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.
