Aromatic compounds and amines

Question 1

Wha is the formula of benzene?

Answer 1

C6H6

Question 2

Describe the structure of benzene

Answer 2

• planar cyclic structure- all the C-C bonds are of the same length
• each carbon forms single covalent bonds to the carbons on either side and to a single hydrogen atom
• the final unpaired electron is located in the p-orbital, perpendicular to the plane of the ring
• the 6 p electrons are delocalised in a ring structure both above and below the plane of carbon atoms

Question 3

Why is benzene more stable than the theoretical cyclohexa-1,3,5-triene?

Answer 3

The expected enthalpy of hydrogenation of cyclohexa-1,3,5-triene = -360kJ/mol
The experimental enthalpy of hydrogenation of benzene = -208kJ/mol- less exothermic than expected
The increase in stability is thought to be due to the delocalised ring of electrons

Question 4

Why does benzene only undergo electrophilic substitution reactions and not electrophilic addition?

Answer 4

Benzene has a high electron density and therefore attracts electrophiles.
It does not undergo addition reactions because this would involve breaking up the delocalised system

Question 5

What are the conditions for the nitration of benzene?

Answer 5

conc. nitric acid
conc. sulfuric acid(catalyst)
temperature below 55 degrees

Question 6

Give the equation for the formation of the NO2(+) electrophile.

Answer 6

HNO3 + 2H2SO4 → NO2(+) + 2HSO4(-) + H3O(+)

Question 7

Why is the nitration of benzene important?

Answer 7

• nitro compounds can be reduced to form aromatic amines which are used to manufacture dyes and pharmaceuticals
• some nitro compounds can be used as explosives such as TNT

Question 8

What are the conditions for the acylation of benzene?

Answer 8

anhydrous aluminium chloride catalyst
heat under reflux
50 degrees

Question 9

Give the equation for the formation of the electrophile

Answer 9

AlCl3 + RCOCl → RCO(+) + AlCl4(-)

Question 10

Why is the acylation of benzene important in organic synthesis?

Answer 10

It introduces a reactive functional group on to the benzene ring

Question 11

What are the conditions for the reduction of a nitro compound to an aromatic amine?

Answer 11

Tin and HCl, heat under reflux and NaOH

Question 12

What are the uses of quaternary ammonium salts?

Answer 12

Cationic surfactants

Question 13

How does a cationic surfactant work and where is it used?

Answer 13

the hydrocarbon tail will bind to nonpolar substances such as grease, whilst the cationic head will dissolve in water
uses- hair products, fabric cleaners

Question 14

How does the ammonium ion get rid of static and where is it used?

Answer 14

The positively charged ammonium ion binds to negatively charged surfaces such as hair and fibre.
uses- fabric conditioners

Question 15

What are amines?

Answer 15

Weak bases

Question 16

How do amines act as bases?

Answer 16

There is a lone pair of electrons on the nitrogen atom that can form a dative covalent bond with a H+ ion

Question 17

Why are primary aliphatic amines stronger bases than ammonia which is a stronger base than aromatic amines?

Answer 17

Aromatic amines
-the benzene ring draws the electrons towards itself and the nitrogen lone pair gets partially delocalised onto the ring. The electron density on the nitrogen decreases, making the lone pair, much less available

Primary aliphatic amines
-the alkyl groups have a positive inductive effect on the lone pair on the nitrogen. The electron density on the nitrogen atom increases, making the lone pair more available

Question 18

How can aliphatic amines be made?

Answer 18

reduction of a nitrile

heating a haloalkane with excess ammonia

Question 19

What are the conditions for the reduction of a nitrile?

Answer 19

Lithium aluminium anhydride in dry ether followed by some dilute acid
or
Catalytic Hydrogenation
high temperature and pressure
Nickel catalyst

Question 20

What is the functional group of an amide?

Answer 20

-CONH2