Aromatic Chemistry
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What is the molecular formula of benzene?
C6H6
Is benzene saturated or unsaturated?
Unsaturated
Why was the structure and bonding of benzene a puzzle for a long time?
Even though it is unsaturated, it doesn’t readily undergo addition reactions. All the carbon bonds were equivalent implying that all the carbon-carbon bonds were the same.
What does delocalisation mean?
The electrons are spread over more than two atoms.
How many covalent bonds does each carbon in benzene have?
3. 1 to hydrogen 2 to carbons The 4th electron of each carbon atom is in a p-orbital and there are 6 of these, one on each carbon. The p orbitals overlap and the electrons in them are delocalised. They form a region of electron density above and below the ring.
What is the importance of the delocalised ring of electrons?
Stabilises the benzene
How does benzene look at room temperature (298K)?
Colourless liquid
Why is the boiling point of benzene much higher than that of hexane?
Benzene’s flat hexagonal molecules pack together very well in the solid state. This makes them harder to separate and this must happen for the solid to melt.
What are the dangers of benzene?
It is carcinogenic (may cause cancer)
What do arenes mix with?
Other hydrocarbons and non-polar solvents.
What two factors are important to the reactivity of benzene?
The ring is an area of high electron density because of the delocalised bonding and thus is attacked by electrophiles. The aromatic ring is very stable it needs energy to be put in to break the ring before the system can be destroyed. This is called the delocalisation energy.
What reactions do most aromatic systems undergo?
Electrophilic substitution
How do arenes combust?
In air with flames that are noticeable smoke. This is because they have a high carbon:hydrogen ratio compared with alkanes. There is usually unburnt carbon remaining when they burn in the air and this produces soot. A smoky flame suggests an aromatic compound.
Why isn’t addition used with benzene?
It would require the input of the delocalisation energy to destroy the aromatic system.
How does the mechanism of the electrophilic substitution of benzene occur?
1) Arrow from delocalised ring to electrophile. 2) Part of the delocalised ring breaks and there is a positive charge in the ring. 3) Arrow from the C-H bond to the + in the delocalised ring. 4) Delocalised ring fully reforms and electrophile is added to the bond where hydrogen was removed.
What is nitration?
The substitution of a NO2 group for a hydrogen atom on the benzene ring.
What is the electrophile used in nitration?
NO2+
How is the electrophile NO2+ generated?
In the mixture of concentrated nitric acid and concentrated sulfuric acid. At 55 degrees celsius. H2SO4 + HNO3 -> H2NO3+ + HSO4- H2NO3+ -> NO2+ + H2O Overall: H2SO4 + HNO3 -> NO2 + H2O + HSO4-
What is NO2+ called?
Nitronium ion.
What is nitration an important step for?
The production of explosives like TNT. It is the first step in making aromatic amines which are used to make industrial dyes.
What is the catalyst in Friedel Crafts Acylation reactions?
Aluminium chloride.
Where does the RCO group come from?
Acyl chlorides
What is getting substituted in the Friedel Crafts Acylation reactions?
RCO
What is the electrophile in the acylation reactions?
RCO+