Lecture 7
Wednesday, January 21, 2015
12:19 PM
Aromatic cations and anions.
In some cases aromatic compounds can be made by adding an empty p orbital to the conjugated
system. That happens with hepta triene. By adding a empty p orbital we get a conjugated cyclic pi
system with 6 electrons. That's aromatic.
antiaromatic
In some cases adding a p orbital with two electrons (carbaninon) to the system can make it aromatic. With
cyclopendadiene (shown below) deprotonation gives us an extra pi orbital that with other p orbitals in the molecule
makes conjugated pi system with 6 electrons and the anion is aromatic. Becvause deprotonation leads to the formation
of an aromatic compound pKa of the diene is low relative to other alkanes. Notice that the cycloheptariene anion shown
above is antiaromatic because it has 8 electrons in the pi system.
aromatic
Aromatic systems can also have heteroatoms. Then we call them heteroaromatics.
pyridine
pyrrole
Pyridine has a nitrogen that is sp2 hybridized and makes a double bond with a carbon as a part of the aromatic system. It also
has an extra pair of electrons in a sp2 orbital that is perpendicular to the aromatic system and it can use it to react with
protons. So it is a reasonable base. Pyrrole has a nonbopnding pair of electrons that are part of the aromatic system. As a
result that pair of nonbonding electrons cannot be used in a reaction with a proton and pyrrol is a very weak base.
Protonated pyrrole is not arromatic anymore. We have to take 2 electrons from the aromatic system to protonate pyrrol.
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Lecture 7
Wednesday, January 21, 2015
4:20 PM
Electrophilic aromatic substitution:
Arenium ion
or
Sigma complex
Arenes are less nucleophilic than alkenes. They give up electrons less easily than alkenes. That is because reaction with
electrophiles to make arenium ion destroys aromaticity.
Instead of nucleophilic addition to arenium ions we observe elimination which restores the aromaticity. Notice that the
product of electrophilic addition to benzene is not aromatic. The product of the substitution is.
So, again the aromaticity effects the reacticvity and selectivity of the reaction.
Arenes are less reactive than alkene so they do not react with boranes or peroxyacids.
All electrophilic aromatic substitutions have the same mechanism. The question is what is the reactive electrophile and
what picks up the proton at the end.
Nitration:
Nitronium ion
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Lecture 7
Wednesday, January 21, 2015
4:28 PM
Sulfonation:
Bromination:
Bromine does not react with benzene because it is not reactive enough. When activated by a Lewis acid (in this case FeBr3) it
becomes reactive enough and reacts with benzne. Lewis acid wants electrons, takes them away from Br2 and makes Br2 more
electrophilic. Notice that at the end of the reaction with get HBr and regenerate FeBr3. FeBr3 is not consumed in the reaction
and acts as a catalyst.
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