The nitroso group, -N=O, is one of the few nonhalogens that is an orthoand para-directing deacti

The nitroso group, -N=O, is one of the few nonhalogens that...

Key Concepts

Electrophilic Aromatic Substitution

This is a fundamental reaction mechanism in aromatic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. The process involves the formation of a sigma complex (carbocation intermediate) that temporarily disrupts the aromatic system, followed by deprotonation to restore aromaticity. Understanding this mechanism is key to predicting the outcome of reactions involving aromatic compounds and explaining the behavior of various substituents on the ring.

Resonance Structures in Carbocation Intermediates

Resonance stabilization is crucial in understanding the stability of carbocation intermediates in electrophilic aromatic substitution. The positive charge that develops during the formation of the sigma complex is delocalized over the aromatic ring through resonance, distributing the charge over several atoms. Drawing these resonance structures helps illustrate how certain positions (ortho, meta, para) are stabilized to different extents, influencing the reactivity pattern and site selectivity of the electrophilic attack.

Directing Effects of Substituents

Substituents on an aromatic ring influence both the reactivity and the site of electrophilic attack. They are classified as either ortho/para directing or meta directing based on their electronic effects. These directing effects arise because substituents can either donate or withdraw electron density through resonance or inductive effects, thereby stabilizing or destabilizing the carbocation intermediate at different positions relative to the substituent.

Deactivating Substituents

Deactivating groups decrease the overall reactivity of an aromatic ring toward electrophilic aromatic substitution by withdrawing electron density. Although they make the ring less reactive, the deactivating groups can still direct the incoming electrophile to specific positions (typically ortho and para, or meta, depending on the mechanism of stabilization afforded by resonance). Understanding deactivating effects helps in predicting both the rate and regioselectivity of the electrophilic attack on substituted aromatic systems.

Transcript

00:01 So you're about to learn about a new type of directing group that is orthoparidirecting, but is in fact another deactivating species like those halogens are.

00:17 Right.

00:17 So why is that? and we have to draw resident structures of the carbocadion intermediates when they react with another electrophile.

00:28 Right.

00:29 So if you look at it, i boxed it.

00:32 The corner up here.

00:33 If you take a look, you'll notice that in a lot of my videos, i will say that when you have a hetero atom that is able to donate a lone pair into the benzene ring, it's generally an electron donating group, which is true.

00:50 So i can't get my marker to go up there.

00:54 There we go.

00:55 That's not where the loan pair was.

00:57 Anyway, so if you have, i'll eventually do it in some of the residence forms, but you can, also show by residents that this is an electron withdrawing group.

01:06 So you'll, you can push the carbonyl.

01:11 It's not really a carbonyl.

01:12 Now it's a nitracil.

01:14 You can push it up so that you arrive at a resident structure that gives you a positive nitrogen and a negative oxygen, just like you can with carbonyles.

01:24 So that positive charge would tell you that it's actually an electron withdrawing group, because now if you have a benzene ring, now your positive charge, if you want to treat it like a carbocanion, your positive charge is going to need electron density to be stabilized.

01:45 So it's going to be withdrawing electron density into what is now an empty p orbital on that nitrogen atom.

01:55 Also, the strong dipole moment, these nitrousils aren't as, polar as carbonials in the sense that the dipole moment between a nitrogen -oxygen bond isn't as polar as that of a nitrogen -oxygen bond, because carbon has a much lower electronegativity than the nitrate does.

02:18 Right.

02:18 So that explains why it's a deactivator, right? so in terms of the induction outweighs the effects from residents.

02:28 But why is it a electron? why is it a orthopor director.

02:34 So you're going to see, we're able to draw a similar resident structure with the ortho and para intermediates that we can't draw with the meta.

02:46 And this is going to look very familiar.

02:49 Right...

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