Activating and Deactivating
Every electrophilic aromatic substitution reaction has the same plot.
A pi bond from the aromatic group attacks an electrophile, creating a carbocation. Next, a base removes a proton to make it aromatic again.
The rate limiting step is breaking aromaticity to form the carbocation.
Now: The more stable that carbocation is, the lower will be the activation energy required to form that carbocation – and the faster that reaction will be!
So think back to the post from two days ago about carbocation stability.
- If we change the H on the benzene to OCH3, that should make for a more stable carbocation, right? So we’re going to increase the rate of the reaction.
- If we replace H with CF3, we’re going to make a more unstable carbocation. So we’ll decrease the rate of the reaction.
Again: the rate is proportional to the stability of the carbocation.
Let’s say that for benzene the rate is equal to 1.
- If we replace H with OCH3, the rate will be greater than 1 (faster)
- If we replace H with CF3 the rate will be lower than 1 (slower)
There are lots of other functional groups that will increase or decrease the rate, relative to H. Any group which increases the rate (relative to H) is called anactivating group. Any group with decreases the rate (relative to H) is called adeactivating group.
Common activating groups (not a complete list): Alkyl, NH2, NR2, OH, OCH3, SR.
Common deactivating groups (not a complete list): NO2, CF3, CN, halogens, COOH, SO3H.
If you think about it, you’ll also see that where you put the group E on the ring will also influence the carbocation.
Tomorrow: let’s take a breath and go through the weeks’ worth of emails.
Thanks for reading! James