Aromatic compounds 1 Flashcards
What is an arene?
A molecule that contains at least one benzene ring. In the past they were called aromatic compounds because of their distinctive smells.
Benzene molecular formula and shape
planar molecule with 6 carbons arranged in a hexagonal ring
120 bond angle
Suggested that benzene molecules contain a ring of six carbons with alternating single and double bonds
The modern structure of benzene
All carbon bonds the same length, between the length of c-c and c=c bonds
6 carbon atoms joined by single bonds with delocalized electrons above and below the plane
Each carbon atom contributes one electron to form a bond that is spread over all six C atoms, these electrons are delocalised
Benzene is a planar (flat) molecule, allowing the p orbitals to overlap, forming pi bonds
How does delocalization of the electrons in benzene affect its stability?!?
The delocalization of the electrons in the pi bonds means that benzene is more stable than might be expected. Benzene doesn’t readily undergo addition reactions as this would require lots of energy to break up the delocalized electron system.
Instead, most of the reactions of benzene are substitution reactions. In these reactions a new group replaces a hydrogen atom and the delocalized electron system is maintained
What does the enthalpy change of hydrogenation of benzene show??
That the kekule module is not correct. If it was the enthalpy change for the hydrogenation of benzene should be 3 times that of cyclohexene but it isn’t.
This is because benzene does not contain 3 doubles bonds, it instead has a delocalised electron system which makes it more stable. This means more energy is required to break the delocalized electron system than is required to break 3 double bonds. As a result the enthalpy change for reaction is much lower than expected.
Electrophilic subsitution of benzene:
What is an electrophile?
Why is benzene stable?
Benzene undergoes electrophilic subsitution reactions with electrophiles such as NO2+, Cl+ and CH3+.
An electrophile is a species that accepts a lone pair of electrons.
Benzenes delocalized electron system makes it very stable. By undergoing substitution rather than addition reactions benzene retains the stability associated with the delocalized system.
2 steps of electrophilic substitution for benzene
1) An electrophile, E+, is added to the benzene ring
- A dative covalent bond is formed between a carbon atom in the benzene molecule and the electrophile
2) A hydrogen ion is eliminated
- the bond between the carbon atom in the benzene ring and the hydrogen atom is broken. A hydrogen ion is released and the pair of electrons is used to restore the delocalized electron system.
The nitration of benzene
Nitrobenzene forms from the nitration of benzene. During nitratition a nitro group, N02, is substititued onto the benzene molecule. The overall equation for the reaction is:
C6H6 + NO2(+) —> C6H5N02 + H(+)
1) Sulfuric acid protonates the nitric acid
2) the protonised nitric acid decomposes to form a nitronium ion (N02(+))
3) the nitronium ion electrophile reacts with benzene, the reaction takes place at about 50 degrees, if higher temp. then further nitration may occur
4) the sulfuric acid catalyst is regenerated
overall, a nitro group is added to the benzene and a hyrdogen is lost
What is a nitronium ion?
N02(+). It is an electrophile which is generated by using a nitrating mixture.
What is a nitrating mixture?
A mixture of conc. sulfuric acid and conc. nitric acid
Equation for sulfuric acid protonating nitric acid
H2S04 + HNO3 —> HSO4(-) +H2NO3(+)
Equation for nitric acid decomposing to form a nitronium ion
H2NO3(+) + H2SO4 —> NO2(+) + H3O(+) HSO4(-)
the nitronium electrphile then reacts with benzene. The reaction takes placce at about 50 degress.
Equation for regeneration of sulfuric acid catalyst
H(+) + HSO4(-) —> H2SO4