25.2 Electrophillic substitution reactions of benzene Flashcards
What type of reactions does benzene undergo?
Substitution reactions
What does benzene typically react with?
Electrophiles by electrophilic substitution
What happens during electrophilic substitution reactions of benzene?
A hydrogen atom on the benzene ring is replaced by another atom or group of atoms
What is the general equation representing electrophilic substitution in benzene?
What is formed from the reaction of benzene with nitric acid?
Nitrobenzene, water
What are the reagents and conditions for the nitration of benzene?
concentrated sulfuric acid (H2SO4), heated at 50°C
What is the equation for the nitration of benzene?
What happens if the temperature rises above 50°C in the nitration of benzene?
Further substitution reactions occur, leading to production of dinitrobenzene
What is the equation showing further substitution in the nitration of benzene?
What is nitrobenzene useful for?
Important starting material in preparation of dyes, pharmaceuticals and pesticides. Starting material in preparation of paracetamol
What is the name of the mechanism for nitration of benzene?
Electrophilic substitution
What is the electrophile used for nitration of benzene?
Nitronium ion NO2+
How is a nitronium ion produced?
Reaction of nitric acid with concentrated H2SO4
Draw the mechanism for the nitration of benzene
When do halogens react with benzene?
When a halogen carrier is present
What are common halogen carriers?
AlCl3, FeCl3, AlBr3, FeBr3
How are halogen carriers generated?
In-situ (in a reaction vessel) from the metal and the halogen
What is the name of the mechanism for the bromination of benzene?
Electrophilic substitution
What are the reagents and conditions for the bromination of benzene?
Br2 halogen carrier (eg. FeBr3, AlBr3), RTP
What are the reagents and conditions for the chlorination of benzene?
Cl2, halogen carrier (eg. FeCl3, AlCl3), RTP
What is the equation for the bromination of benzene?
Draw the mechanism for the bromination of benzene
What happens in stage 2 of bromination of benzene?
Bromonium ion accepts a pair of electrons from the benzene ring to form a dative covalent bond
What is the electrophile for bromination of benzene
Br+
Why does benzene not react with non-polar bromine molecule?
Benzene is too stable
What is the equation for the chlorination of benzene?
Give an example of a halogen carried used for chlorination of benzene
FeCl3, AgCl3
What is formed from the chlorination of benzene?
chlorobenzene, HCl
What is formed from the bromination of benzene?
Bromobenzene, HBr
What is the alkylation of benzene?
The substitution of a hydrogen atom in the benzene ring by an alkyl group
What are the regents and conditions required for the alkylation of benzene (Friedel-Crafts alkylation)?
Haloalkane, in presence of AlCl3
What is the purpose of AlCl3 in alkylation of benzene (Friedel-Crafts alkylation)?
Acts as a halogen carried catalyst generating the electrophile
What does Friedel-Crafts alkylation do?
Increases the number of carbon atoms in a compound by forming carbon-carbon bonds
What is the equation for the reaction of benzene with chloroethane?
What is formed when benzene reacts with an acyl chloride?
Ketone, HCl
What is the name of the mechanism for Acylation of benzene?
Electrophilic substitution
What is the equation for the reaction of benzene with ethanoyl chloride?
What are the regents and conditions required for the acylation of benzene?
Acyl chloride, AlCl3
What is the mechanism for the reaction between cyclohexene and bromine?
What is the mechanism for the reaction of alkenes with bromine?
Electrophillic addition
What is the mechanism for the reaction of benzene with bromine?
Electrophilic substitution
What is the equation for the reaction of cyclohexene with bromine?
What is the difference between the reactions of alkenes and arenes?
-Alkenes decolourise bromine by an electrophilic addition reaction, bromine adds across the double bond
-Benzene does not react with bromine unless a halogen carrier catalyst is present, this is electrophilic substitution
Why is the reactivity of benzene low?
-Due to the delocalised 𝛑-electrons spread above and beyond the plane of carbon atoms. The electron density around two C atoms is less than a C=C in alkene
-When a non-polar molecule (eg.bromine) approaches benzene there is insufficient 𝛑-electron density to polarise the bromine molecule, preventing any reaction taking place
Why is the reactivity of alkenes high for reaction with Br2?
-The 𝛑-bond in alkene contains localised electrons above and below the plane of the two carbon atoms, this produces an area of high electron density
-The localised electrons in the 𝛑-bond induce a dipole in the non-polar bromine molecule making a one bromine slightly positive and another slightly negative
-The slightly positive bromine atom enables bromine to act as an electrophile
