Topic 4.2 - Aromacity Flashcards
Give the molecular and skeletal formula of benzene
How can the structure of benzene be represented? Which model is more suitable?
Describe the bonding in Benzene
Benzene has six carbon atoms which each form a single covalent bond to the carbons next to it and to a hydrogen atom. This means each carbon atom has one unpaired electron in the outer p-orbital. These unpaired electrons combine to form a delocalised ring of electrons.
Explain the delocalised electron system of benzene in terms of the types of bonds involved
The p orbital on each carbon atom in benzene overlaps with the p orbitals either side of them. This series of overlaps produces a pi bond system. These electrons are spread out over the whole carbon ring and therefore are described as being delocalised.
What is the shape and bond angle of benzene?
Benzene is a planar, regular hexagon
and the shape around each carbon atom
is trigonal planar. The bond angle is 120*
What is the general name for compounds containing a benzene ring?
Arenes or aromatic compounds.
Why does benzene not undergo electrophilic addition reactions?
Benzene’s delocalised ring of electrons is a region of high electron density which attracts electrophiles. However, the ring of electrons is very stable so benzene does not undergo electrophilic addition as this would break up the delocalised ring of electrons.
What are the two common electrophilic substitution mechanisms for benzene?
Friedel-Crafts acylation
Nitration reaction
How is the acyl chloride electrophile made stronger for Friedel-Crafts acylation?
An acyl chloride is reacted with AlCl3. The AlCl3 accepts a lone pair of electrons from the acyl chloride. This increases the polarisation in the acyl chloride and it forms a carbocation. This is a much stronger electrophile so it is able to react with the benzene ring.
Give the equation for the reaction between aluminium chloride and ethanoyl chloride
Draw the electrophilic substitution mechanism for the reaction between benzene and methanoyl chloride
Explain how AlCl3 behaves as a catalyst in the Friedel-Crafts acylation of benzene
AlCl3 initially reacts with an acyl chloride to produce a reactive carbocation intermediate and AlCl -. At the end of 4
the mechanism a hydrogen ion is released from the benzene ring. This reacts with the AlCl - to produce HCl 4
and AlCl3. Therefore AlCl3 does not get used up during the reaction and so acts as a catalyst.
What conditions are required for Friedel-Crafts acylation reactions?
The reactants should be heated under reflux in a non-aqueous solvent.
Give the reactants and catalyst required for the nitration of benzene
How is the electrophile, NO +, produced
for the nitration of benzene?
Draw the electrophilic substitution mechanism for the nitration of benzene
How does sulfuric acid act as a catalyst in the nitration of benzene?
Draw the electrophilic substitution mechanism for the reaction between benzene and chlorine
