The N, N, N -trimethylammonium group, -N^+(CH3)3, is one of the few groups that is a meta-direct

The N, N, N -trimethylammonium group, -N^+(CH3)3, is one of...

Key Concepts

Inductive Withdrawal by Positively Charged Groups

Positively charged groups, such as the N,N,N-trimethylammonium group, withdraw electrons by the inductive effect, pulling electron density away from the aromatic ring through sigma bonds. This electron deficiency deactivates the ring toward electrophilic attack, and due to the lack of resonance interaction, the meta position becomes the relatively less hindered site for substitution, leading to meta-directing behavior.

Resonance and Inductive Effects

Resonance (or mesomeric) effects involve the delocalization of electrons through ? systems of conjugated molecules, while inductive effects involve electron withdrawal or donation through sigma bonds based on electronegativity differences. In aromatic substitution, these effects determine how the substituent influences the electron density of the aromatic ring. A substituent that does not participate in resonance electron withdrawal can still exert a strong effect through the inductive withdrawal of electrons if it bears a positive charge.

Substituent Effects: Activating/Deactivating and Directing Groups

Substituents on an aromatic ring affect its reactivity by either donating or withdrawing electron density. Activating groups increase the ring's electron density and typically direct incoming electrophiles to the ortho and para positions, while deactivating groups reduce the electron density of the ring, making it less reactive and often directing electrophiles to the meta position. Understanding whether a group is electron donating or electron withdrawing explains its directing effect in substitution reactions.

Electrophilic Aromatic Substitution (EAS)

Electrophilic aromatic substitution is a fundamental reaction in aromatic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity and the site of substitution on the ring are significantly influenced by the nature of the substituents already attached to the ring, which can either activate or deactivate the ring toward further electrophilic attack.

Transcript

00:01 Okay, this is a pretty interesting question, right? so why is it that the, if you look at that range of electron withdrawing groups on your, in the chapter that introduces substituent effects, you'll notice that there's something more deactivating than the nitro group, and that's the trimethoammonium or really any type of alcohol ammonium.

00:25 So that is when nitrogen has a fourth bond to carbon.

00:30 So its lone pair electrons are being taken.

00:38 And so the loan payer is gone, is what i'm trying to say.

00:41 They're involved in a bond that really cannot be broken easily, a nitrogen carbon bond.

00:46 So now there's a formal positive charge on nitrogen.

00:49 So because of that, we can't do resonance anymore in the ring, right? so first of all, resonance requires that you have, um, electrons to push, right? i can't, because there's no loan pair anymore, i don't have a lone pair to push down into the ring, which is why we can show through residents that amino groups, for example, are electron donating.

01:18 That's for starters.

01:20 There's also no way that we can do this, because if there was some carbonyon, we were cycling in the ring, there is no way we could push the pie bonds up because, one, there's no vacant p orbital, right? so residence also requires things to be flat.

01:39 So there's no orbital for the nitrogen to accept those electrons into, but also we've been making five bonds to nitrogen, which is illegal.

01:48 Then it would have 10 valence electrons, which is not possible, right? so that's the resonance argument.

01:56 What about just think about the kind of reaction.

02:00 That i've drawn at the top here, right? so think about what happens if you were going to add something to this ring.

02:07 So this notation where you have the substituent coming out of the center of the ring just means that it's a general, it's a vague general scheme for that electron could be anywhere.

02:18 That electron, sorry, that substituent can be anywhere.

02:20 That electrofile can be at any position on that ring.

02:23 It doesn't matter where, for the sake of the scheme that it's in, you just have two substituents and that e is at whatever position with respect to the triatholomino group, triathill ammonium, sorry.

02:36 So we're going to draw the orthometro and para resident structures with upon addition of a second substituent to the benzene ring.

02:49 Right? so i'm going to make the ortho ones appear here.

02:53 There we go.

02:54 So you'll notice that if we start so if we add the electrophile to the ring right here and we start pushing the electrons around, we're going to get to an intermediate that puts both of those the carbocadion on the same atom as the nitrogen that also has a positive charge.

03:15 So when in chemistry have you seen two formal positive charges next to each other and have that be low energy? right...

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