Hydrocarbons and Halogen compounds

Question 1

Why are arenes very stable compounds

Answer 1

the delocalisation of π electrons in the ring spreads out the negative charge of the molecule instead of confining it to a small area so it’s not very reactive

Question 2

Why do arenes predominantly undergo substitution reactions over addition reactions?

Answer 2

due to aromatic stabilisation (the presence of delocalised π electrons)

Question 3

What kind of reactions maintains the delocalised ring of electrons in arenes and how

Answer 3

-substitution reactions
-the aromaticity is restored by heterolytic cleavage of the C-H bond

Question 4

What kind of reactions disrupt the aromatic stabilisation with example

Answer 4

-in addition reactions, the aromaticity is not restored and is in some cases wholly lost
-for example in the hydrogenation of benzene a less energetically cyclohexane is produced and aromaticity is completely lost

Question 5

Series of reactions that arenes can undergo

Answer 5

*Substitution
*Nitration
*Friedel-Crafts alkylation
*Friedel-Crafts acylation
*Complete Oxidation
*Hydrogenation

Question 6

Three steps in all electrophilic substitution

Answer 6

*Generating the electrophile
*Electrophilic attack on the benzene ring
*Regenerating aromaticity of the benzene ring

Question 7

electrophilic attack on the benzene ring

Answer 7

-A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond
-This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms

Question 8

restoring aromaticity

Answer 8

aromaticity is restored by heterolytic cleavage of the C-H bond so that bond electrons in this bond go into the benzene π bonding system

Question 9

Halogenation reactions are what type of reaction

Answer 9

Electrophilic Substitution

Question 10

Arenes undergo substitution with halogens to form

Answer 10

halogenoarenes(aryl halides)

Question 11

Benzene reacts with chlorine in the presence of

Answer 11

anhydrous aluminium chloride (AlCl3) catalyst

Question 12

Benzene reacts with bromine in the presence of

Answer 12

anhydrous aluminium bromide (AlBr3) catalyst

Question 13

In substitution reactions with arenes chlorine and bromine act as

Answer 13

an electrophile that replaces a hydrogen atom on the benzene ring

Question 14

Why is a catalyst needed for arenes to react with halogens

Answer 14

due to the stability of the benzene structure

Question 15

How electrophilesare generated during halogenation of arenes

Answer 15

halogen is reacted with a halogen carrier so the halogen molecules form a dative bond with the halogen carrier by donating a lone pair of electrons from one of its halogen atoms into an empty 3p orbital of the halogen carrier
For example:
Cl2 + AlCl3 → Cl+ + [AlCl4]-

Question 16

Alkylarenes undergo halogenation on which positions

Answer 16

2 or 4 positions since alkyl groups are electron-donating groups that activate these positions

Question 17

halogenation of alkylarenes result

Answer 17

in the formation of two products
(2 halogenoarene and 4 halogenoarene)

Question 18

when excess halogen is used

Answer 18

multiple substitutions occur (2,4 halogenoarene, 2,6 halogenoarene and 2,4,6 halogenoarene)

Question 19

nitration of arenes is what type reaction

Answer 19

Electrophilic Substitution

Question 20

What occurs in the nitration of benzenes

Answer 20

a nitro (-NO2) group replaces a hydrogen atom on the arene

Question 21

Reactants and conditions of nitration reaction

Answer 21

benzene is reacted with a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) at a temperature between 25 and 60 degrees Celsius

Question 22

How electrophilesare generated during nitration of arenes

Answer 22

electrophile NO2+ ion is generated by reacting it with concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)
HNO3 + H2SO4 → NO2+ 2HSO4- + H3O+

Question 23

nitration of alkylarenes will occur in what position

Answer 23

2 or 4 positions due to the electron-donating properties of alkyls

Question 24

Friedel-Crafts reactions are what type of reactions

Answer 24

Electrophilic Substitution

Question 25

To use arenes as starting materials for the synthesis of other organic compounds

Answer 25

their structure needs to be changed to turn them into more reactive compounds

Question 26

Friedel-Crafts reactions can be used to

Answer 26

substitute a hydrogen atom in the benzene ring for an alkyl group (Friedel-Crafts alkylation) or an acyl group (Friedel-Crafts acylation)

thus turning benzene into more reactive organic cmpounds

Question 27

Friedel-Crafts alkylation

Answer 27

-an alkyl chain is substituted into the benzene ring
-benzene ring is reacted with a chloroalkane in the presence of an AlCl3 catalyst

Question 28

Friedel-Crafts acylation

Answer 28

-an acyl group is substituted into the benzene ring
-benzene ring is reacted with an acyl chloride in the presence of an AlCl3 catalyst

Question 29

Why can alkyl arenes be oxidised

Answer 29

presence of the benzene ring in alkyl arenes affect the properties of the alkyl side-chain

Question 30

product and conditions of oxidising alkyl side-chains

Answer 30

carboxylic acids when refluxed with alkaline potassium manganate(VII) and then acidified with dilute sulfuric acid (H2SO4)

Question 31

hydrogenation of benzene is what type of reaction

Answer 31

Addition reaction

Question 32

Hydrogenation of benzene (reactants and catalysts)

Answer 32

benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane

Question 33

Depending on the reaction conditions, halogenation can occur

Answer 33

In the aromatic ring or in the side chain

Question 34

When does halogenation of alkylarenes in the aromatic ring occur

Answer 34

when a halogen and anhydrous halogen carrier catalyst (such as AlBr3 or AlCl3) is used

Question 35

Who is less reactive:
Halogenoalkanes or Aryl Halides

Answer 35

-Aryl halides are less reactive because the carbon-halogen bond in aryl halides is stronger
-This is due to the partial overlap of the lone pairs on the halogen atom with the π system in the benzene ring
-The carbon-halogen bond, therefore, has a partial double bond character

Question 36

Nucleophilic substitution reactions of halogenolkanes and aryl halides

Answer 36

-Halogenoalkanes can take part in nucleophilic substitution reactions. A nucleophile, such as a hydroxide (OH-) ion, will attack the slightly positive carbon atom. A covalent bond is formed between that carbon atom and the nucleophile which causes the carbon-halogen bond to break. Overall, the halogen is replaced by the nucleophile
-Halogenoarenes do not readily undergo nucleophilic substitution reactions. Only under extremely harsh conditions, such as temperatures of 200 degrees Celsius and a pressure of 200 atmospheres, will the chlorine in chlorobenzene get replaced by a nucleophile such as a hydroxide (OH-) ion. This is because the carbon-chlorine bond is very strong and cannot be easily broken.

Question 37

Explain the unreactivity of halogenoarenes

Answer 37

-Unreactivity can be explained by the delocalisation of a lone pair on the halogen over the benzene
-This causes additional stabilisation of the system and strengthens the carbon-halogen bond, which affects the reactions that halogenoarenes will undergo
-It gets harder to break the carbon-halogen bond in halogenoarenes, which decreases reactivity

Question 38

When does halogenation of alkylarenes in the side chain occur

reactant and conditions

Answer 38

-when the halogen is passed into boiling alkylarene in the presence of ultraviolet (UV) light
-no substitution into the benzene ring occurs under these conditions

Question 39

What type of reaction is halogenation of alkylarenes in the side chain

Answer 39

free-radical substitution reaction

Question 40

What happens when excess halogen is used during the halogenation of alkylarenes in the side chain

Answer 40

all hydrogen atoms on the alkyl side-chain will be substituted by the halogen atoms

Question 41

Substituents that are already present on the arenes can

Answer 41

-Affect where the substitution of the hydrogen atom on the arene takes place
-These groups are said to direct substitution reactions to different ring positions

Question 42

substituents in the arenes can either be

Answer 42

electron-withdrawing or electron-donating groups

Question 43

Electron-withdrawing substituents

Answer 43

-Remove electron density from the π system in the benzene ring making it less reactive
-These groups deactivate attacks by electrophiles and direct the incoming electrophile to attack the 3 and/or 5 positions

Question 44

Electron-donating substituents

Answer 44

-Donate electron density into the π system of the benzene ring making it more reactive
-These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2, 4 and/or 6 positions