4.1 Halogenation and the Role of Lewis Acids

If we try to do the same reaction with benzene, what will happen?

• There is no reaction when benzene is heated.

• Benzene is an aromatic compound.
• Due to its aromaticity, it has a special stability.

• The reaction wouldn't happen because it would be going "uphill" in energy.

• An important concept in organic chemistry is brought about by this.
• The difference in the electron density between the two compounds is one of the driving forces for any reaction.
• The electrophile is electron-poor.
• They are attracted to each other in space.
• If the reaction isn't going well, we can try to make the attraction stronger between the nucleophile and the electrophile.
• We can either do it in one of two ways.
• We can either make the nucleophilic even more electron-rich or the electrophilic even more electron-poor.

• We will look at both scenarios in this chapter.
• We should try to make the electrophile a better one.

• It's important to remember why Br is an enthusiast in the first place.

• It would be even better if we had br+ instead of br.

• Lewis acids come into the picture.

• Consider the compound.
• The atom in this structure is aluminum.

• The aluminum atom does not have an octet.
• There are six electrons around the aluminum atom.
• That means that there is one empty space in aluminum.
• The empty orbital can accept electrons.

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• It is likely not accurate to think of this as a free solution.

• The important point is that this complex can function as a delivery agent of Br+, and that is what we needed in order to force a reaction between benzene and bromine.
• Let's try our reaction again.
• A reaction is observed when benzene is treated with bromine in the presence of a Lewis acid.
• It is not what we expected.

• This isn't an addition reaction.
• The aromatic protons was replaced with bromine.

• Let's look at the accepted mechanism to see how this happens.
• It is crucial that you fully understand this mechanism, because we will soon see that all other aromatic substitution reactions follow a similar mechanism.

• The intermediate generated by this step is not aromatic.

• It's important to remember what resonance structures are.
• From the first semester, resonance is not a molecule flipping back and forth between different states.
• resonance is the way we deal with drawing inadequacy.
• There is no single drawing that captures the essence of the intermediate, so we draw three drawings and combine them to get a better understanding of the intermediate.

• There are some special names for this intermediate.
• It's often called a sigma complex, or an Arrhenium ion.

• The base is where the protons are removed.

• When drawing a deprotonation step, you should show the base that is removing the protons.
• It might be a good idea to use br- to remove the protons.
• It is not a good base.
• The base that removes the protons is made of aluminum.
• There is a "delivery agent" of Br-.

• The Lewis acid is regenerated in the end.
• The Lewis acid isn't being consumed by the reaction.

• The presence of a small amount of Lewis acid will suffice.

• It might seem like a lot of steps on the surface.
• resonance structures are not steps.
• The three resonance structures in the center of the mechanism are necessary to understand the nature of the one and only intermediate.
• There are only two steps to the mechanism.
• In the first step, benzene acts as a nucleophile attacking Br+ to form the sigma complex, and in the second step, a protons is removed from the ring to reestablish aromaticity.
• The two steps are attack and deprotonate.
• H+ comes off when Br+ comes on.

• If you can't see the mechanism above, try to change it on a separate sheet of paper.
• There are two steps: E+ on and H+ off.