Aromatics 3.3.10

Question 1

what shape is benzene

Answer 1

Planar molecule

Question 2

what’s the formula for benzene

Answer 2

C6H6

Question 3

how many valent electrons does carbon have and how are they bonded to the carbon atom

Answer 3

4
- each carbon is bonded to 2 other carbons and 1 hydrogen
- the final lone electron is in a p-orbital which sticks out above and below the planar ring

Question 4

what do the lone electrons in the p orbital combine to form

Answer 4

a delocalised ring of electrons

Question 5

why are all the C-C bonds in the molecule of benzene the same

Answer 5

Due to the delocalised electron structure the C-C bonds have the same bond length

Question 6

what type of bond length is the C-C bond in benzene

Answer 6

lies between a single bond and a double bond

Question 7

What did kekule say about his structure of benzene

Answer 7

He thought there was alternating double and single bonds

Question 8

what is benzene more stable than

Answer 8

the theoretical alternative cyclohexa-1,3,5-triene

Question 9

how do we measure stability of benzene

Answer 9

By comparing the enthalpy change of hydrogenation in benzene and cyclohexa-1,3,5-triene

Question 10

Why would we expect benzene to have an enthalpy change of hydrogenation of -360kjmol-1

Answer 10

•Because cyclohexane has 1 double bond and its enthalpy change of hydrogenation is -120kjmol-1
• If benzene had 3 double bonds
- 3 x -120 = -360

Question 11

Does benzene or cyclohexa-1,3,5-triene require more energy to break bonds

Answer 11

Benzene requires more energy to break the bonds

Question 12

Why is benzene more stable than the theoretical cyclohexa-1,3,5-triene with 3 double bonds

Answer 12

• Less exothermic so
  -More energy is required to break bonds in benzene than in cyclohexa-1,3,5-triene
• The stability is due to the delocalised electron structure
• All have the same bond length

Question 13

what are arenes

Answer 13

Also known as aromatic compounds .
They are molecules that contain a benzene ring

Question 14

how are aromatic compounds named

Answer 14

1) Name with “benzene” at the end .
- With a halogen , alkyl group or nitro
2) Named as if phenyl is a functional group ( C6H5) . Benzene ring can be regarded as a substituent side group on another molecule , like alkyl groups are
E.g alkene , alkane , ketones , aldehyde , carboxylates
Only if 7 or more carbons are attached to benzene ring

Question 15

what’s the chemical formula for a phenyl group

Answer 15

C6H5

Question 16

what are the specific examples for naming aromatic compounds

Answer 16

1) Phenol ( a benzene ring with OH attached )
2) Phenylamine ( a benzene ring with NH2 attached )
3) benzoic acid ( carboxylic acid group attached to benzene ring )
4) benzaldehyde ( benzene ring with aldehyde group attached )

Question 17

what are the two mechanisms you need to know for reactions of arenes

Answer 17

1) friedel-crafts acylation
2) nitration reaction

Question 18

how do we make a powerful electrophile in friedel-crafts acylation

Answer 18

Use halogen carrier AlCl3 ( aluminium chloride ) and react it with an acyl chloride

Question 19

what is the change in functional group of friedel crafts acylation

Answer 19

Benzene => phenyl ketone

Question 20

what reagents and conditions are needed for friedel crafts acylation

Answer 20

Reagent :
Acyl chloride in the presence of anhydrous AlCl3 catalyst

Condition :
Heat under reflux (50 C)
Anhydrous AlCl3

Question 21

why is using friedel craft acylation useful

Answer 21

Can help benzene to react as it is difficult due to its stability

Question 22

why is nitration of benzene useful

Answer 22

Allows us to make dyes for clothing and explosives such as TNT (trinitrotoluene)

Question 23

what is the reaction needed to make an electrophile for nitration of benzene

Answer 23

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

Question 24

what do we do after we make an electrophile for nitration of benzene

Answer 24

Use nitronium ion ( NO2+) ( electrophile ) and react with benzene to produce nitrobenzene

Question 25

what’s the electrophile needed in nitration of benzene

Answer 25

NO2+ (nitronium ion )

Question 26

what is the change in functional group for nitration of benzene

Answer 26

benzene => nitrobenzene

Question 27

what are the reagents and conditions needed for nitration of benzene

Answer 27

reagents :
Concentrated HNO3
Concentrated H2SO4 ( catalyst )

Conditions :
50 C

Question 28

how are dyes and pharmaceuticals formed from nitrobenzene compounds

Answer 28

Can be made by reducing nitrobenzene’s to aromatic amines

Question 29

What is the overall reaction for nitration of benzene

Answer 29

C6H6 + HNO3 => C6H5NO2 + H2O

Question 30

What’s the reaction for the generation of electrophile using acyl chloride in friedel crafts acylation

Answer 30

CH3COCl + AlCl3 => CH3C^+=O + AlCl4^-

Question 31

What’s the reaction for the generation of electrophile using acid anhydrides in friedel crafts acylaction

Answer 31

CH3COOCOCH3 + AlCl3 => CH3CO^+ + [ Cl3AlCOOCH3]-

Question 32

What is the reaction for the regeneration of AlCl3 (catalyst) using the acyl chloride in friedel crafts acylation

Answer 32

AlCl4 ^- + H^+ => AlCl3 + HCl

Question 33

What’s the reaction for the regeneration of AlCl3 ( catalyst) using the acid anhydrides in friedel crafts acylation

Answer 33

[ Cl3AlCOOCH3 ] ^ - + H^+ => AlCl3 + COOHCH3

Question 34

what’s the reaction mechanism called for aromatics

Answer 34

electrophilic substitution

Question 35

EQ :
explain the bonding in and the shape of a benzene molecule . Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule . Use data in your answer (6)

Answer 35

Bonding :
• Each C has 3 bonds
• Spare electrons in a p orbital overlap
• delocalisation
Shape :
• planar
• 120 bond angle
• C-C bonds equal in length
Stability :
• Expected Enthalpy change of hydrogenation of cyclohexa-1,3,5-triene = 360kJmol-1
• hydrogenation of benzene is less exothermic by 152kJmol-1
• Benzene is more stable than cyclohexa-1,3,5-triene

Question 36

EQ : the enthalpy of hydrogenation of cyclohexa- 1,3-diene is not exactly double that of cyclohexane
Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value . (3)

Answer 36

•Between -239 to -121
• Alternating double and single bonds
• some delocalisation