3.10 Benzene: Aromatic Hydrocarbons / Arenes

Question 1

Give the definition of aliphatic compounds.

Answer 1

Straight or unbranched chain of organic substances.

Question 2

Give the definition for aromatic or arene compounds.

Answer 2

Includes one or more ring of six carbon atoms with delocalised bonding.

Question 3

What is benzenes molecular formula?

Answer 3

C6H6

Question 4

Describe the structure of benzene.

Answer 4

Its basic structure is six carbon atoms in a hexagonal ring, with one H atom bonded to each C atom.
Each C atom is bonded to two other C atoms and one H atom by single covalent sigma-bonds. This leaves one unused electron on each C atom in a p orbital, perpendicular to the plane of the ring.
The six p electrons are delocalised in a ring structure above and below the plane of carbon atoms.
The 6 electrons in the pi bonds are delocalised and spread out over the whole ring.

Question 5

Draw the abbreviated formula for benzene.

Answer 5

(see page 1 in the chemrevise revision guide)

Question 6

Draw the displayed formula for benzene.

Answer 6

(see page 1 in the chemrevise revision guide)

Question 7

Draw the structure of benzene with the delocalised electrons displayed on it.

Answer 7

(see page 1 in the chemrevise revision guide)

Question 8

What shape of molecule is benzene?

Answer 8

Benzene is a planar molecule.

The evidence suggests all the C-C bonds are the same and have a length and bond energy between a C-C single and C=C double bond.

Question 9

What is the bond angle of benzene?

Answer 9

The H-C-C bond angle is 120 degrees.

Question 10

Why is the enthalpy of hydrogenation for benzene less than that of cyclohexane-1,2,5-triene?

Answer 10

The 6 pi electrons are delocalised and not arranged in 3 double bonds.

Question 11

What does ‘delocalisation energy’ mean?

Answer 11

The increase in stability connected to delocalisation of electrons.

Question 12

Explain how benzene is more thermodynamically stable than cyclohexane-1,2,5-triene. Use diagrams in your explanation.

Answer 12

(see page 1 in the chemrevise revision guide)

Question 13

Describe why cyclohexane-1,3-diene would have more thermodynamic stability than cyclohexa-1,4-diene. Use diagrams in your answer.

Answer 13

(see page 1 in the chemrevise revision guide)

Question 14

Is benzene an aliphatic molecule or an aromatic/arene molecule?

Answer 14

Benzene belongs to the aromatic class.

Question 15

Draw methylbenzene.

Answer 15

(see page 2 in the chemrevise revision guide)

Question 16

Draw ethylbenzene.

Answer 16

(see page 2 in the chemrevise revision guide)

Question 17

Draw chlorobenzene.

Answer 17

(see page 2 in the chemrevise revision guide)

Question 18

Draw bromobenzene.

Answer 18

(see page 2 in the chemrevise revision guide)

Question 19

Draw nitrobenzene.

Answer 19

(see page 2 in the chemrevise revision guide)

Question 20

Draw benzoic acid.

Answer 20

(see page 2 in the chemrevise revision guide)

Question 21

Draw benzaldehyde.

Answer 21

(see page 2 in the chemrevise revision guide)

Question 22

Draw 1,3-dimethylbenzene.

Answer 22

(see page 2 in the chemrevise revision guide)

Question 23

Draw 1-chloro-4-methylbenzene.

Answer 23

(see page 2 in the chemrevise revision guide)

Question 24

Draw 4-hydroxybenzoic acid.

Answer 24

(see page 2 in the chemrevise revision guide)

Question 25

Draw 2,4,6-trinitromethylbenzene.

Answer 25

(see page 2 in the chemrevise revision guide)

Question 26

Draw phenylamine.

Answer 26

(see page 2 in the chemrevise revision guide)

Question 27

Draw phenylethene.

Answer 27

(see page 2 in the chemrevise revision guide)

Question 28

Draw 2-phenylbutane.

Answer 28

(see page 2 in the chemrevise revision guide)

Question 29

Draw phenylethanone.

Answer 29

(see page 2 in the chemrevise revision guide)

Question 30

Draw phenylethanoate.

Answer 30

(see page 2 in the chemrevise revision guide)

Question 31

Draw 1-phenylpropane-1,2-diol.

Answer 31

(see page 2 in the chemrevise revision guide)

Question 32

Draw 3-phenylpropanal.

Answer 32

(see page 2 in the chemrevise revision guide)

Question 33

What mechanism reactions does benzene usually undergo and why?

Answer 33

Benzene does not generally undergo addition reactions because these would involve breaking up the delocalised system. Most of Benzenes reactions involve substituting one H for another atom or group of atoms. Benzene has a high electron density and so attracts electrophiles.

The reactions are usually electrophilic substitutions.

Question 34

Why is benzene banned for use in schools?

Answer 34

Benzene is a carcinogen.

cancers causing molecules

Question 35

Why does methylbenzene react more readily than benzene?

Answer 35

Methyl benzene is less toxic and also reacts more readily than benzene as the methyl side group releases electrons into the delocalised system making it more attractive to electrophiles.

Question 36

What are the reagents for the nitration of benzene?

Answer 36

Concentrated nitric acid in the presence of concentrated sulphuric acid (catalyst).

Question 37

What is the reaction mechanism for the nitration of benzene?

Answer 37

Electrophilic substitution.

Question 38

What is the electrophile for the nitration of benzene?

Answer 38

N(O2)+

Question 39

What is the equation for the formation of the electrophile in the nitration of benzene?

Answer 39

HNO3 + 2H2SO4 -> N(O2)+ + 2HS(O4)- + H3O+

This is an acid base reaction.

The HNO3 acts as a base.

Question 40

Why is the nitration of benzene an important reaction?

Answer 40

Nitration of benzene and other arenas is an important step in synthesising useful compounds e.g. explosive manufacture (like TNT, trinitrotoluene / 2,4,6-trinitromethylbenzene) and formation of amines from which dyestuffs are manufactured.

Question 41

Draw the overall equation for the nitration of benzene.

Answer 41

(see page 3 in the chemrevise revision guide)

Question 42

Draw the mechanism for the reaction of the nitration of benzene.

Answer 42

(see page 3 in the chemrevise revision guide)

Question 43

What temperature is the nitration of benzene done under?

Answer 43

60º.

On using higher temperatures a second nitro group can be substituted.

Question 44

What is the change in the functional group for the nitration of benzene?

Answer 44

benzene -> nitrobenzene

Question 45

What are the reagents for the friedal crafts acylation?

Answer 45

Acyl chloride in the presence of anhydrous aluminium chloride catalyst.

Question 46

What are the conditions for the friedal crafts acylation?

Answer 46

Heat under reflect (50º)

Question 47

What is the mechanism reaction for the friedal crafts acylation?

Answer 47

Electrophilic substitution.

Question 48

What is the equation for the formation of the electrophile for the friedal crafts acylation?

Answer 48

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

Question 49

What is the overall equation for the friedal crafts acylation?

Answer 49

(see page 3 in the chemrevise revision guide)

Question 50

Draw the mechanism for the friedal crafts acylation.

Answer 50

(see page 3 in the chemrevise revision guide)

Question 51

Why is the freidal crafts acylation an important reaction?

Answer 51

In organic synthesis they introduce a reactive functional group on to the benzene ring.

Question 52

What is the change in the functional group in the friedal crafts acylation?

Answer 52

benzene -> phenyl ketone

Question 53

What are the reagents in the reaction for reducing a nitroarene into aromatic amines?

Answer 53

Sn and HCl
or
Fe and HCl

Question 54

What are the conditions in the reaction for reducing a nitroarene into aromatic amines?

Answer 54

Heating

Question 55

What is the mechanism in the reaction for reducing a nitroarene into aromatic amines?

Answer 55

Reduction

Question 56

Draw the overall equation for reacting nitrobenzene to form phenylamine.

Answer 56

(see page 4 in the chemrevise revision guide)

Question 57

What will be formed in the reaction of nitrobenzene to form phenylamine? How can we reduce this?

Answer 57

As the reaction is carried out in HCl, the ionic salt C6H5N(H3)+Cl- will be formed, which is soluble in water. Reacting this salt with NaOH will give the insoluble phenylamine.

Question 58

What happens to the molecules properties and reactions when a Cl atom is directly attached to a benzene ring?

Answer 58

The C-Cl bond is made stronger. Typical halogenoalkane substitution and elimination reactions do not occur. Also the electron rich benzene ring will repel nucleophiles.

Question 59

What happens to the molecules properties and reactions when a -OH group is directly attached to a benzene ring?

Answer 59

Delocalisation makes the C-O bond stronger and the O-H bond weaker. Phenol does not act like an alcohol - it is more acidic and does not oxidise.

Question 60

What happens to the molecules properties and reactions when a NH2 group is directly attached to a benzene ring?

Answer 60

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

Question 61

Draw chlorobenzene.

Answer 61

(see page 4 in the chemrevise revision guide)

Question 62

Draw phenol.

Answer 62

(see page 4 in the chemrevise revision guide)

Question 63

Draw phenylamine.

Answer 63

(see page 4 in the chemrevise revision guide)

Question 64

Look at page 5 in chemrevise revision guide, not sure how to make notes on these, ask monty about each reaction.

Answer 64

understand? memorised?