Today’s post has two big images, but has a really simple message.
The products you get from an electrophilic aromatic substitution are directly related to the stability of the carbocation intermediate.
There are 2 main cases to worry about.
Take a benzene with a group like OCH3 attached to it. Now draw reaction arrows from a Pi bond on the ring towards an electrophile (E) so that it ends up on the ortho, meta, and para substitutents respectively. The most stable carbocations in each case will look like those below.
Make sure you know how to obtain these using curved arrows!
- Note how the carbocations for the “ortho” and “para” cases are the most stable (since every atom has a full octet). This means they’ll be faster to form than the “meta” carbocation, which is less stable. That’s why the major products are ortho and para .
- Note that the para is a little more stable due to reduced steric interactions – (not resonance)
- What I just said for OCH3 also applies for other activating groups like NH2, SR, OH, etc. as well as alkyl groups (which lack lone pairs but are still activating groups).
Now let’s look at CF3. Here it’s the opposite case.
- The carbocations for the “ortho” and “para” cases are the most unstable, since we have a carbocation adjacent to the electron withdrawing CF3. This means that they will be higher in energy (more unstable) than the “meta” carbocation, which is less bad. So the meta product is formed preferentially.
- This is why CF3 is a “meta director” (although I prefer to call groups like CF3 “ortho-para avoiders”.
- This also applies to other meta directors such as NO2, CN, SO3H, ketones, and so on.
I will spare you from putting a third big diagram in here to describe halogens like F, Cl, Br, and I, but they are often a source of confusion. Experiments show us that they are ortho-para directors. So the fact that they can contribute to resonance (like OCH3) is what stabilizes the ortho-para products relative to meta.
The bottom line for today is that groups that can donate electrons will stabilize the intermediate carbocation, favoring ortho-para products. Groups that withdraw electrons will destabilize the intermediate carbocation, favoring meta products. (Halogens are the weird exception: they slow down aromatic substitution, but favor ortho-para products).
Question for you: what do you think happens when you have more than one directing group? Which one “wins” ?