Chapter 25 - Aromatic Compounds

Question 1

What is benzene?

Answer 1

A cyclic hydrocarbon compound C6H6

Question 2

What are some properties of benzene?

Answer 2

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

Question 3

Define arene

Answer 3

An aromatic hydrocarbon containing one or more benzene rings

Question 4

Define aromatic

Answer 4

Any compound containing one or more benzene rings

Question 5

What are the three arguments against the kekule structure?

Answer 5

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.

Question 6

Describe the structure of Benzene

Answer 6

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.

Question 7

What are benzenes unusual properties?

Answer 7

Under normal conditions benzene does not:

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

Question 8

Why does benzene undergo substitution reaction rather than addition reactions?

Answer 8

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.

Question 9

What is aliphatic and alicyclic?

Answer 9

Aliphatic - organic molecules containing chains and/or rings

Alicyclic - molecules that have rings which are not benzene.

Question 10

How do you name compounds with benzene?

Answer 10

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

Question 11

What structures do you need to learn?

Answer 11

• Benzoic acid
• Phenol
• Methylbenzene
• phenylamine
• Benzaldehyde

Question 12

Define electrophile

Answer 12

Electron pair acceptor

Question 13

What does the general electrophilic substitution of benzene look like?

Answer 13

Question 14

What are the conditions for nitration of benzene?

Answer 14

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

Question 15

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

Answer 15

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

Question 16

Nitration of benzene equation?

Answer 16

Benzene + conc HNO3 –> nitrobenzene + H2O

Question 17

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

Answer 17

Overall equation products: Nitrobenzene + H2O

Question 18

How is the H2SO4 catalyst regenerated?

Answer 18

HSO4- + H+ -> H2SO4

(H+ from benzene)

Question 19

Outline acylation of benzene

Answer 19

Benzene + RCOCl with FeCl3
Forms Phenylalkanone + HCl

example
benzene + ethanoyl chloride –> phenylethanone + HCl

Acyl group = RCO

Question 20

How does a nitrating mixture produce an electrophile?

Answer 20

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

Question 21

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

Answer 21

Halogen carrier which acts as a catalyst

Question 22

Give two examples of chlorine carriers

Answer 22

AlCl3
FeCl3

Question 23

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

Answer 23

H-X

(X= a halogen)

Question 24

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

Answer 24

C6H6 + Br2 –> C6H5Br + HBr

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

Question 25

What is the bromination of benzene mechanism? 3 steps.

Answer 25

Question 26

Chlorination of benzene reaction?

Answer 26

C6H6 + Cl2 --> C6H5Cl + HCl

Question 27

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

Answer 27

Alkylation Acylation

Question 28

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

Answer 28

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

Question 29

Outline alkylation of benzene

Answer 29

Benzene + RCl in presence of FeCl3 Forms alkylbenzene + HCl

Question 30

What is acylation? What is required?

Answer 30

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

Question 31

Outline electrophilic aromatic substitution

Answer 31

Question 32

Compare the reactivity of benzene and alkenes. Explain.

Answer 32

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.

Question 33

what is the mechanism for electrophilic addition in alkenes?

Answer 33

Question 34

What is the structure of phenol?

Answer 34

C6H6O OH attached to benzene ring

Question 35

Are phenols acids or bases?

Answer 35

phenols are a weak acid

Question 36

Why are phenols less soluble in water than alcohols?

Answer 36

Due to the presence of a non-polar benzene ring

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

What are the products of phenol reacting with NaOH?

Answer 39

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

Question 40

Why does phenol not react with Na2CO3?

Answer 40

Too weak of an acid to react with weak base

Question 41

What is the test for phenols vs carboxylic acids?

Answer 41

- 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

Question 42

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

Answer 42

C6H5OH + 3Br2 --> H3OBr3 + 3HBr Bromination phenol decolourises bromine and the 2,4,6-tribromophenol forms a white precipitate

Question 43

What are the reaction conditions of bromination of phenol?

Answer 43

Room temperature No halogen carrier catalyst required

Question 44

What is the equation for the Nitration of phenol?

Answer 44

C6H5OH + HNO3 --> C6H4OHNO2 (2-nitrophenol) + H2O note: HNO3 must be DILUTE nitric acid

Question 45

Why does phenol react so readily with bromine?

Answer 45

OH group acts to increase reactivity of benzene ring

Question 46

What is the term for OH increasing benzenes' reactivity?

Answer 46

Activating group

Question 47

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

Answer 47

Group donates electron density into the benzene ring. More reactive than benzene. -NH2 -OH

Question 48

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

Answer 48

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.

Question 49

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

Answer 49

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

Question 50

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

Answer 50

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

Question 51

What 3 directing groups do you need to know?

Answer 51

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

Question 52

Outline the literal differences between phenol and nitrobenzene reacting with bromine

Answer 52

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.

Question 53

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

Answer 53

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.

Question 54

Why do alkenes react more readily with bromine than benzene.

Answer 54

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.