Hydrocarbons and Halogen compounds Flashcards
Why are arenes very stable compounds
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
Why do arenes predominantly undergo substitution reactions over addition reactions?
due to aromatic stabilisation (the presence of delocalised π electrons)
What kind of reactions maintains the delocalised ring of electrons in arenes and how
-substitution reactions
-the aromaticity is restored by heterolytic cleavage of the C-H bond
What kind of reactions disrupt the aromatic stabilisation with example
-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
Series of reactions that arenes can undergo
*Substitution
*Nitration
*Friedel-Crafts alkylation
*Friedel-Crafts acylation
*Complete Oxidation
*Hydrogenation
Three steps in all electrophilic substitution
*Generating the electrophile
*Electrophilic attack on the benzene ring
*Regenerating aromaticity of the benzene ring
electrophilic attack on the benzene ring
-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
restoring aromaticity
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
Halogenation reactions are what type of reaction
Electrophilic Substitution
Arenes undergo substitution with halogens to form
halogenoarenes(aryl halides)
Benzene reacts with chlorine in the presence of
anhydrous aluminium chloride (AlCl3) catalyst
Benzene reacts with bromine in the presence of
anhydrous aluminium bromide (AlBr3) catalyst
In substitution reactions with arenes chlorine and bromine act as
an electrophile that replaces a hydrogen atom on the benzene ring
Why is a catalyst needed for arenes to react with halogens
due to the stability of the benzene structure
How electrophilesare generated during halogenation of arenes
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]-
Alkylarenes undergo halogenation on which positions
2 or 4 positions since alkyl groups are electron-donating groups that activate these positions
halogenation of alkylarenes result
in the formation of two products
(2 halogenoarene and 4 halogenoarene)
when excess halogen is used
multiple substitutions occur (2,4 halogenoarene, 2,6 halogenoarene and 2,4,6 halogenoarene)
nitration of arenes is what type reaction
Electrophilic Substitution
What occurs in the nitration of benzenes
a nitro (-NO2) group replaces a hydrogen atom on the arene
Reactants and conditions of nitration reaction
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
How electrophilesare generated during nitration of arenes
electrophile NO2+ ion is generated by reacting it with concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4)
HNO3 + H2SO4 → NO2+ 2HSO4- + H3O+
nitration of alkylarenes will occur in what position
2 or 4 positions due to the electron-donating properties of alkyls
Friedel-Crafts reactions are what type of reactions
Electrophilic Substitution
To use arenes as starting materials for the synthesis of other organic compounds
their structure needs to be changed to turn them into more reactive compounds
Friedel-Crafts reactions can be used to
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
Friedel-Crafts alkylation
-an alkyl chain is substituted into the benzene ring
-benzene ring is reacted with a chloroalkane in the presence of an AlCl3 catalyst
Friedel-Crafts acylation
-an acyl group is substituted into the benzene ring
-benzene ring is reacted with an acyl chloride in the presence of an AlCl3 catalyst
Why can alkyl arenes be oxidised
presence of the benzene ring in alkyl arenes affect the properties of the alkyl side-chain
product and conditions of oxidising alkyl side-chains
carboxylic acids when refluxed with alkaline potassium manganate(VII) and then acidified with dilute sulfuric acid (H2SO4)
hydrogenation of benzene is what type of reaction
Addition reaction
Hydrogenation of benzene (reactants and catalysts)
benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane
Depending on the reaction conditions, halogenation can occur
In the aromatic ring or in the side chain
When does halogenation of alkylarenes in the aromatic ring occur
when a halogen and anhydrous halogen carrier catalyst (such as AlBr3 or AlCl3) is used
Who is less reactive:
Halogenoalkanes or Aryl Halides
-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
Nucleophilic substitution reactions of halogenolkanes and aryl halides
-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.
Explain the unreactivity of halogenoarenes
-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
When does halogenation of alkylarenes in the side chain occur
reactant and conditions
-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
What type of reaction is halogenation of alkylarenes in the side chain
free-radical substitution reaction
What happens when excess halogen is used during the halogenation of alkylarenes in the side chain
all hydrogen atoms on the alkyl side-chain will be substituted by the halogen atoms
Substituents that are already present on the arenes can
-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
substituents in the arenes can either be
electron-withdrawing or electron-donating groups
Electron-withdrawing substituents
-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
Electron-donating substituents
-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
