Section 6 - Aromatic Compounds and Amines

Question 1

What is the formula for benzene?

Answer 1

C₆H₆

Question 2

What is the structure of benzene?

Answer 2

Planar cyclic:

• each carbon atom forms a covalent bond to two other carbons and one hydrogen atom
• the unpaired electron is located in a p-orbital that sticks out above and below the plane of the ring
• these orbitals all combine to make a ring of delocalised electrons - pi bond.

Question 3

Does the pi bond make benzene stable or unstable?

Answer 3

Stable

Question 4

Describe the stability of benzene in relation to theoretical cyclohexa-1,3,5-triene:

Answer 4

• cyclohexene has one double bond and an enthalpy change of -120kJmol^-1 when hydrogenated.
• we would expect cyclohexatriene to have an enthalpy change of -360kJmol^-1.
• the experimental enthalpy change of benzene is -208kJmol^-1 suggesting its bonds are harder to break than cyclohexatriene.
• benzene is much more stable - probably because of the pi bonds.

Question 5

What kind of reactions do arenes undergo?

Answer 5

Electrophilic Substitution

Question 6

Why is the benzene ring so good at attracting electrophiles?

Answer 6

Because of the high electron density of the pi bond.

Question 7

Why doesn’t benzene undergo electrophilic addition reactions?

Answer 7

This would destroy the ring of delocalised electrons.

Question 8

How reactive is benzene?

Answer 8

Fairly unreactive - it’s so stable

Question 9

What is a halogen carrier?

Answer 9

A catalyst used to make electrophiles stronger.

Question 10

Give an example of a halogen carrier:

Answer 10

AlCl₃

Question 11

How does AlCl₃ make the acyl chloride electrophile stronger?

Answer 11

• it accepts a lone pair from the acyl chloride
• as the lone pair is pulled away, the polarisation of the acyl chloride increases and a carbocation forms (C+)

Question 12

What are the conditions for Friedel-Crafts Acylation?

Answer 12

• heat
• reflux
• non-aqueous solvent

Question 13

What are the conditions for nitration of benzene?

Answer 13

• conc sulfuric acid
• conc nitric acid
• 50^oc (for only 1 NO₂)

Question 14

What are the equations for the formation of the nitronium ion in nitration of benzene?

Answer 14

HNO₃ + H₂SO₄ —> H₂NO₃⁺ + HSO₄^-

H₂NO₃⁺ —> NO₂⁺ + H₂O

Question 15

Why are aromatic amines useful?

Answer 15

Used to manufacture dyes and pharmaceuticals

Question 16

How can aromtic amines be formed from nitro compounds?

Answer 16

Reduction

Question 17

What is the IUPAC name for TNT?

Answer 17

trinitrotolouene

Question 18

What are amines?

Answer 18

Organic derivatives of ammonia

Question 19

Why are quaternary ammonium salts used as cationic surfactants?

Answer 19

• The hydrocarbon tale binds to non-polar substances like grease
• The cationic head will dissolve in water
• The ammonium ion can also bind to negatively charged sufraces like hair and fibre to prevent static.

Question 20

Are amines acidic or basic? Why?

Answer 20

Basic - they accept protons.

Question 21

What kind of bond can the lone pair of the electrons on the ntrogen atom form with a H⁺ ion?

Answer 21

A dative covalent bond

Question 22

What determines the strength of an amine as a base?

Answer 22

The availability of the lone pair of electrons on the Nitrogen atom.

Question 23

Why are primary amines more basic than ammonia?

Answer 23

• Alkyl group has a positive inductive effect towards the Nitrogen atom
• Lone pair becomes more available

Question 24

Why is ammonia more basic than phenylamine?

Answer 24

• The lone pair on the Nitrogen atom merge with the delocalised cloud of electrons.
• The lone pair is less available.

Question 25

Are amines electrophiles or nucleophiles?

Answer 25

Nucleophiles

Question 26

What is the mechanism for the reaction of ammonia and halogenoalkanes?

Answer 26

Nucleophilic Substitution

Question 27

What is the product of the reaction of ammonia with halogenoalkanes?

Answer 27

A mixture of primary, secondary and tertiary amines and quaternary ammonium salt.

Question 28

Why is the product of heating halogenoalkanes with ammonia a mixture?

Answer 28

The primary amine you first produce can react with other remaining halogenoalkane in a nucleophilic substitution reaction.

Question 29

What are the conditions of the reaction between halogenoalkanes and ammonia?

Answer 29

• heat
• excess ammonia

Question 30

What is an apropriate lab method of reducing a nitrile to make a primary amine?

Answer 30

• Lithium Aluminium Hydride (LiAlH₄) - a strong reducing agent
• Non-aqueous solvent (eg. dry ether)
• Dilute acid

Question 31

Why is LiAlH₄ inapropriate for industrial reduction of nitriles?

Answer 31

Too expensive

Question 32

What is an apropriate industrial method of reducing nitriles?

Answer 32

Catalytic Hydrogenation

• hydrogen gas
• metal catalyst (platinum or nickel)
• high temp and pressure

Question 33

How do you make aromatic amines?

Answer 33

Reduction of a nitro-compound e.g. nitrobenzene

Question 34

What are the conditions for the reduction of nitrobenzene?

Answer 34

1) - heat
- tin metal
- concentrated HCl
- reflux

(makes a salt)

2) - alkali (eg. sodium hydroxide solution)

Question 35

What are aromatic amines useful for?

Answer 35

• dyes
• pharmaceuticals

Question 36

What is an amide?

Answer 36

Carboxylic acid derivative (group -CONH₂)

Question 37

Why do amides behave differently to amines?

Answer 37

The carbonyl group pulls electrons away from the NH₂ group.

Question 38

What is an N-substituted amide?

Answer 38

When one of the hydrogens in an amide is replaced with an alkyl group.