Aromatics (Benzene) Flashcards
What is the molecular formula of
Benzene ?
C6H6
What structure
of Benzene is readily accepeted?
Resonance Structure
What structure
of Benzene is disputed?
because it might suggest that benzene behaves like an alkene
Kekulé’s Structure
What are the problems with
Kekulé’s Structure?
because it might suggest that benzene behaves like an alkene
- Bond Length disparities expected (due to presence of both C-C and C=C bonds)
- High reactivity expected
- Irregular molecular shape expected (Benzene is planar)
These are seen to not be the case
What is
the hybdrisation of all the carbons in the benzene molecule?
sp2 hybridised
In alkenes, only the carbon atoms in the C=C are sp2 hybridised
What happens to
the unhybridised pz orbital in the 6 Benzene carbons?
The pz orbitals overlap at the the top and bottom, creating a ring-like shape
Recall that the p orbitals have a dumb-bell shape, hence the top and bottom overlap
Hence,
what are the bond angles about each carbon centre?
120°
Benzene is a planar molecule
What is
the carbon-to-carbon bond length in Benzene?
139 pm
The bond length is shorter than the C-C bond but not as short as the C=C bond. The smaller the bond length, the stronger the bond.
Is Benzene very reactive?
No
What is the reason for the (low) reactivity of Benzene?
It has a resonance stability
Means that the pi electrons are not readily available
What are resonance structures?
These are structures that are used to descibe the presence of delocalised e- in compounds
Due to the delocalised e- the compounds can take on more than one form depending on their rel. positioning.
Why do resonance structures form?
When there is more than one way to represent the double bonds or lone pairs in a molecule
due to the presence of delocalised electrons
How does resonance dispute Kekulé’s structure
Under Kekulé’s structure, ‘Benzene’ can either be represented as 1,3,5-hexatriene or 2,4,6-hexatriene. Because these are 2 diff. structures of the same thing, Benzene is given a resonance structure
Which is more reactive:
Benzene or a(n) (cyclo)alkene ?
The (cyclo)alkene
What is the reason for this difference in reactivity?:
between benzene and a(n) (cyclo)alkene ?
In the benzene ring, the 6 pi electrons (1 donated by each of the 6 carbons) are delocalised, hence its resonance structure. In (cyclo)alkenes, however, the pi electrons are localised in the C=C bond
Localisation increases electron density
Which is correct when speaking about delocalised electrons?:
They are:
* at lower energy levels
* at higher energy levels
They are at lower energy levels
This makes them more stable. The lower the energy level of an electron, the closer it is to the nucleus of the atom
What kind of reactions
does Benzene undergo?
Electrophilic Substitution Reactions
What is the movement of electrons
in the first reacting step of electrophilic substitution reactions in Benzene?
After the electrophile is formed
The electrophile attacks the electron rich benzene ring. 2 pi electrons move from the benzene ring to the electrophile.
Generally speaking,
are catalysts needed for Benzene to react?
Yes
Name some
Electrophilic Substitution Reactions of Benzene
- Halogenation
- Nitration
- Alkylation
- Acylation
Under what body of reactions
do alkylation and acylation fall?
Friedel-Crafts reactions
What are the reaction conditions, (reagents, catalyst & electrophile),
for the reaction of Benzene with a Halogen?
take Chlorine as an example
REAGENTS: Benzene & Chorine gas
CATALYST: Iron (III) / Aluminum Chloride
ELECTROPHILE: Cl+ (chloronium ion)
the halide is peculiar to the reacting halogen
What will be physically observed
in the reaction of Benzene with Chlorine?
In the presence of catalyst
Dense fumes of white gas (HCl(g))are seen
Hydrogen halides escape as white fumes when produced
What will be physically observed
in the reaction of Benzene with Bromine?
In the presence of catalyst
There will be a decolorisation of the reacting system
To show that benzene reacts with the bromine gas
