Benzene Flashcards
Explain the overlapping of orbitals in Benzene
Sideways overlap of p-orbitals
Leading to delocalised pi bond above and below plane of molecule
Describe the bonding in benzene
Planar molecule with a ring of 6 carbons
Each C is covalently bonded to 2 other carbons and 1 H
Sideways overlap of p-orbitals
Leading to delocalised pi bonds above and below plane of molecule
Why is benzene less ready to undergo addition reactions than alkenes?
Alkenes have localised C=C with a greater electron density to attract electrophiles more easily
Benzene has a stable delocalised system that would be disturbed/broken with addition
What is meant by delocalisation energy?
The difference between experimental ΔΗ for benzene and ΔΗ reaction according to the Kékule structure
OR
The extra stability in benzene structure due to electron delocalisation
The extra stability in Benzene is due to what?
It’s delocalisation energy due to the π system
Does Benzene or alkenes have a greater electron density?
Alkenes due to the localised double bond (C=C)
What does delocalisation mean?
Electron pairs are shared between three of more carbon atoms- eg in aromatic compounds such as benzene and naphalene
How is benzene formed?
For catalytic reforming of crude oil
What does electrophilic substitution involve in benzene?
The replacement of ‘ring hydrogen atoms’ by an electron deficient species or group (eg NO2+ or CH3+)
What are the conditions and reagents for the nitration of benzene?
Conc HNO3
Conc H2SO4
Reflux at 50-55°C (to avoid thermal decomposition)
Give the equation for the nitration of benzene
C6H6 —conc HNO3 conc H2SO4 reflux 50-55°C –> C6H5NO2 + H2O
How do you form nitro benzene?
By refluxing benzene with conc HNO3 & conc H2SO4 at 50-55°C
How is the nitronium cation (NO2+) formed in the nitration of benzene?
The stronger acid (H2SO4) forces HNO3 to act as a base and accept a proton, resulting in NO2+ and H2SO4- and water
What is NO2+?
A nitronium cation
A powerful electrophile
What conditions are needed to form bromo benzene?
Br2 FeBr3
In the absence of sunlight
Anhydrous
What reaction is FeBr3 involved in and what is its function?
In the halogenation of benzene
As a halogen carrier
Polarises Br-Br bond
How do you form chloro benzene?
From benzen with- AlCl3, Cl2 No UV Room temp Anhydrous
What does polarisation mean?
An unequal electron distribution in a covalent bond
In what reaction is AlCl3 used?
Chlorination of benzene- as a halogen carrier and catalyst
In the alkylation of Benzene
What are the conditions for the alkylation of benzene?
Ch3Cl AlCl3
Anhydrous
Warm
Give the equation (including conditions) of the alkylation of benzene
Benzene —CH3Cl AlCl3 anhydrous warm —> C6H5CH3 + HCl
Explain the bonding in chloro benzene
P/π orbital overlap
Benzene-Cl bond very strong so not easily substituted
What is the only substitution reaction that chloro benzene will undergo?
OH (hydrolysis)
NaOH (aq) 300°C increased pressure
How is chloro-nitro benzene formed?
From chloro benzene with the use of H2SO4/HNO3
How can chlorophenylmethane be formed from methylbenzene?
Cl2 UV
Free radical substitution
