3.3.10-11 Aromatics and amines

Question 1

Describe the bonding and structure of benzene.

Answer 1

• 6 carbon molecules in a hexagonal ring
• Forms planar structure (120° bond angle)
• Alternating single and double bonds, p orbitals overlap, delocalised π system.

Question 2

What are the relative bond lengths of the following: C-C, C=C, delocalised π system in benzene

Answer 2

C=C < benzene < C-C

Question 3

What makes benzene more stable than the theoretical molecule cyclohexa-1,3,5-triene?

Answer 3

Delocalised p electrons

Question 4

What is the reaction for the nitration of benzene?

think: conditions, reaction, mechanism

Answer 4

Conditions: conc. HNO3, conc. H2SO4

HNO3 + 2H2SO4 ⇌ NO2+ + 2HSO4- + H3O+

Electrophilic substitution of NO2+

Question 5

What are the functional groups of aromatic compounds?

Answer 5

Arenes, phenol, (chloro)benzene

Question 6

What is the reaction for the hydrogenation of benzene?

think: reaction, conditions

Answer 6

Addition reaction

C6H6 + 3H2 → C6H12

Conditions: Ni catayst and heat

Question 7

What is the reaction for Friedel-Crafts alkylation?

Answer 7

Haloalkane + benzene → alkylbenzene + HCl

Condition: anhydrous AlCl3 and heat

Question 8

What is the reaction for Friedel-Crafts acylation?

Answer 8

Acyl chloride + bezene → acylbenzene + HCl

Condition: anhydrous AlCl3 and heat

Question 9

Why is it surprising that 1-chlorobenzene undergoes nucleophilic substitution with ammonia?

Answer 9

• Ammonia is a nucleophile
• Benzene repels nucleophiles

Question 10

What is the difference between an aliphatic and aromatic amine?

Answer 10

Aliphatic = alkyl group

Aromatic = phenyl, benzene ring

Question 11

What are the different classifications of amines?

Answer 11

• Ammonia
• Primary amine
• Secondary amine
• Tertiary amine
• Quaternary ammonium salt

Question 12

How can you produce an amine?

Answer 12

• Nucleophilic substitution, haloalkane + excess hot and ethanolic ammonia
• Reduction of nitriles (-CN) using LiAlH4, dry ether (nucleophilic add)

Question 13

Why do amines have a higher bp than alkanes but smaller bp than alcohols?

Answer 13

• Amines have hydrogen bonds between the N-H, stronger IMF than that which is present in alkanes (Van der Waal’s)
• Alcohols have hydrogen bonds between O-H, strongers hydrogen bonds as oxygen is more electronegative

Question 14

What shape are all amines, what is the bond angle?

Answer 14

Trigonal pyrimidal
107°

Question 15

How can you produce phenylamine from benzene?

Answer 15

Benzene → Nitrobenzene
(conc. H2SO4/HNO3)

Nitrobenzene → Phenylamine
(heat under reflux, conc. HCl/Sn catalyst)
(then add NaOH)

Question 16

What is the functional group of axo dyes?

Answer 16

R-N≡N-R

Question 17

How do you make an azo dye from phenylamine?

Answer 17

Phenylamine + HNO2 + HCl
Then add phenol

Question 18

How do amines act as a base?

Answer 18

Proton acceptor, donate lone pair of electrons to H+

Question 19

What 2 factors affect the basicity of amines?

Answer 19

• Positive inductive effect- alkyl groups will donate e- density so the nitrogen atom and lone pair become more negative
• Delocalisation- aromatic rings cause lone pair on nitrogen to become delocalised into the ring. Lone pair less available.

Question 20

What is the trend in the strongest/weakest bases across the amines?

Answer 20

3° > 2° > 1° > NH3 > phenylamine

Question 21

How can you produce a quaternary ammonium salt?

Answer 21

Nuc sub
Tertiary amine + haloalkane

Question 22

Explain why methylamine is more likely to react with an haloalkane than ammonia.

Answer 22

• Methylamine has a stronger positive inductive effect as it has more alkyl groups, so more electron density donated to the N atom and lone pair.

Question 23

What is the role of quaternary ammonium salts?

Answer 23

• Cationic surfacents (conditioner, fabric softener)
• Used in cleaning products to reduce static charge build-up

Question 24

How do you produce an N-sub amide?

Answer 24

Nucleophilic addition elimination
1° amine + RCOCl
(acylation)

Question 25

How do you produce an amide?

Answer 25

Nucleophilic addition elimination
Ammonia + RCOCl
(acylation)