How to think about Organic Chemistry and the MCAT
If chlorobenzene is treated with sulfur trioxide (SO3) and concentrated sulfuric acid, what will be the major product(s)?
A. benzenesulfonic acid
B. m-chlorobenzenesulfonic acid
C. o– and p-chlorobenzenesulfonic acid
D. benzene
Write down what you think before moving on
Essentially, the question is saying that chlorobenzene is going to be reacting with something. First, let’s draw the structure:
What we are dealing with here is an aromatic compound; therefore, it must be electrophilic aromatic substitution. Now, we need to think about….what does the addition sulfur trioxide (SO3) and concentrated sulfuric acid allow us to do? Let’s look at what happens when we mix the two together:
The acidic proton on sulfuric acid can hydrogen bond with one of the oxygen atoms on sulfur trioxide. This creates a perfect situation for a nucleophile to attack the S atom in SO3. Let’s see how our aromatic halide compound can act as a nucleophile:
The electrons from the aromatic ring can swing out (like a hinge on a door) onto the ortho-position. Now, these electrons can attack the electrophilic sulfur atom pushing the electrons onto the oxygen atom, which now can create a formal bond with the hydrogen atom. For clarity sake, let’s rewrite the structure in the following way:
Now, let’s finish the mechanism:
At this point, we need to reestablish the aromatic system. The sulfate group that was part of sulfuric acid can now come in and deprotonate the hydrogen atom (act as a base) that is on the same carbon as the sulfonic acid group. The hydrogen is removed and the electrons form a double bond, which gives us our substituted product.
But wait…are we done yet??
Not just yet! The chlorine group is not only an ortho-directing group but a para-directing group; therefore, we need to place a sulfonic acid group in the para-position of another molecule:
See if you can draw the mechanism for the para-product.
The answer is C
EXTRA
Why do we not get ortho and para substitution on the same molecule? The sulfonic acid group is a meta-director and is an electron withdrawing group. Although chlorine is a ortho/para-director, it is a very electrophilic atom, which means it is also electron withdrawing. With both of these groups withdrawing electron density away from the aromatic ring, the ring is now not reactive enough to add another sulfonic acid group.