M6 Chapter 25 - Aromatic Compounds

Question 2

What is the structure and formula of Benzene?

Answer 2

C6H6

Question 3

What is the state of benzene at room temperature?

Answer 3

Liquid

Question 4

What is the bond angle of benzene?

Answer 4

120

Question 5

What are the 3 features of benzene that don’t support Kekule’s model?

Answer 5

• Benzene is resistant to addition reactions.
• Enthalpy change of hydrogenation of benzene is more stable than predicted.
• All the carbon bonds are the same length.

Question 6

What technique was used to find the bond lengths of benzene?

Answer 6

X-ray diffraction

Question 7

What happens to the 4th electron in the p-orbital of each Carbon atom in benzene.

Answer 7

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

Question 8

How do the rings of electron density affect the stability of Benzene?

Answer 8

Makes benzen very stable, even though it is unsaturated (aromatic stability)

Question 9

Why does Benzene have a relatively high melting point?

Answer 9

Close packing of flat hexagonal when solid.

Question 10

Is benzene soluble in water?

Answer 10

No because it is nonpolar.

Question 11

What are the dangers of Benzene - why is it not used in schools?

Answer 11

It is a carcinogen.

Question 12

How do you name compounds containing a benzene ring?

Answer 12

-benzene, or phenyl-; this can designate position on the ring using numbers if there is more than one substituent.

Question 13

Why is benzene attacked by electrophiles?

Answer 13

High electron density above/below ring due to delocalised electrons.

Question 14

Draw a general electrophillic substitution mechanism of benzene, using El+ to represent an electrophile.

Answer 14

Coming out from Benzene curly arrow of El+

Two lines formed; El and H

From H line, curly arrow going into + in the benzene system.

Question 15

Nitration of benzene is what type of reaction?

Answer 15

Electrophillic substitution reaction

Question 16

Which ion is used to nitrate benzene?

Answer 16

NO2+

Question 17

What is the catalyst in nitration of benzene?

Answer 17

Sulfuric Acid

Question 18

How is this NO2+ ion generated? (conditions and equations)

Answer 18

Concentrated H2SO4 and concentrated HNO3.

H2SO4 + HNO3 –> H2NO3+ HSO4-

H2NO3+ –> H2O + NO2+

Overall: H2SO4 + HNO3 –> HSO4- + NO2+ + H2O

Question 19

How is the H2SO4 catalyst regenerated in the nitration of benzene?

Answer 19

HSO4- + H+ –> H2SO4 (H+ from benzene ring)

Question 20

Draw a mechanism and write an overall equation for the nitration of benzene.

Answer 20

C6H6 + HNO3 –> C6H5NO2 + H2O

Benzene ring with curly arrow coming off with ‘NO2+’.

This goes to Benzene with NO2 and labelled H bond. H bond has curly arrow going into + inside benzene ring.

Question 21

What type of catalyst is used for a friedel crafts reaction?

Answer 21

A halogen carrier (eg: AlCl3)

Question 22

Why does benzene not react directly with halogens?

Answer 22

The aromatic ring is too stable.

Question 23

What is happening when AlCl4- is formed in terms of electrons?

Answer 23

The lone pair of electrons on the chlorine atom is forming a coordinate bond to Al.

Question 24

How is the AlCl3 catalyst reformed?

Answer 24

AlCl4- + H+ –> HCl + AlCl3 (H+ from benzene)

Question 25

How could you use a friedel-crafts mechanism to add a methyl group to a benzene ring?

Answer 25

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene.

Question 26

Draw the mechanism for the acylation of Benzene from RCO+.

Answer 26

Same type of reaction. Arrow out into COR+ which then goes to COR and H lines.

Question 27

What is the structure of phenol?

Answer 27

Benzene with OH.

Question 28

What reactions can you carry out to show the weak acidity of phenol?

Answer 28

A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates.

Question 29

Write the equation of the reaction between phenol with bromine to 2,4,6-tribromophenol?

Answer 29

Phenol + 3Br2 --> 2,4,6-tribromophenol + 3HBr

Question 30

Write the equation for the reaction between phenol with dilute nitric acid.

Answer 30

Phenol with dilute HNO3 --> 2-Nitrophenol + 4-Nitrophenol.

Question 31

What is the relative ease of electrophilic substitution of phenol compared to benzene and why?

Answer 31

It is easier for electrophillic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into the ring system of electrons therefore, this increases the electron density of the ring hence, electrophiles are more attracted to phenol.

Question 32

What is the directing effect of electron donating groups OH and NH2?

Answer 32

They direct group to the 2 and 4 position of the ring in electrophillic substitution of aromatic compounds.

Question 33

What is the directing effect of electron withdrawing group NO2?

Answer 33

NO2 directs atoms to the 3 position of the ring in electrophillic substitution of aromatic compounds.