[25.2] electrophilic substitution reactions of benzene Flashcards
describe the structure and bonding of a benzene molecule [6]
- it is a planar ring of 6 carbon atoms
- each carbon forms a sigma bond with the 2 adjacent carbon atoms and single hydrogen atom
- the remaining 1 electron of each carbon is in a p-orbital that is at right angles to the plane of the ring
- adjacent p-orbtials overlap above and below the ring
- this overlapping creates a system of π bonds that spreads over the 6 carbons
- the 6 electrons in this π bond system are delocaised
what are the 3 steps for electrophilic addition?
- inducing a dipole
- attacking the electrophile
- attacking the carbocation
what happens in step 1 of electrophilic addition?
the high electron density in the alkene pi bond induces a temporary dipole in the halogen, making the halogen an electrophile
what happens in step 2 of electrophilic addition?
- the pi bond electrons attack the electrophile and form a bond
- the bond between halogen atoms break by heterolytic fission, generating a nucleophilic anion and a carbocation
what happens in step 3 of electrophilic addition?
the nucleophile attacks the cation to form the addition product
why won’t benzene react with bromine?
- due to the stability of the delocalised ring structure, benzene and its derivatives do not readily take part in the typical reactions of alkenes
- aromatics usually take part in substitution rather than addition reactions - this allows the product to retain the stability of the benzene ring
- in typical reactions, the benzene ring reacts with an electrophile to undergo electrophilic substitution
how does benzene react during electrophilic substitution?
- benzene will react with an electrophile if the electrophile is generated first. this is called electrophilic substitution
- one (or more) of the Hs is replaced by another group
- the energetically stable delocalised π-electron ring
what is the general equation for electrophilic substitution in benzene?
C₆H₆ + E⁺ -> C₆H₅E + H⁺
what catalysts are needed for the electrophilic substitution of benzene with Br₂, Cl₂, HNO₃, C₂H₅Cl and CH₃COCl?
- Br₂ - FeBr₃ or AlBr₃
- Cl₂- FeCl₃ or AlCl₃
- HNO₃ - H₂SO₄, 50 °C
- C₂H₅Cl - AlCl₃
- CH₃COCl - AlCl₃
why is 50 °C needed for ES of benzene with HNO₃?
- if the temperature of the reaction rises above 50 °C, further substitution reactions may occur leading to the production of dinitrobenzene
- this shows the importance of temperature control in the preparation of organic compounds
describe alkylation
- friedel-crafts reaction
- increases number of C-C bonds
what occurs in step 1 of ES?
- the electrophile is attracted to the ring due to he high electrol density caused by delocalised π-electrons
- a pair of electrons from the ring are donated to the electrophile to form a covalent bond
what occurs in step 2 of ES?
- an unstable intermediate forms with an incomplete ring structure and a positive charge
> the incomplete ring structure must be over halfway around the ring - the bond witht he hydrogen atom undergoes heterolytic fission to donate electrons to the ring structure
what occurs in step 3 of ES?
the stable ring structure returns leaving the substituted product and a hydrogen ion
why are halogen carriers needed?
- halogens, haloalkanes and alcyl chlorides do not react with benzene unless a catalyst (a halogen carrier is present)
- halogen carriers make the electrophiles
- the balogen carrier must also be regenerated at the end (eg. H⁺ + AlCl₄⁻ -> AlCl₃ + HCl)
