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Chemistry > Chapter 25 - Aromatic Compounds > Flashcards

Chapter 25 - Aromatic Compounds Flashcards

Introducing benzene, Electrophilic substitution reactions of benzene, The chemistry of phenol, Directing groups.

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1
Q

What is benzene?

A

A cyclic hydrocarbon compound C6H6

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2
Q

What are some properties of benzene?

A

benzene is a sweet smelling, colourless, highly flammable liquid and is a carcinogen (cancer causing),

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3
Q

Define arene

A

An aromatic hydrocarbon containing one or more benzene rings

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4
Q

Define aromatic

A

Any compound containing one or more benzene rings

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5
Q

What are the three arguments against the kekule structure?

A

1) benzenes low chemical reactivity - doesn’t decolourise bromine like an alkene
2) carbon lengths in benzene are all the same but c=c is different length than c-c
3) benzenes enthalpy of hydrogenation is lower than expected. so its more stable than kekule structure.

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6
Q

Describe the structure of Benzene

A

The neighbouring carbons p-orbitals overlap and form a delocalised π system ABOVE and BELOW the carbon structure. The electrons spread throughout the entire ring.

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7
Q

What are benzenes unusual properties?

A

Under normal conditions benzene does not:

1) decolourise bromine water
2) react with strong acids like HCl
3) react with halogens

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8
Q

Why does benzene undergo substitution reaction rather than addition reactions?

A

In an addition reaction: the electrons in the pi-system would have to bond with the atom/group being added but this would make it more unstable.

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9
Q

What is aliphatic and alicyclic?

A

Aliphatic - organic molecules containing chains and/or rings

Alicyclic - molecules that have rings which are not benzene.

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10
Q

How do you name compounds with benzene?

A

Benzene suffix when ring contains:
- alkyl group (<7 Cs)
- Halogen
- Nitro (NO2)

Phenyl prefix when ring contains:
- alkyl group (>7 Cs)
- functional group (e.g. carbonyl, alcohol etc)
e.g. 2-phenylethanol

When alcohol and carboxylic acid, prefix -hydroxy.
e.g. 2-hydroxybenzoic acid

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11
Q

What structures do you need to learn?

A
  • Benzoic acid
  • Phenol
  • Methylbenzene
  • phenylamine
  • Benzaldehyde
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12
Q

Define electrophile

A

Electron pair acceptor

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13
Q

What does the general electrophilic substitution of benzene look like?

A
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14
Q

What are the conditions for nitration of benzene?

A

Conc HNO3 (reagent)
Conc H2SO4 (catalyst)
50C

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15
Q

What happens in nitration if it gets hotter than 50 degrees?

A

More than one nitro (NO2) group may be added to the ring

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16
Q

Nitration of benzene equation?

A

Benzene + conc HNO3 –> nitrobenzene + H2O

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17
Q

What is the Nitration of benzene mechanism? there are 3 steps…

A

Overall equation products: Nitrobenzene + H2O

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18
Q

How is the H2SO4 catalyst regenerated?

A

HSO4- + H+ -> H2SO4

(H+ from benzene)

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19
Q

Outline acylation of benzene

A

Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl

example
benzene + ethanoyl chloride –> phenylethanone + HCl

Acyl group = RCO

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20
Q

How does a nitrating mixture produce an electrophile?

A

H2SO4 + HNO3 -> NO2+ + HSO4- + H2O
Nitronium ion (NO2)+ is electrophile

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21
Q

As halogens cannot react on their own with benzene, what is required?

A

Halogen carrier which acts as a catalyst

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22
Q

Give two examples of chlorine carriers

A

AlCl3
FeCl3

23
Q

What is formed as a by-product of halogenation of benzene?

A

H-X

(X= a halogen)

24
Q

What is the equation for bromination of benzene? Temperature/conditions?

A

C6H6 + Br2 –> C6H5Br + HBr

happens at room temprature and in the presence of a halogen carrier

25
Q

What is the bromination of benzene mechanism? 3 steps.

A
26
Q

Chlorination of benzene reaction?

A

C6H6 + Cl2 –> C6H5Cl + HCl

27
Q

What are the two types of Friedel Crafts reactions of benzene?

A

Alkylation
Acylation

28
Q

What is alkylation?
Conditions/what is required?
Equation example?

A

In alkylation, a haloalkane is reacted with an aromatic compound to introduce an alkyl group to the benzene group. a halogen carrier is required.

C6H6 + C2H5Cl –> C6H5C2H5 + HCl

29
Q

Outline alkylation of benzene

A

Benzene + RCl in presence of FeCl3
Forms alkylbenzene + HCl

30
Q

What is acylation?
What is required?

A

In acylation an acyl chloride (RCOCl) is reacted with an aromatic compound to form an aromatic ketone. A halogen carrier is required.

31
Q

Outline electrophilic aromatic substitution

A
32
Q

Compare the reactivity of benzene and alkenes. Explain.

A

Benzene is much less reactive than alkenes.

e.g. alkenes react readily with bromine but benzene needs a catalyst (halogen carrier).

  • The pi-bond in alkenes is localised so has a high electron density wheras the pi-system in benzene is delocalised with a lower electron density so harder to polarise Br2 etc.
33
Q

what is the mechanism for electrophilic addition in alkenes?

A
34
Q

What is the structure of phenol?

A

C6H6O
OH attached to benzene ring

35
Q

Are phenols acids or bases?

A

phenols are a weak acid

36
Q

Why are phenols less soluble in water than alcohols?

A

Due to the presence of a non-polar benzene ring

37
Q

Is phenol more or less acidic than carboxylic acids and alcohols?

A

Phenol is more acidic than alcohol but less acidic than a carboxylic acid.

38
Q

What do alcohols, phenol and carboxylic acid react with? (bases)

A

Alcohols do not react with any bases.
Alcohol = least acidic

Phenol reacts with strong bases but not weak.
Phenol = more acidic

Carboxylic acid reacts with both strong and weak bases.
COOH = most acidic

39
Q

What are the products of phenol reacting with NaOH?

A

phenol + NaOH = Sodium phenoxide( C6H5O-Na+) + Water

40
Q

Why does phenol not react with Na2CO3?

A

Too weak of an acid to react with weak base

41
Q

What is the test for phenols vs carboxylic acids?

A
  • Both will turn an indicator orange
  • Add Na2CO3 (a weak base). It reacts with the carboxylic acid to produce CO2 (bubbles) but phenols do not.

2 CH3COOH + Na2CO3 –> 2CH3COONa + CO2 + H2O

42
Q

What is the reaction of phenol with excess Br2? what is seen?

A

C6H5OH + 3Br2 –> H3OBr3 + 3HBr

Bromination

phenol decolourises bromine and the 2,4,6-tribromophenol forms a white precipitate

43
Q

What are the reaction conditions of bromination of phenol?

A

Room temperature
No halogen carrier catalyst required

44
Q

What is the equation for the Nitration of phenol?

A

C6H5OH + HNO3 –> C6H4OHNO2 (2-nitrophenol) + H2O

note: HNO3 must be DILUTE nitric acid

45
Q

Why does phenol react so readily with bromine?

A

OH group acts to increase reactivity of benzene ring

46
Q

What is the term for OH increasing benzenes’ reactivity?

A

Activating group

47
Q

What is an activating group? What are the activating groups you need to know?

A

Group donates electron density into the benzene ring. More reactive than benzene.

-NH2
-OH

48
Q

What is a deactivating group? Example you need to know?

A

Group that withdraws electron density from the benzene ring. Less reactive than benzene.

-NO2

(COOH and Halogens are also deactivating).

So benzoic acid less reactive than benzene.

49
Q

Compare the reactivity of benzene vs phenol for bromination and Nitration?

A

Bromine and nitric acid react more readily with phenol than benzene.

50
Q

What 2 and 4 directing groups do you need to know?

A

Activating groups:
- OH
- NH2
- R (alkyl)
- Halogens

51
Q

What 3 directing groups do you need to know?

A

Deactivating groups:
- NO2
- COOH
- CHO (ketone)

52
Q

Outline the literal differences between phenol and nitrobenzene reacting with bromine

A

Phenol is rapid reaction, nitro is slower.
Nitro needs halogen carrier
Phenol at room temp, nitro needs high temp.
Phenol can have multiple substitutions, nitro can have only one.

53
Q

Why do bromine and nitric acid react more readily with phenol than benzene?

A

The lone pair of electrons from the oxygen p-orbital of the OH group is being donated into the π system of phenol. This increases the electron density of phenol. This means electrophiles are more strongly attracted to phenol hence the aromatic ring is more susceptible to attack.

54
Q

Why do alkenes react more readily with bromine than benzene.

A

Alkenes have a high electron density because they have a localised pi-bond. This electron density highly polarises Br2.

Benzene has a lower electrong density (due to delocalised pi-bond) so is not high enough to polarise Br2.

Chemistry (22 decks)

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