(2) Aromatic Chemistry

Question 1

What is the molecular formula for Benzene?

Answer 1

C6H6

Question 2

What name is given to benzene as a substituent group?

Answer 2

Phenyl group.

Question 3

Name the following:

C6H5NH2

C6H5CHCH2

C6H5OH

Answer 3

• Phenylamine
• Phenylethene
• Phenol

Question 4

Reasons Benzene is NOT an alkene?

Answer 4

1. Reactivity with Br2
2. Enthalpy of hydrogenation
3. Bond length

Question 5

Describe the reactivity of Br2 with Cyclehexa-1,3,5-triene (kekule).

Answer 5

• Bromine water turns orange to colourless with presence of double bonds
• electrophilic addition

Question 6

Describe the reactivity of Br2 with Benzene.

Answer 6

• Benzene undergoes substitution reactions and NOT addition (Br2)
• Benzene only reacts with bromine at high temps and with catalyst

Question 7

What is the Enthalpy Change when one double bond is hydrogenated.

Answer 7

enthalpy change is -120 Kjmol-1

Question 8

Cyclohexa-1,3,5-triene hydrogenation value?

Answer 8

energy released is about -360 Kjmol-1 (-120x3).

Question 9

Benzene hydrogenation value?

Answer 9

energy released -208 Kjmol-1

about -152 less exothermic than expected

Question 10

What does the -208 suggest about benzene

Answer 10

• suggests benzene is more stable due to electrons being delocalised

Question 11

How do bond lengths show benzene is NOT an alkene?

Answer 11

• benzene bond lengths that are all the same

- bond lengths are intermediate between short C=C and long C-C

Question 12

Summary of reasons to doubt kekule model?

Answer 12

• Benzene is less reactive than alkene, doesn’t undergo addition and decolourise bromine
• Benzene undergoes substitution reaction in order to maintain ring of delocalised electrons
• bond lengths are intermediate between short C=C and long C-C
• enthalpy change of hydrogenation less exothermic than expected as benzene is more stable

Question 13

Bonding of Benzene key facts?

Answer 13

• Each C has 3 covalent bonds
• the spare electrons in p orbitals overlap and form pi cloud
• This is a delocalised electron system

Question 14

Shape of Benzene key facts?

Answer 14

• planar molecule
• 120 bond angle
• C-C bonds equal in length

Question 15

Stability of Benzene Key facts?

Answer 15

• Expected enthalpy change of cycloexan-1,2,3-triene = -360
• Benzene enthalpy change is less by 152 (-208)
• Benzene is more stable as its at lower energy than cyclohexatriene

Question 16

What are the physical properties of Arenes?

Answer 16

• Benzene is a colourless liquid at room temp
• Benzene has a MP higher than hexane as its a planar molecule
• enables it to stack together very well in the solid state meaning Van der Waals forces harder to break.

Question 17

What is the reactivity of Aromatic Compounds?

Answer 17

• high electron density due to the delocalisation. This means arenes react with electrophiles.
• The aromatic ring is very stable. It remains intact at the end of reactions.

Question 18

How are Arenes combusted?

Answer 18

• they are unsaturated,

- tend to undergo incomplete combustion, solid carbon particles form

Question 19

Outline Electrophilic Substitution (nitration reagent and overall equation of how electrophile is formed)

Answer 19

Reagents: Conc H2SO4 and Conc HNO3

Overall Reaction: H2SO4 + HNO3 —> HSO4- + H2O + NO2+

• NO2+ is called nitromium ion

Question 20

Outline the Nitration mechanism and name product.

Answer 20

• Nitrobenzene

check pg 15 in notes

Question 21

How can H2SO4 be reformed after nitration?

Answer 21

H+ + HSO4- —> H2SO4

Question 22

Uses of nitrated arenes?

Answer 22

• can be used to produce explosive

- methyl benzene can be nitrated three times to form TNT used in explosives

Question 23

How is Friedel-Crafts Acylation different to Electrophilic Addition?

Answer 23

• Requires acyl chloride

Question 24

How is the electrophile used in Friedel- Crafts Acylation formed?

Answer 24

RCOCl + AlCl3

—>RCO+ + AlCl4-

** check pg 18**

Question 25

What is the purpose of AlCl3 in formation of electrophile RCO+

Answer 25

• catalyst

Question 26

Outline Friedel-Crafts Acylation with benzene and methanoyl chloride.

Answer 26

** check pg 19**

Question 27

What does the H+ form at end of Friedel-Crafts Acylation?

Answer 27

H+ + AlCl4- —> AlCl3 + HCl

Question 28

What is the Friedel-Crafts Acylation overall equation and Nitration Overall Equation?

Answer 28

** check pg 19 and 15**

Question 29

Suggest how rate of reaction of propanone with HCN would compare with rate of reaction of propanal with HCN.

Answer 29

• propanone slower

- easier to attack at end of chain

Question 30

Why is the enthalpy of hydrogenation of cyclo-1,3-diene is not double of cyclohexene?

Answer 30

The double bonds in cyclo-1,3-diene are close enough to allow delocalisation, so more stability.

Question 31

Explain why a catalyst is needed for the Acylation of benzene.

Answer 31

Benzene ring has electrons above and below plane of atoms.

Delocalised electrons repel attacking species so catalyst is needed.

Question 32

How can you determine stability of molecule based on double bonds?

Answer 32

• if molecule has alternating double and single bonds then double bonds are close enough for delocalisation
• less exothermic and therefore more stable