Electron Withdrawing Groups - Online Tutor, Practice Problems & Exam Prep

In electrophilic aromatic substitution (EAS), the first substituent on a benzene ring significantly influences the reactivity and direction of subsequent reactions. Electron donating groups (EDGs) activate the ring and direct new substituents to ortho and para positions, while electron withdrawing groups (EWGs) deactivate the ring and direct substitutions to meta positions. Understanding these directing effects is crucial for predicting reaction outcomes in organic synthesis. The EAS activity chart categorizes substituents based on their electron-donating or withdrawing properties, highlighting their impact on reactivity and regioselectivity.

Activity and Directing Effects

Now we know how to add a single substituent to a benzene ring using an EAS mechanism. We're pretty good at figuring out how to add different types. You know how to add nitros and R groups and ketones. But now the question we have to ask ourselves is, what happens if you want to do a second reaction on that benzene? What if you already have something there and now you're adding a second EAS reagent? Where is it going to add? How is it going to add? How is that first substituent going to affect the second? It turns out that it really has a huge effect on the second one and that brings us to the EAS of monosubstituted benzene. Now we're not talking about just regular benzene. We're talking about benzene with a substituent already. This section also is called directing groups or activity of benzene. All of that is covered in this section. It turns out that that first substituent is going to really alter the electron density of the benzene ring. So it's going to affect the reactivity towards subsequent reactions and it's going to affect the direction of subsequent reactions.

The first thing that it's going to do is it's going to affect activity. It turns out that this is our first introduction to these terms of electron donating groups or electron withdrawing groups. These are extremely important principles in organic chemistry and they're going to come up not only for the rest of this course but also for the rest of your professional career. If you want to go into anything pre-health and you need to take more classes in the sciences and in the life sciences, then you're going to need to know about electron donating groups and electron withdrawing groups.

First of all, if that first substituent that you add happens to be an electron donating group, that means it's giving more electrons to the benzene. Do you think it's going to make it more reactive or less reactive towards another EAS reaction? Remember, the benzene acts as a nucleophile in the reaction. The more electrons you pump into it, the more you're going to activate it to react. They activate the ring towards more reactions. So if you add an electron donating group, it's going to want to react even more the second time.

However, if you add an electron withdrawing group, that's going to pull electron density out of the ring, making it less nucleophilic. That's going to deactivate the ring towards future reactions. That means the second reaction will be more difficult to perform than the first, meaning it's actually less reactive than benzene by itself.

But that's not it, guys. You might be wondering, how do I know if something is electronegative? We'll get there. Just hold on. Also, guys, they have directing effects because it turns out that electron donating groups tend to be what we call ortho para directors. They tend to direct towards the ortho and para positions, whereas electron withdrawing groups tend to be meta directors, meaning that they direct subsequent EAS reactions to happen only at the meta positions.

Here I have a picture of these 2 benzenes and an electron donating group, we would expect to add the second EAS reagent in the ortho positions or in the para positions. Hence, OP director. It actually means that it directs to all of those positions. Whereas meta directors, electron withdrawing groups, they pull electrons out of the ring, so they're going to tend to add in the meta positions.

I forgot to draw the dipole of the electron donating. It's going to push electrons into the ring. It directs towards OP whereas electron withdrawing groups direct towards the meta positions. The scientific explanation of why that happens is going to be for another video. We're not going to really talk about the scientific technical definition right now. It has to do with resonance structures. But you could also read your textbook if you want more information on that. What I'm going to focus on for right now is really just memorizing and really just knowing which groups are your electron donating and which groups are electron withdrawing groups. So let's move on to the chart that's going to help us with this information.

Badass Activity Chart

And this brings us to the badass EAS activity chart. This chart is going to be the go-to that you can use to learn what groups are electron donating and electron withdrawing, and also if they're going to be ortho, para, or meta directors in regards to EAS. First of all, let's just learn how to navigate this chart. There are 2 major lines that you need to understand here. One, that's the activity line and the other one is the director line. Now the activity line is really just comparing to that of an unsubstituted benzene. What we're saying is that any group that is above that line, above that arrow is going to activate towards further EAS more than regular benzene. Any group that's below that line is going to deactivate it. So as you can see, that's the first set of arrows going up and down. The second arrow is the director arrow. The director arrow says that anything that's above that line is going to be an ortho, para director, and anything that's below that line is going to be a meta director. As you can see, there's a general trend that the activators tend to be ortho, para. The deactivators tend to be meta with basically one noted exception and we'll get there in a second.

Now before talking about the individual groups, let's also talk about some generalizations we can make. Well, it turns out that electron donating groups all have something in common, which is that they give electrons to the benzene. How do they do that? Well, first of all, anything with a negative charge is electron donating. Just keep that in mind. Electron donating by definition, if it has a negative charge, it's going to be giving electrons away. But you can also have basically donated effects by just being an R group. Remember that R groups, through a phenomenon called hyperconjugation, can share some of their electron density with the benzene. Remember that that has to do with the same principle that stabilizes carbocations. The more R groups, the more stable the carbocation. The same thing happens with the benzene ring. The more R groups, the more donating it is. This effect is from hyperconjugation. But the other way that you could donate electrons is through a lone pair, which I've drawn here. An electron donating group could just be any group that has a lone pair right next to the ring and that would be through resonance because that lone pair could enter the ring through resonance structures and it would spend a lot of its time in the ring. That's the general features of electron donating groups. And even if you don't remember a single group here, we're going to go through the groups one by one. But even if you don't remember a single group, you could use that trend to know, hey, is this electron donating? You could just look at it and tell.

Now we've got our electron withdrawing groups. The generalization we can make about electron withdrawing groups is that they have a positive charge. Obviously, anything with a full positive charge right outside the benzene ring is going to pull electrons towards it and deactivate it. But also this goes for partial charges. Anything that has a partial positive charge is going to deactivate the ring. A really easy example of this would be carbonyls. Carbonyls have a partial positive and a partial negative due to dipoles. So if you have a carbonyl carbon right next to a benzene ring, that partial positive is going to pull electrons away. So again, even if you don't remember a single group here, you could use that pattern to deduce, is this going to be electron donating or electron withdrawing. It's really as simple as positive and negative.

Now what I want to do is get into the specific groups that you need to memorize and the order that they're in because the order is actually going to be important for a lot of reasons in this course. Like I told you guys earlier, electron donating, electron withdrawing groups are some of the most popular concepts in organic chemistry too. You definitely want to be familiar with them. We said that anything that's above the line of benzene or reactivity is going to be more reactive than benzene. That definitely holds the case. For example, an R group, let's just say it was a methyl group. That would be called toluene. If that was toluene, so CH₃, this is just an example. Toluene is 24 times more reactive than benzene. We can see that it's definitely a good activator because of the fact

Predict the major product of the following EAS reaction.

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Predict the product of the following multi-step synthesis. 

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Here’s what students ask on this topic:

What are electron withdrawing groups (EWGs) in organic chemistry?

Electron withdrawing groups (EWGs) are substituents that pull electron density away from the rest of the molecule through either inductive or resonance effects. This makes the molecule, particularly aromatic rings like benzene, less nucleophilic and less reactive towards electrophilic aromatic substitution (EAS) reactions. Common EWGs include nitro groups (NO₂), carbonyl groups (C=O), and halogens (F, Cl, Br, I). EWGs typically direct new substituents to the meta position on a benzene ring due to their electron-withdrawing nature.

How do electron withdrawing groups affect the reactivity of benzene in EAS reactions?

Electron withdrawing groups (EWGs) decrease the electron density of the benzene ring, making it less nucleophilic and less reactive towards electrophilic aromatic substitution (EAS) reactions. This deactivation occurs because EWGs pull electron density away from the ring, reducing its ability to donate electrons to an incoming electrophile. As a result, the benzene ring becomes less reactive compared to an unsubstituted benzene. Additionally, EWGs direct new substituents to the meta position, as this position is less destabilized by the electron-withdrawing effect.

What is the difference between electron donating groups (EDGs) and electron withdrawing groups (EWGs) in terms of their directing effects in EAS reactions?

Electron donating groups (EDGs) and electron withdrawing groups (EWGs) have opposite effects on the reactivity and regioselectivity of benzene in electrophilic aromatic substitution (EAS) reactions. EDGs increase the electron density of the benzene ring, making it more reactive and directing new substituents to the ortho and para positions. In contrast, EWGs decrease the electron density, making the ring less reactive and directing new substituents to the meta position. This difference in directing effects is due to the way these groups influence the electron distribution within the benzene ring.

Why are halogens considered electron withdrawing groups but still direct ortho and para in EAS reactions?

Halogens are unique in that they are electron withdrawing due to their high electronegativity, which pulls electron density away from the benzene ring through inductive effects. However, they also possess lone pairs of electrons that can participate in resonance with the benzene ring. This resonance donation partially offsets their inductive electron withdrawal, making them ortho and para directors in electrophilic aromatic substitution (EAS) reactions. Despite being deactivating overall, the resonance effect stabilizes the ortho and para positions, leading to their unusual directing behavior.

How can you identify if a substituent is an electron withdrawing group (EWG) or an electron donating group (EDG)?

To identify if a substituent is an electron withdrawing group (EWG) or an electron donating group (EDG), consider the following general rules: EWGs typically have electronegative atoms or groups with partial or full positive charges, such as nitro (NO₂), carbonyl (C=O), and halogens (F, Cl, Br, I). These groups pull electron density away from the benzene ring. EDGs, on the other hand, often have lone pairs or alkyl groups that can donate electron density to the ring, such as hydroxyl (OH), amino (NH₂), and alkyl groups (R). These groups increase the electron density of the benzene ring.