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Edexcel Chemistry Unit 5 > 5.4.1 Arenes: Benzene > Flashcards

5.4.1 Arenes: Benzene Flashcards

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1
Q

What is the Kekulé structure of bonze?

A

Alternating single and double bonds that constantly flip between the two carbons.

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2
Q

Give two bits of evidence against the Kekulé structure of benzene.

A

X-ray diffraction showed all the carbon-carbon bonds in benzene are the same length and infrared studies have shown that none of the bonds are normal single or double bonds because they absorb energy of different frequencies.

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3
Q

Explain the delocalised model of benzene.

A

The p-orbitals of all six carbon atoms overlap to create π bonds. This creates two ring shaped clouds of electrons above and below the ring. The bonds are all equal in length and the p electrons are delocalised.

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4
Q

How are the delocalised rings represented in the skeletal formula of benzene?

A

A circle within the hexagon.

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5
Q

How do the delocalised electrons in benzene affect its stability?

A

They increase it.

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6
Q

The enthalpy change of hydrogenation of cyclohexene is -120kJ mol^1 what would you expect the enthalpy change of hydrogenation of benzene to be?

A

-360kJ mol^-1

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7
Q

What is the experimental enthalpy value for the hydrogenation of benzene?

A

-208kJ mol^-1

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8
Q

Explain why a less negative enthalpy value of hydrogenation means that the delocalised electrons give benzene stability.

A

More energy must have been put into the break the bonds in benzene than would have been needed to break the bonds in the Kekulé structure.

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9
Q

Why does benzene burn with a smokey flame?

A

There isn’t enough oxygen in the aorta burn benzene completely. Many carbon atoms stay as carbon producing soot in the hot gas.

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10
Q

Give the equation for the complete combustion of benzene.

A

2C6H6 + 15O2 → 12CO2 + 6H2O

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11
Q

What does the benzene ring attract and why?

A

It has a high electron density so it attracts electrophiles.

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12
Q

Why does benzene usually undergo substitution reactions rather than addition reactions?

A

To preserve the delocalised ring.

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13
Q

How do you make nitrobenzene?

A

Warm benzene with concentrated nitric and sulfuric acids.

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14
Q

What is the role of sulfuric acid in the nitration of benzene?

A

It acts as a catalyst to help make the nitronium ion, NO2+ which is the electrophile.

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15
Q

What is the electrophile in the nitration of benzene?

A

NO2+

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16
Q

Give two equations to show how the electrophile is formed for the nitration of benzene.

A

HNO3 + H2SO4 → H2NO3+ + HSO4-

H2NO3+ → NO2+ + H2O

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17
Q

Where does the pair of electrons move in the first stage of the mechanism for the nitration of benzene?

A

From the benzene ring to the electrophile, NO2+.

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18
Q

What is the charge on the benzene ring of the intermediate product for the nitration of benzene?

A

+1

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19
Q

Where does the pair of electrons move int he second stage of the mechanism for the nitration of benzene?

A

From the bond C-H with hydrogen back to the ring.

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20
Q

Where does the lost H+ ion go after the nitration of benzene?

A

Back to the HSO4- to reform the catalyst, H2SO4

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21
Q

How do you ensure mononitration of benzene?

A

Keep the temperature below 55ºC.

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22
Q

What happens if the temperature is above 55ºC when nitrating benzene?

A

You will get multiple substitutions.

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23
Q

How do you make benzenesulfonic acid?

A

Warm benzene to 40ºC with fuming sulfuric acid for about 30 minutes.

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24
Q

What is fuming sulfuric acid?

A

Sulfur trioxide, SO3 dissolved in sulfuric acid.

25
Q

What is the electrophile in the production of benzenesulfonic acid?

A

Sulfur trioxide.

26
Q

What is the charge of the electrophile sulphur trioxide?

A

Neutral, however the oxygens are slightly negative and the sulfur is slightly positive.

27
Q

What happens in the first stage of the mechanism for making benzenesulfonic acid?

A

The sulfur from the SO3 attracts a pair of electrons from the delocalised ring.

28
Q

What happens in the second step of the mechanism for making benzenesulfonic acid?

A

The negative O atom on SO3- takes the H atom from the benzene.

29
Q

What happens to the electron pair in the second step of the mechanism for making benzenesulfonic acid?

A

They move from the C-H bond to the delocalised ring.

30
Q

What is the formula for benzenesulfonic acid?

A

C6H5SO3H

31
Q

What are halogen carriers? Give an example.

A

Something that accepts a lone pair of electrons from the halogen in an electrophile, sometimes forming a carbocation, making the electrophile a lot stronger. AlCl3.

32
Q

What does Friedel-Crafts alkylation reaction produce?

A

Alkyl benzenes.

33
Q

Give the equation for the formation of the formation of the carbocation R+ from a chloroalkane

A

RCl + AlCl3 → R+ + AlCl4-

34
Q

How is the halogen carrier regenerated in the alkylation of benzene?

A

A chlorine reacts with the H+ that left the benzene ring to form HCl.

35
Q

What conditions are halogen carriers used in and why?

A

Dry conditions, such as refluxing in dry ether because AlCl3 is very sensitive to hydrolysis.

36
Q

What does Friedel-Crafts acylation of benzene produce?

A

Phenylketones.

37
Q

What happens to acyl chlorides when they react with halogen carriers?

A

They form an RCO+ carbocation.

38
Q

How reactive are alkylbenzenes compared to benzene? What effect does this have?

A

More reactive, resulting in polyalkylation.

39
Q

How reactive are phenylketones compared to benzene? What effect does this have?

A

Less reactive, resulting in only one substitution.

40
Q

What happens to a hydrogen in benzene after acylation?

A

It is substituted for an acyl group.

41
Q

What happens when you shake cyclohexene with bromine water?

A

Bromine adds across the double bond to give a dibromoalkane. The water turns colourless.

42
Q

Why won’t benzene decolourise bromine water?

A

Because it takes too much energy to break up the delocalised ring.

43
Q

What can you add to Br2 to make it a strong enough electrophile to attack the benzene ring?

A

FeBr4-

44
Q

Give the equation for the formation of the electrophile Br+.

A

Br2 + FeBr3 → Br+ + FeBr4-

45
Q

Give an example of an addition reaction that benzene does undergo?

A

Hydrogenation.

46
Q

Give the conditions required for the hydrogenation of benzene to form cyclohexane.

A

150ºC in the presence of a finely divided nickel catalyst.

47
Q

Explain why reactions with benzene are often very slow.

A

The delocalised ring makes it very stable.

48
Q

What makes methylbenzene and methoxybenzene more reactive than benzene?

A

They donate electrons to the delocalised ring, increasing its electron density.

49
Q

Why isn’t benzene often used in labs?

A

Because it is highly carcinogenic.

50
Q

What is phenol?

A

A benzene ring with an OH group substituted for one of the H atoms.

51
Q

What is the formula of phenol?

A

C6H5OH

52
Q

How does the OH group in phenol increase the electron density of the benzene ring?

A

One of the electron pairs in the p-orbitals of the oxygen overlaps with the delocalised ring of electrons in the benzene ring, increasing electron density.

53
Q

Which positions of phenol are bromine molecules most likely to attack?

A

2, 4 and 6.

54
Q

What happens when you shake phenol with orange bromine water?

A

It decolourises.

55
Q

What are the two products of adding Br2 to phenol?

A

2,4,6-tribromphenol and HBr.

56
Q

Describe 2,4,6-tribromophenol.

A

A white solid that smells of antiseptic.

57
Q

How can phenol be nitrated?

A

With dilute nitric acid.

58
Q

Name the two different products of mixing phenol with dilute nitric acid.

A

2-nitrophenol and 4-nitrophenol.

59
Q

Why is it much easier to nitrate phenol compared to benzene?

A

Because the OH group activates the benzene ring.

Edexcel Chemistry Unit 5 (5 decks)

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