Chapter 27 (Aromatic Chemistry)

Question 1

Define Aliphatic

Answer 1

Straight or branched chain organic substances

Question 2

Define Aromatic/arene

Answer 2

Including one or more ring of six carbon atoms with delocalised bonding

Question 3

What is the molecular formula or benzene?

Answer 3

C6H6

Question 4

What can be inferred from benzene being a planar molecule?

Answer 4

C-C bonds are all the same lengthened the bond energy between C-C and C=C bonds are equal.

Question 5

What is the bond angle of H-C-C?

Answer 5

120

Question 6

Describe the electrons in the π bond?

Answer 6

Delocalised and spread out over the whole ring, they are not attached to a particular atom.

Question 7

How is a benzene ring drawn?

Answer 7

Hexagon with circle inside

Question 8

Define delocalisation energy?

Answer 8

The increase in stability connected to delocalisation

Question 9

Why is the actual enthalpy of hydration of benzene less than the theoretical?

Answer 9

There is an increase in stability due to the delocalised electrons so benzene is more thermodynamically stable than the theoretical structure.

Question 10

Describe and explain the hydration enthalpy of cyclohexane-1,3-diene

Answer 10

Hydrogenation would be less negative than -240 kJ mol-1

As the π bonds are closer together and overlap there is some delocalisation and extra stability.

Question 11

Describe and explain the hydration enthalpy of cyclohexane-1,4-diene

Answer 11

Hydrogenation enthalpy would be -240 kJ mol-1

There would not be delocalisation as the π bonds are too far apart and don’t overlap.

Question 12

What is the molecular formula of a phenyl group?

Answer 12

C6H5

Question 13

When is phenol used as a name?

Answer 13

When the benzene ring is regarded a side group of another molecule.

Question 14

Why does benzene not undergo addition reaction?

Answer 14

These would involve breaking up delocalised systems.

Question 15

Why does benzene undergo electrophilic substitution?

Answer 15

A H atom can be substituted for another group of atoms.

Question 16

Why is methylbenzene used instead of benzene?

Answer 16

It is less toxic (benzene is carcinogen) and reacts more readily that benzene

Question 17

Why is methyl benzene more reactive than benzene?

Answer 17

The methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.

Question 18

What structure did Kekule suggest benzene had?

Answer 18

Alternating C-C and C=C bonds

Question 19

What was the problem with the shape of Kekule’s benzene?

Answer 19

The structure would be irregular while benzene is planar due to bond lengths.

Question 20

What was the problems with the reactivity of Kekule’s benzene?

Answer 20

The structure would undergo reactions similar to alkenes but benzene does not undergo addition reactions readily.

Question 21

What was the problems with the halogenation of Kekule’s benzene?

Answer 21

Would expect isomeric products but benzene only produces one product.

Question 22

What was the problems with the energy of hydrogenation of Kekule’s benzene?

Answer 22

A ∆H of -360 kJ mol-1 but benzene has an actual value of -208 kJ mol-1.

Question 23

What type of reaction is the nitration of benzene, what is the electrophile, the reagent, the conditions and the catalyst?

Answer 23

Reaction = Electrophilic substitution

Electrophile = nitronium ion/NO2+

Reagent = Concentrated HNO3

Conditions = 50º

Catalyst = Concentrated H2SO4

Question 24

How is a nitronium ion formed?

Answer 24

Reacting concentrated nitric acid and concentrated sulphuric acid.

Question 25

What is used to reduce a nitro group to an amine group?

Answer 25

Tin and HCL OR Nickel and Hydrogen

Question 26

What is the equation for the formation of a nitronium electrophile?

Answer 26

HNO3 + 2H2SO4 > NO2+ + 2HSO4- + H3O+

OR

H2SO4 + HNO3→ HSO4– +NO2+ +H2O

Question 27

What are the uses for nitro compounds?

Answer 27

• Explosives

* Make aromatic amines (used to make dyes)

Question 28

What is Friedel Crafts Acylation?

Answer 28

Reaction between benzene and acyl group

Question 29

What temperature is needed to add the first nitronium ion to benzene?

Answer 29

60º

Question 30

What is the equation for the formation of the electrophile for an acyl chloride?

Answer 30

AlCl3 + Acyl chloride (CH3COCl) > CH3CO+ + AlCl4-

Question 31

What is formed when benzene reacts with an acyl chloride?

Answer 31

Phenyl Ketone + HCl (+ AlCl3)

Question 32

What type of reaction is between acyl chloride and benzene?

Answer 32

Electrophilic substitution

Question 33

What are the reagents and conditions for a reaction between acyl chloride and benzene?

Answer 33

Heat under reflux at 50º

Anhydrous aluminium chloride catalyst

Question 34

What are the reagents used for reducing an nitroarene to an amine?

Answer 34

Sn and HCl OR Fe and HCl OR Ni and H2

Question 35

What is the equation for the reaction of nitrobenzene to a phenyl amine?

Answer 35

Nitrobenzene + 6[H] > Phenylamine + 2H2O

Question 36

Describe and explain the difference in properties between a phenol and other alcohols?

Answer 36

It is more acidic and does not oxidise. Delocalisation makes the C-O bond stronger and the O-H bond weaker.

Question 37

Describe and explain the difference in properties between a chlorobenzene and other chlorine haloalka/enes?

Answer 37

The C-Cl bond is made stronger and typical haloalkane substitution and elimination reactions will not occur as the electron rich benzene ring will repel nucleophiles.

Question 38

Describe and explain the difference in properties between a phenylamine and other amines?

Answer 38

Less basic than aliphatic amines as lone pair is delocalised and less available for accepting a proton.