reflux

formation of electrophile:
FeBr3 + Br2 —> FeBr4- + Br+

~~MECHANISM~~

Question 7

Reactions of benzene - Friedel-Crafts: Alkylation (reagents&cond + formation of electrophile)

~~MECHANISM~~

Answer 7

Alkylation:
Adding an R group/ alkyl group

Reagents: AlCl3 + R-Cl (haloalkane)
cond: heat under reflux

formation of electrophile:
AlCl3 + R-Cl –(dry cond)–> AlCl4- + R+

~~MECHANISM~~

Question 8

Reactions of benzene - Friedel-Crafts: Acylation (reagents&cond + formation of electrophile)

~~MECHANISM~~

Answer 8

Acylation:
Adding C(=O)R groups (acyl groups)

Reagents: AlCl3 + RC(=O)Cl
cond: heat under reflux

formation of electrophile:
AlCl3 + RC(=O)Cl –(dry cond)–> AlCl4- + RC+=O

~~MECHANISM~~

Question 9

In Electrophilic substitution: nitration of benzene, why do we heat under reflux to <55deg?

Answer 9

To prevent further substitutions of NO2 groups

Question 10

Phenol reactions - Phenol and water

Answer 10

BOH + H2O —> BO- + H3O+

Question 11

Why can phenol act as an acid but cyclohexanol is a neutral alcohol?

Answer 11

• Phenol can donate a proton to form the phenoxide ion
• the phenoxide ion is stable as the negative charge can be delocalised into the ring

Question 12

Phenol reactions - Phenol and sodium hydroxide (NaOH)

Answer 12

BOH + NaOH —> BO-Na+ + H2O
phenol dissolves in NaOH

Question 13

Why is water sparingly soluble in water but soluble in sodium hydroxide (NaOH)?

Answer 13

• phenol has a lot of electrons, therefore strong london forces, and doesn’t dissolve well in water
• it can H-bond with water but its outweighed by london forces
• Charged sodium phenoxide ions can h-bond with water, helping it dissolve

Question 14

Which reactions of phenol show that it is more reactive than benzene & why?

Answer 14

1. Phenol and bromine water
2. Phenol and dilute nitric acid

Phenol is more reactive than benzene:
1. lone pair of electrons on the oxygen atom are delocalised into the Pi system
2. ring has higher electron density
3. increasing reactivity

Question 15

Reactions of phenol: Phenol and bromine water

Answer 15

BOH + 3Br2 (aq) —> B(with br at 2,4,6 and OH at 1) + 3HBr
3 SUBSTITUTIONS

Question 16

Reactions of phenol: Phenol and dil. nitric acid

Answer 16

BOH + HNO3 —> B(1-OH 2-NO2) / B(1-OH, 4-NO2) + H2O

forms either 2-nitrophenol or 4-nitrophenol

1 SUBSTITUTION

Question 17

Compare benzene and phenol reacting with bromine water

Answer 17

Compare
* Both form HBr
* Both undergo electro subst

Contrast
* benzene requires heating, phenol does not
* 3 substitutions in phenol, 1 in benzene

Question 18

Compare benzene and phenol reacting with nitric acid (HNO3)

Answer 18

Compare
* Both form H2O

Contrast
* benzene needs conc H2SO4 catalyst
* benzene needs heat

Question 19

How to tell if something-benzene is more/less reactive than benzene

Answer 19

MORE reactive than benzene
* R group -> donates e- density
* O directly bonded -> lp on O delocalised to Pi system, increasing e- density

LESS reactive than benzene
* O NOT directly bonded -> electronegative, pulls e- density away from ring
* Cl/I/Br -> electronegative, pulls e- density away from ring