CD.4 Flashcards
Electrophilic substitution reactions
Because benzene molecules contain areas of high electron density, it reacts with which type of molecules?
Electrophiles
What type of mechanism is it when alkenes react with electrophiles?
Electrophilic addition
What type of molecule is the product when alkene undergo addition reaction with electrophile?
Saturated molecule
Why are reaction mechanisms different for alkene and benzene with electrophiles?
Because benzene contains a system of delocalised electrons making it very stable.
Why won’t benzene undergo addition reaction with electrophiles?
Because a ring with two double bonds is higher in energy than just the benzene ring, therefore it is energetically unfavourable to disrupt the delocalisation in an addition reaction.
What type of mechanism is it when benzene reacts with electrophiles?
Electrophilic substitution.
Why would benzene undergo substitution with electrophiles?
Because at the end of the substitution, the stable delocalisation of benzene is kept, only a hydrogen is replaced by the electrophile group.
What is the first step of the mechanism of electrophilic substitution of benzene?
The delocalised electrons in benzene attacks the electrophile, one pair of double bond opens up to allow the electrophile to join. As a result, the ring contains two double bonds, one carbon is attached to the electrophile and a hydrogen, and the other is a carbocation. The delocalisation is disrupted.
Is the first step of the mechanism fast or slow?
Slow, because the stable benzene doesn’t like the delocalisation to be disrupted, setting off this step requires energy, therefore the presence of a catalyst and heat is often required.
What is the second step of the mechanism of electrophilic substitution of benzene?
The hydrogen donates its bonding electrons to reform the delocalised system and leaves as a hydrogen ion.
Is the second step of the mechanism fast or slow?
Fast, because the benzene would preferred the delocalisation to be reformed therefore this step happens fairly spontaneously.
What is the reaction known as when benzene reacts with liquid bromine in the presence of a catalyst?
Bromination
What catalyst is often used in bromination?
Iron filling or iron(III) bromide, FeBr3.
What condition is required for bromination?
Heat under reflux.
What product is produced other than bromobenzene in bromination?
Hydrogen bromide, which is a gas.
What acts as the electrophile in bromination?
Br+ ions.
How is the electrophile Br+ generated in bromination from bromine molecules?
The catalyst, iron(III) bromide, FeBr3, removes a Br- ion from a bromine molecule to for FeBr4-, leaving a Br+ ion, which is better electrophile than Br2.
How is the FeBr4- regenerated back to iron(III) bromide, the original catalyst?
The H+ ion substituted of the benzene ring reacts with FeBr4- to produce HBr and iron(III) bromide.
What is the reaction known as when benzene reacts with chlorine in the presence of a catalyst?
Chlorination
What catalyst is often used in chlorination?
Aluminium chloride, AlCl3.
What condition is required for chlorination and why?
Anhydrous conditions, because the catalyst aluminium chloride reacts violently with water
Why is heat not required for chlorination?
Because the chlorine is very reactive and AlCl3 is a powerful catalyst therefore no heat is required to provide energy for the first step of the mechanism.
What product is produced other than chlorobenzene in chlorination?
Hydrogen chloride, which is a gas.
What acts as the electrophile in chlorination?
Cl+
Molecules like FeBr3 and AlCl3 which are able to catalyse electrophilic reaction are known as what?
Halogen carriers.
What is the reaction known as when an alkyl group is introduced onto a benzene ring?
Alkylation
What is reacted with benzene in alkylation?
alkyl chloride, or other alkyl halide molecules.
What catalyst is used in alkylation?
Anhydrous aluminium chloride, or similar halogen carriers to suit the alkyl halide.
What condition is required for alkylation?
Heat under reflux.
What products are formed in alkylation?
Alkyl benzene, and a hydrogen halide, depending on what alkyl halide was used.
What is the reaction known as when an acyl group is introduced onto a benzene ring?
Acylation
What is reacted with benzene in acylation?
Acyl chloride or acid anhydride.
What catalyst is used in acylation?
Anhydrous aluminium chloride.
What condition is required for acylation?
Heat under reflux.
What products are formed in acylation
Acyl benzene, and hydrogen chloride.
What are Friedel-Crafts reactions (alkylation and acylation) important for synthesis?
They provide a way of forming new C-C bonds and to add side groups to benzene rings.
