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Flashcards in Module 6.1 Deck (160)
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1

What did Kekule' say about Benzene?

- Ring of carbon atoms with alternating single and double bonds between them
- Each carbon is bonded to 2 other carbon atoms and a hydrogen giving a formula of C6H6.
- Benzene has a planar shape/cyclic shape

2

What model came and replaced Kekule' model?

The delocalised model of benzene.

3

What does the delocalised model of benzene say?

- Each of the 6 carbon bonds donate one electron from it's p-orbital. These electrons combine to form a ring of delocalised electrons ABOVE and BELOW the plane of the molecules. This overlapping of p-orbitals forms a pi-system.
- All the bonds are the same length
- Electrons are said to be delocalised because they do not belong to a specific carbon atom.

4

Give 3 reasons why Kekule' model was disapproved

- Bond lengths
- Resistance to reaction
- Enthalpy change of hydrogenation

5

Using the concept of bond lengths, explain why Kekule' model was disapproved

One reason why Kekule' model was disapproved is due to the bond lengths. Under Kekule' structure of benzene, there are 3 C=C bonds (length of 135pm each) and 3 single bonds (length of 147pm each).
However, X-ray diffraction techniques have show that ALL 6 carbon bonds in benzene are the same length at 140pm (between 135 and 147pm)., disapproving Kekule' model.

6

Using the concept of resistance to reaction, explain why Kekule' model was disapproved

Under Kekule' model of benzene, you would expect Benzene to go through similar reactions to alkenes (because of the double bond). For example, alkenes such as propene undergo electrophilic addition. However, benzene tends to undergo electrophilic substitution reactions such as halogenation and nitration rather than electrophilic addition.

7

Using the concept of enthalpy change of hydrogenation, explain why Kekule' model was disapproved

-Cyclohexene has one double bond and its enthalpy change of hydrogenation is -120kJ/mol. Therefore, as Kekule' benzene model has 3 double bonds, we can assume that it has an enthalpy change of hydrogenation of -360kJ/mol.
However, experiments have proved that Benzene's enthalpy change of hydrogenation is actually -208kJ/mol, meaning that benzene is LESS EXOTHERMIC than Kekule' had originally expected and so it is more ENERGETICALLY STABLE.

8

What is meant by electrophilic substitution?

The substitution of an atom/group with another atom/group using an electrophile.

9

What is meant by an electrophile?

An electron pair acceptor.

10

Give all the details for the nitration of benzene.

Reaction Name: Nitration of Benzene
Reagents: Conc nitric acid and conc sulfuric acid (the CATALYST)
Conditions : Reflux at 50 degrees celsius
Electrophile used : NO2^+
Generation of electrophile: HNO3 + H2SO4 ⇄ NO2^+ + HSO4^- + H20.
Regeneration of catalyst : H^+ + HSO4^- → H2S04.
Equation: C6H6 + HNO3 + (H2SO4) → C6H5NO2 + H2O
Product name: Nitrobenzene

11

Give all the details for the bromination of benzene

Reaction Name : Bromination
Reagents : Bromine and a halogen carrier e.g. AlBr3
Conditions : Halogen carrier (catalyst) present.
Electrophile: Br^+
Generation of electrophile : Br2 + AlBr3 ⇄ AlBr4^- + Br^+
Regeneration of catalyst: H^+ +AlBr4^- → HBr + AlBr3
Product name : Bromobenzene

12

Give the generic details for the halogenation of benzene

Reaction Name : Halogenation
Reagents : Halogen and a halogen carrier e.g. AlX3
Conditions : Halogen carrier (catalyst) present and Reflux
Electrophile: X^+
Generation of electrophile : X2 + AlX3 ⇄ AlX4^- + X^+
Regeneration of catalyst: H^+ +AlX4^- → HX + AlX3
Product name : _______benzene

Where X is a halogen

13

What should you always include when answering questions about electrophilic substitution?

What reacts?
Reagents?
Equation?
Electrophile and its generation?
Catalyst and its regeneration?
Conditions?
Mechanism shown?
Charges?
Curly arrows?
Final H^+
5/6's covered on intermediate?

14

Why do we need a halogen carrier in order for a halogen to react with benzene?

Benzene's low electron density means that it cannot induce a dipole in the electrophile. Also, benzene's fully delocalised ring is STABLE meaning that it will resist attack. Therefore, a halogen carrier is needed to POLARISE the electrophile, making it strong enough to react with benzene.

15

Give examples of halogen carriers

Iron halides - FeX3
Aluminium halides - Alx3
iron (Fe)

Where X is a halogen

16

When halogen carriers are regenerated what is added to them, and what is produced?

Added = H^+
Produced = Halogen carrier + Hydrogen halide.

17

When electrophilic substitution occurs, what is actually happening between bonds?

- The electrophile attacks benzene causing a pair of electrons to leave the delocalised system to form a bond with the electrophile.
- This disrupts the delocalised system and forms an intermediate.
- To restore the delocalised ring, the pair of electrons in the C=H bond moves back into the ring.

Overall, there is substitution of hydrogen and the electrophile. This reaction is known as electrophilic substitution.

18

What is a Friedel-Crafts reaction?

A reaction that forms C-C bonds.

19

How do you carry out Friedel-Crafts reactions?

Reflux benzene with a halogen carrier and then reacting it with either a haloalkane (alkylation) or an acyl chloride (acylation). with ANHYDROUS conditions

20

What is meant by Friedel-Crafts alkylation?

A reaction where any alkyl group is put onto a benzene ring using a haloalkane and a halogen carrier under anhydrous conditions. The resultant product is an alkylbenzene.

21

Give the general equation for Friedel- Crafts alkylation

C6H6 + R-X → C6H5R + HX
(Benzene) + (Haloalkane) → Alkylbenzene + Hydrogen halide

Conditions: Reflux and a halogen carrier.

22

Give all the details of a Friedel-Crafts alkylation reaction

Reagents : A haloalkane and an anhydrous halogen carrier (catalyst)
Conditions: Reflux in the presence of an anhydrous halogen carrier. (CATALYST)
Products: Alkylbenzene + Hydrogen halide.
Generation of electrophile :RX + FeX3 → R^+ + FeX4^-
Regeneration of catalyst : H^+ + FeX4^- → FeX3 + HX

23

Give the general equation for the generation of the electrophile in a alkylation reaction

RX + FeX3 → R^+ + FeX4^-

24

Give the general equation for the regeneration of the catalyst for alkylation

H^+ + FeX4^- → FeX3 + HX

25

What is meant by Friedel-Crafts acylation?

A reaction where any acyl group is put onto a benzene ring using an acyl chloride and an halogen carrier under anhydrous conditions. The resultant product is an phenylketone + HCl.

26

Give the general equation for Friedel- Crafts acylation

C6H6 + RCOCl → C6H5COR + HCl
(Benzene) + (Acyl Chloride) → (Phenylketone) + (HCl)

27

Give all the details of a Friedel-Crafts acylation reaction

Reagents : An acyl chloride and an anhydrous halogen carrier
Conditions: Reflux in the presence of an anhydrous halogen carrier. (CATALYST)
Products: Phenylketone + Hydrochloric acid
Electrophile: NO2+
Generation of electrophile: RCOCl + FeCl3 → FeCl4^- + R^+CO
Regeneration of catalyst : H+ + FeCl4^- → FeCl3 + HCl

28

Give the general equation for the generation of the electrophile in a acylation reaction

RCOCl + FeCl3 → FeCl4^- + R^+CO

29

Give the general equation for the regeneration of the catalyst for acylation

H+ + FeCl4^- → FeCl3 + HCl

30

What is the friedel-crafts alkylation product naming system?

alkyl-benzene

e.g. methylbenzene
e.g. pentylbenzene