CbD - Reactions of aromatic compounds Flashcards
What reactions do arenes undergo?
Electrophilic substitution reactions.
What electrophilic substitution reactions does benzene undergo?
- Bromination
- Chlorination
- Nitration
- Sulfonation
Friedel-craft reactions:
- Alkylation
- Acylation
Bromination
Benzene reacts with liquid bromine in the presence of a catalyst, such as iron filings or iron(III) bromide.
Heat is necessary.
The bromine is decolourised and fumes of HBr are given off.
What catalyst is usually used in bromination and why?
Iron filings or iron(III) bromide - to form the Br+ ion.
What conditions are needed for bromination?
Br2 is needed (usually from FeBr3 or formed in situ from Fe).
Reflux.
Chlorination
Benzene reacts in the same way as in bromination but in the presence of an aluminium chloride catalyst.
Anhydrous conditions.
What catalyst is usually used in chlorination and why?
Aluminium chloride (AlCl3) as it is used as the halogen carrier.
What conditions are needed for chlorination?
Cl2 from AlCl3
What is the role of the iron catalyst in bromination?
Br+ is generated from the action of the catalyst.
- Iron reacts with bromine to form iron(III) bromide, FeBr3.
2Fe + 3Br2 –> 2FeBr3 - The FeBr3 then interacts with a neighbouring bromine molecule - a Br- ion is removed from it to form the Br+ electrophile.
- Br+ then bonds to a carbon atom in the benzene ring and an H+ ion is lost from the ring.
- The H+ reacts with the FeBr4^- to produce HBr and regenerate the FeBr3 catalyst.
H+ + FeBr4^- –> HBr + FeBr3
What are halogen carriers?
Substances that are able to remove halogen atoms from molecules.
Substances that are able to remove halogen atoms from molecules are known as what?
Halogen carriers.
What equation shows how AlCl3 is a halogen carrier in chlorination?
Cl2 + AlCl3 –> AlCl4^- + Cl+
At what temperature does chlorination occur and why?
Room temperature because of the combination of reactive chlorine and the powerful halogen carrier aluminium chloride.
AlCl3 reacts violently with water, so the reaction must be carried out under anhydrous conditions.
What do halogen carriers do?
Polarise halogen molecules such as Br2 or Cl2
The positively charged end of the halogen molecule then acts as an electrophile and reacts with the benzene ring in the usual electrophilic substitution reaction.
What does an electrophile need to be/have to be able to attack the stable benzene ring?
A pretty strong positive charge - only positive ions or strongly polar compounds will do this.
How do halogen carriers make good electrophiles?
Most compounds aren’t polarised enough to be good electrophiles but some can be made into stronger electrophiles using a catalyst called a halogen carrier.
A halogen carrier accepts a lone pair of electrons from a polar molecule containing a halogen - the electrophile. As the lone pair of electrons is pulled away, the polarisation in the electrophile increases and sometimes a carbocation forms. This makes the electrophile loads stronger.
Examples of halogen carriers
Aluminium halides, iron halides and iron.
What is a carbocation?
An organic ion with a positively charged carbon atom.
What is the importance of halogenated benzene derivatives?
Benzene rings with halogens attached have been found to be important intermediates in synthetic routes.
The carbon atom attached to the halogen can be made to attack other carbon atoms and this ability to form carbon-carbon bonds is crucial if chemists wish to make large complex molecules from simpler starting materials.
Nitration
When you warm benzene with concentrated nitric and sulfuric acid, you get nitrobenzene.
Sulfuric acid is a catalyst - it helps to make the nitronium ion, NO2^+, which is the electrophile.
Need to keep temperature below 55 degrees.
Conditions required for nitration
HNO3
c. H2SO4, below 55 degrees.
Why is sulfuric acid needed in nitration?
It is a catalyst - helps in the formation of the nitronium ion, NO2^+, which is the electrophile.
What catalyst is needed in nitration and why?
Sulfuric acid, H2SO4.
Helps in the formation of the nitronium ion, NO2^+, which is the electrophile.
Electrophile in bromination
Br+
Electrophile in chlorination
Cl+
Electrophile in nitration
NO2^+
Electrophile in sulfonation
SO3
Sulfonation
When you boil benzene under reflux with concentrated sulfuric acid for several hours, or, warm benzene to 40 degrees with fuming sulfuric acid for half an hour, you make benzenesulfonic acid.
Need sulfur trioxide, SO3 as the electrophile.
Product of nitration
nitrobenzene
Product of sulfonation
benzenesulfonic acid
Conditions needed for sulfonation
c. H2SO4 as it contains SO3
Reflux
Why is sulfuric acid needed in sulfonation?
As it contains SO3 because it breaks up like this:
H2SO4 –> H2O + SO3
Why does fuming sulfuric acid warmed to 40 degrees with benzene produce benzenesulfonic acid?
Fuming sulfuric acid is basically just a lot of SO3 molecules dissolved in sulfuric acid.
Why is the reaction of warming benzene to 40 degrees with fuming sulfuric acid for half an hour ‘better’ than boiling benzene under reflux with concentrated sulfuric acid for several hours to produce benzenesulfonic acid?
Fuming sulfuric acid is basically just a lot of SO3 molecules dissolved in sulfuric acid. It is much richer in SO3 than c. H2SO4 which is why the reaction is quicker and needs less heating.
Example of a carbon-carbon bond forming reaction
Friedel-Crafts reaction
What type of reaction is it when AlCL3/FeCl3 are used as halogen carrier catalysts to help polarise organic molecules that contain halogens and cause them to substitute in a benzene ring?
Friedel-Crafts reaction
Alkylation
Benzene is heated under reflux with a chloroalkane and anhydrous aluminium chloride to form an alkylbenzene.
How can alkyl groups be substituted for H atoms on benzene rings?
By refluxing a chloroalkane and benzene with a halogen carrier such as AlCl3.
What does reflux mean?
Boiling reactants in a flask fitted with a condenser to stop them boiling away.
Conditions for alkylation
Chloroalkane
Anhydrous AlCl3
Reflux
Electrophile in alkylation
Carbocation formed from the chloroalkane and AlCl3
Why do compounds such as AlCl3 and FeBr3 help benzene to react with halogens?
AlCL3/FeCl3 are used as halogen carrier catalysts to help polarise organic molecules that contain halogens and cause them to substitute in a benzene ring.
They are friedel-crafts catalysts.
What part attaches to the benzene ring in alkylation and acylation?
The chlorine end.
The chlorine is removed then the chain/molecule is added.
Conditions for acylation
Acyl chloride
Anhydrous AlCl3
Reflux
Electrophile in acylation
Acylium ion formed from the acyl chloride and AlCl3
Acylation
When benzene is treated with an acyl chloride (or acid anhydride) and aluminium chloride under anhydrous conditions acylation occurs as an acyl group becomes attached to the benzene ring.
What does acylation produce?
Phenylketones (+ HCl).
What is the importance of Friedel-Crafts alkylations and acylations?
They are particularly useful to synthetic chemists, because they provide a way of adding carbon atoms to the benzene ring and building up side chains.
What is the importance of nitration reactions?
Inserting nitro groups into a benzene ring that is part of a chromophore will modify the properties of the chromophore. It will change the wavelength of light that it absorbs and the molecule will have a different colour.
What is the importance of sulfonation reactions?
Because the sulfonic acid group is able to form ionic salts, sulfonation provides a way of forming more soluble derivatives of aromatic compounds. Dyes are often made more soluble by adding sulfonic acid groups.
