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A-level Chemistry Yr 13 Mine > 25 - Aromatic Compounds > Flashcards

25 - Aromatic Compounds Flashcards

1
Q

What is the formula for benzene?

A

C6H6

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2
Q

What are the two ways of representing the benzene ring?

A

The Kekulé model and the delocalised model.

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3
Q

Explain what the Kekeulé model of Benzene looks like.

A

Made up of planar (flat) ring of carbon atoms with alternating single and double bonds between them, one hydrogen is attached to each carbon.

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4
Q

Explain the delocalised model for benzene.

A

The p-orbitals of all six carbons overlap to create a π-system.
The π-system is made up of two ring-shaped clouds of electrons above and below the plane of the six carbons.
All the bonds are the same length as all the bonds are the same.
The electrons are delocalised because they do not belong to a specific carbon atom, represented as a circle.

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5
Q

Why is the kekule structure not completely right from X-ray diffraction studies?

A

If Kekule structure was correct you’d except three bonds with length of 154 pm of a C—C bond and three bonds with a length of 134 pm of a C=C bond.

X-ray studies show that the carbon to carbon bonds in benzene have the same length of 140 pm, between a length of a single bond and double bond of carbon.

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6
Q

Explain enthalpy change of hydrogenation with an alkene.

A

If you react an alkene with hydrogen gas, two atoms of hydrogen add across a double bond, this called hydrogenation, and the enthalpy change of this is the enthalpy change of hydrogenation.

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7
Q

Explain the evidence for the delocalised model instead of the kekule model, using Enthalpy Changes of Hydrogenation.

A

Cyclohexene has one double bond, when hydrogenated the enthalpy change is -120 kj mol-1. So if benzene had three double bonds you would expect the enthalpy of hydrogenation to be (3x120=-360).

But the actual enthalpy of hydrogenation of benzene is -208 kJ mol-1, which is far less Exothermic.

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8
Q

What do we learn from benzene’s enthalpy change of hydrogenation being less Exothermic then expected from the kekule model?

A

More energy is required to break the bonds in benzene than would be need to be put in to break the kekule structure.

This difference indicates benzene is more stable than the kekule structure, due to the resistance to reaction. This is thought to be from the delocalised ring of electrons.

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9
Q

What is the experimental evidence for a delocalised rather than Kekulé?

A
  1. bond lengths
  2. enthalpy change of hydrogenation
  3. resistance to reaction
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10
Q

What is the experimental evidence of benzene’s resistance to reaction to prove to favour model of benzene?

A

Alkenes usually react easily with bromine water at room temperature, they decolourises by adding across the double bond. However with benzene it is a struggle to react with bromine water, you need hot benzene and ultraviolet light.

Reluctance of undergoing addition reaction proves this.

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11
Q

Why is it a struggle for benzene to react with bromine water?

A

Due to the π system in benzene of the delocalised electron rings above and below the plane of carbon atoms. The spread out negative charge makes the benzene ring stable, so benzene is reluctant to undergo addition reactions which would destroy the stable ring.

In Alkenes Alkenes the π bond in C=C is a area of high electron density which strongly attracts electrophiles but in benzene this attraction is reduced to the negative charge being spread.

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12
Q

Draw out nitrobenzene.

A

nitrobenzene

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13
Q

Name this molecule

A

Chlorobenzene

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14
Q

Name this molecule.

A

Ethylbenzene

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15
Q

Name this molecule.

A

Phenol

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16
Q

Draw phenylamine.

A
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17
Q

Name this molecule.

A

Benzoic Acid

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18
Q

Name this molecule.

A

Nitrobenzene

19
Q

Name this molecule.

A

Phenylamine

20
Q

Name this molecule.

A

Benzaldehyde

21
Q

If we have more than one functional group attached to the benzene ring how do you number the functional groups on the carbons?

A
  • If the functional groups are all the same, pick any group to start from and count round the way which gives the smallest numbers.
  • If the functional groups are different, start from which functional group gives the the molecule the suffix (e.g -OH group giving phenol) and count round the way that gives the smallest numbers.
22
Q

What groups are prefixes (in front) to benzene? What

A

Alkyl (CH3, C2H5 up until seven or more carbons)
Halogens (F,Br,Cl)
Nitro (NO2)

23
Q

When does benzene become a prefix and what is the prefix called?

A

When the benzene ring is attacked to an alkyl group with a functional group of an alkyl chain site seven or more carbon atoms.

The prefix of benzene in phenyl.

24
Q

Name this molecule.

A

2 - phenyloctane

25
Q

What are the exception for the prefix phenyl for benzene and name them?

A

Benzoic acid - (benzene carboxylic acid)
Phenylamine
Benzaldehyde

26
Q

Name this molecule.

A

4-Chloro-3,5-dimethylphenol

27
Q

Draw 2,4,6-trinitrophenol

A
28
Q

Name this molecule.

A

2-phenylphenol

29
Q

Draw out 2-benzyl-4-chlorophenol

A
30
Q

Draw out an phenylethanone

A
31
Q

Draw out phenyl ethanoate

A
32
Q

What type of reaction does benzene usually undergo?

A

Electrophilic Substitution Reaction

33
Q

What is a result of an electrophilic substitution reaction of benzene?

A

Hydrogen atom being substituted by an electrophile.

34
Q

Draw out the two step general mechanism for a benzene and electrophile, electrophilic substitution reactions?

A
35
Q

Why do you need such a strong electrophile for benzene?

A

Need a strong positive charge to be able to attack the stable benzene ring.

36
Q

What do you need to make a strong electrophile to attack benzene?

A

A halogen carrier like AlCl3, FeBr3

37
Q

Explain how a halogen carrier carrier to make a good electrophile.

A

A halogen carrier accepts a lone pair of electrons from a halogen atom on an electrophile. As the lone pair of electrons is pulled away, the polarisation in the molecule and forms carbocation.

38
Q

Draw out the equation and mechanism for halogen carriers making an electrophile.

A
39
Q

How does the nitration of Benzene happen?

A

When you warm benzene with concentrated nitric acid and concentrated sulfuric acid, you get nitrobenzene

40
Q

How does a nitration of benzene work?

A

A

41
Q

Draw out the three step process with mechanisms for the nitration of benzene.

A
42
Q

Write out the equation for creating the NO2+ ion for the nitration of benzene by reacting Concentrated Nitric Acid and Concentrated Sulfuric acid.

A
43
Q

What is the catalyst in the nitration of benzene and how is it regenerated?

A

Sulfuric Acid, H2SO4

An HSO4- is created in the reaction of nitric acid and sulfuric acid to create the NO2+ ion.
This HSO4- molecule will then reacts with the H+ displaced by the NO2 group on the benzene to form H2SO4 again.

A-level Chemistry Yr 13 Mine (12 decks)

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