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

what does delocalised mean?

Answer 6

delocalised electrons are shared between more than two atoms.

Question 7

What causes benzene to have its unexpected properties?

Answer 7

the neighbouring carbons p-orbitals overlap and form delocalised π orbitals above and below the carbon structure. this is an delocalised electron ring.

Question 8

what are benzenes unusual properties?

Answer 8

under normal conditions benzene does not:

1) decolourise bromine water
2) react with strong acids like HCL
3) react with halogens chlorine, bromine or iodine

Question 9

why does benzene undergo substitution reaction rather than addition reactions?

Answer 9

in an addition reaction the electrons in the pi cloud would have to bond with the atom/group being added but this would make it more unstable hence its not energetically favourable hence it doesn’t happen.

Question 10

Define electrophile

Answer 10

An atom or group of atoms that is attracted to an electron rich centre, where it accepts a pair of electrons to form a new covalent bond

Question 11

what does the general electrophilic substitution of benzene look like?

Answer 11

Question 12

What are the conditions for nitration of benzene?

Answer 12

Benzene reacts with conc HNO3 (Nitric acid) with conc H2SO4 ( sulphuric acid) acting as a catalyst at 50C

Question 13

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

Answer 13

In nitration if it gets hotter than 50 degrees more than one nitro (NO2) group may be added to the ring

Question 14

nitration of benzene equation?

Answer 14

Benzene reacts + conc HNO3 (Nitric acid) = nitrobenzene + H2O

Question 15

nitration of benzene mechanism? there are 3 steps…

Answer 15

Question 16

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

Answer 16

Halogen carrier which acts as a catalyst

Question 17

Give two examples of chlorine carriers

Answer 17

AlCl3
FeCl3

Question 18

What is formed as a byproduct of halogenation of benzene?

Answer 18

H-X

(X= a halogen)

Question 19

bromination of benzene equation? temperature?

Answer 19

C6H6 + Br2 = C6H5Br + HBr

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

Question 20

bromination of benzene mechanism?

Answer 20

Question 21

chlorination of benzene reaction?

Answer 21

C6H6 + Cl2 = C6H5Cl + HCl

Question 22

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

Answer 22

Alkylation
Acylation

Question 23

what is alkylation?

Answer 23

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 24

Outline alkylation of benzene

Answer 24

Benzene + RCl in presence FeCl3
Forms alkylbenzene + HCl

Alkylbenzenes are derivatives of benzene, in which one or more hydrogen atoms are replaced by alkyl groups of different sizes.

Question 25

what is acylation?

Answer 25

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

Question 26

Outline acylation of benzene

Answer 26

Benzene + RCOCl with FeCl3 Forms Phenylalkanone + HCl example benzene + ethanoyl chloride = phenylethanone + Hcl

Question 27

Name the mechanism for bromination of benzene

Answer 27

Electrophilic aromatic substitution

Question 28

Outline electrophilic aromatic substitution

Answer 28

Electrophile near benzene Arrow from benzene to electrophile Intermediate ion of benzene Arrow from H-C bond to ring Product and H+

Question 29

How does a nitrating mixture produce an electrophile?

Answer 29

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

Question 30

How is the H2SO4 catalyst regenerated?

Answer 30

HSO4- + H+ -\> H2SO4 | (H+ from benzene)

Question 31

How is an electrophile formed in reaction with a halogen carrier?

Answer 31

X2 + AlX3 -\> X+ + AlX4-

Question 32

How is the halogen carrier catalyst regenerated?

Answer 32

AlX4- + H+ -\> AlX3 + HX

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 are an acid or base?

Answer 35

phenols are a weak acid

Question 36

why are phenols less soluble in water than alcohols?

Answer 36

phenols are less soluble than alcohols in water 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 are the products of phenol reacting with NaOH?

Answer 38

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

Question 39

Why does phenol not react with Na2CO3?

Answer 39

Too weak of an acid to react with weak base

Question 40

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

Answer 40

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

Question 41

What are the reaction conditions of bromination of phenol?

Answer 41

Room temperature No halogen carrier catalyst required

Question 42

what is the equation for the Nitration of phenol?

Answer 42

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

Question 43

What is HNO3?

Answer 43

HNO3 is nitric acid

Question 44

Why does phenol react so readily with bromine?

Answer 44

OH group acts to increase reactivity of benzene ring

Question 45

What is the term for OH increasing benzenes' reactivity?

Answer 45

Activating group

Question 46

Define activation

Answer 46

An activating group increases the rate of electrophilic aromatic substitution compared benzene

Question 47

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

Answer 47

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

Question 48

name the electron withdrawing and electron donating groups?

Answer 48

electron withdrawing= 3 directing and electron donating and 2,4 directing.

Question 49

Define deactivation?

Answer 49

A deactivating group decreases the rate electrophilic aromatic substitution happens in comparison to benzene.

Question 50

Outline differences between phenol and nitrobenzene reacting with bromine

Answer 50

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 51

where are electron donating groups directing?

Answer 51

electron donating group are 2 and 4 directing.

Question 52

where are electron withdrawing groups directing groups?

Answer 52

electron withdrawing groups excluding halogens are 3 directing

Question 53

What do activating groups do with their electrons in benzene?

Answer 53

Donate electrons

Question 54

What do deactivating groups do with their electrons in benzene?

Answer 54

Withdraw electrons

Question 55

Where do the NH2 and OH groups direct on benzene?

Answer 55

2,4 and 6 positions

Question 56

Where does the NO2 group direct on benzene?

Answer 56

3 and 5 positions

Question 57

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

Answer 57

bromine and nitric acid react more readily with phenol than benzene. This is caused by 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.