Aromatic Chemistry
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Formula of benzene
C6H6
Structure of benzene
Cyclic Planar 4 valent electrons Each C bonded to 2 other C and a H
Structure of final lone electron in benzene
In a p-orbital which sticks out above and below planar ring. Combine to forma delocalised ring of electrons.
Why are all C-C bonds the same length?
Due to the delocalised electron structure
What is the length of C-C bonds in benzene?
139pm In the middle between a single and double bond length
What is the correct structure of benzene?
A cyclic struture with a ring of delocalised electrons within
What was Kekule’s theoretical stucture and name of benzene?
cyclohexane-1,3,5-triene 3 double bonds, 3 single bonds alternating
How can you prove Kekule’s benzene structure incorrect using enthalpy change of hydrogenation?
If cyclohexane (1 double bonds) has an enthalpy change of -120Kjmol-1, benzene should be -360Kjmol-1. However the experimental and actual value is -208Kjmol-1.
Why is benzene’s enthalpy of hydrogenation lower then cyclohexane-1,3,5-triene? What does this effect?
More enery is required to breaksn bands (and so is more exothermic) then expected. Therefore is a more stable molecule due to the delocalised electron structure.
What are arenes?
Molecules containing benzene rings
What are the 2 ways to name an arene?
‘-benzene’ as a suffix ‘Phenyl-‘ as a prefix
Why do arenes have electrophilic substitution reactions?
Benzene has a high electron density due to the delocalised ring of electrons which attracts electrophiles
Why do arenes not undergo electrophilic addition reactions?
Benzene is stable and and electrophilic addition reactions disrupt stable ring of electrons.
Why doesnt electrophilic substition disrupt the stability of the delocalised electron ring?
The H or function group on the benzene ring is instead substituted for the electrophile
What are the 2 mechanisms associated with arenes?
Nitration Friedal-Crafts Acylation
Where is benzene commonly used?
Pharmaceuticals and dye
What is freidal-craft acylation used for?
Allow benzene to react despite being stable
How does Friedal-craft acylation method make Benzene more unstable?
Adding an acyl group (RCO-) to a benzene molecule. This causes the structure to weaken and easier to modify.
Requirement for the electrophile in Friedal-craft acylation
Very strong postive charge
What catalyst is used in Friedal-craft acylation? Why?
AlCl3 or any halogen carrier Produces a much stronger electrophile
What is nitrating benzene useful as?
For dyes and fabric softener
How are dyes made from nitrobenzenes
Are reduced to form aromatic amines
Example of nitrobenze explosive
Trinitrotoluene
How do you make a strong electrophile for nitration
Reacting nitric acid and sulphuric acid: HNO3 + H2SO4 -> H2O + NO2+ + HSO4- Nitronium ion reacts with benzene to produce nitrobenzene