25.1 - 25.4

Question 1

What is benzene

Answer 1

• a cyclic, planar molecule

Question 2

What is the general formula for benzene

Answer 2

• C6H6

Question 3

Describe the bonding in benzene

Answer 3

• each carbon is bonded to 2 other carbons and 1 hydrogen
• the final lone election is in a p-orbital which sticks out about the and below the planar ring
• the lone electrons in the p-orbitals combine to form a delocalised ring of electrons

Question 4

What causes all the bond lengths in benzene to be the same

Answer 4

• because of the delocalised electron structures

Question 5

How is benzene normal drawn

Answer 5

• skeletal formula

Question 6

What did Kekulè think about the structure of benzene

Answer 6

• that there was alternating double single bonds

Question 7

What is the stability of benzene and how do we find it

Answer 7

• more stable then Kekulè’s initial compound of benzene ( lower value=> predicted 360kJmol^-1 but it is 208kJmol^-1)
• we cab measure the stability by comparing the enthalpy change of hydrogenation

Question 8

What is benzene’s stability due to

Answer 8

• the delocalised electron structure

Question 9

What are arenes

Answer 9

• another word to describe a substance with a benzene ring in it

Question 10

What is a NH2 group attached to a benzene ring called

Answer 10

• phenylamine

Question 11

What type of reactions do benzenes undergo

Answer 11

• electrophilic substitution

Question 12

Complete the sentence:
Benzenes don’t react with bromine water..

Answer 12

• like alkenes meaning that they don’t have a double bond at Kekulé proposed

Question 13

Complete the sentence:
Once you have added an acyl group….

Answer 13

• the benzene structure is weaker and it makes it easier to modify it further to make useful products

Question 14

In order to add onto the benzene ring what does the electrophile need to have

Answer 14

• a VERY STRONG positive charge as benzenes don’t react readily

Question 15

As acyl groups aren’t positive enough what do we use

Answer 15

• halogen carries to act as a catalyst (e.g. AlCL3)
• they provide a much stringer electrophile with a stronger positive charge

Question 16

What is an acylation reaction between

Answer 16

• AlCl3 (halogen carrier) and the electrophile

Question 17

What are the products of this reaction and what does this create

Answer 17

• a STRONGER ELECTROPHILE

Question 18

Draw a mechanism for acylation of benzene after the acyl group has reacted with he halogen carrier

Answer 18

Question 19

How is nitration of benzene useful

Answer 19

• use it for dyes for clothing
• used for explosives

Question 20

What are the 3 steps in nitration of benzene

Answer 20

1) generation of the catalyst with H2SO4
2) mechanism
3) regeneration of catalyst

Question 21

Draw the mechanism for the nitration of benzene

Answer 21

Question 22

What functional groups do phenols have

Answer 22

• hydroxyl group (-OH)

Question 23

What is skeletal formula for salicylic acid

Answer 23

Question 24

Describe the reactivity of phenols compared to benzenes

Answer 24

• phenols are more reactive than benzene due to the electron density in the ring being higher

Question 25

Why are electrophilic substitutions more likely to happen with phenol than with benzene

Answer 25

• because of the -OH group and orbital overlap
  (Oxygen overlaps with delocalised ring structure)
• so phenols are more susceptible to attack from electrophiles

Question 26

Why does the position of functional groups on a benzene ring affect the reactivity with electrophiles

Answer 26

• because the substituted benzene rings distort the electron density in the ring

Question 27

What do electron withdrawing groups affect

Answer 27

• they affect the substitution reactions of carbon 3 and 5

Question 28

Where are electronegative groups more likely to withdraw electron density from and what does this mean

Answer 28

• electronegative groups (like NO2) are more likely to withdraw electron density from carbon 2,4 and 6
• this means that electrophiles are more likely to attack carbons 3 and 5

Question 29

What do electron donating groups affect

Answer 29

• affect substitution on carbons 2,4 and 6

Question 30

Where are electronegative groups more likely to withdraw electron density from and what does this mean

Answer 30

• donating groups like (NH2 and OH) donate electrons and increase density in the ring
• specifically to carbon 2,4 and 6
• this means electrophiles are more likely to attack carbons 2,4 and 6 and substitution is more likely to happen at these points

Question 31

When reacting phenol and chloroethane using AlCL3 as a catalyst where is substitution. More likely to happen

Answer 31

• electron donating group so substation is most likely at carbons 2,4 and 6

Question 32

When reacting nitrobenzene and chloroethane using AlCL3 as a catalyst where is substitution more likely to happen

Answer 32

• electron withdrawing group so most likely to get substitution at carbons 3 and 5

Question 33

Why are phenols weak acids

Answer 33

• as it dissociates weakly

Question 34

What do phenols dissociate to form

Answer 34

• phenoxide ion and H+ ion

Question 35

What does phenol and alkalis form

Answer 35

• a salt and water

Question 36

Why can phenols react with bromine water

Answer 36

• because they are much more reactive than benzene because of the overlap of the oxygen molecule in -OH and the p orbital

Question 37

How does phenol react with bromine water

Answer 37

• phenol decolourises bromine water
• producing 2,4,6-tribromophenol (as OH is a electron donating group)and 3HBr

Question 38

How do phenols react with dilute nitric acid

Answer 38

• produces 2-nitrophenols or 4-nitrophenol
• as OH is an electron donating group