Using resonance structures of the intermediates, explain why bromination of biphenyl occurs at o

Using resonance structures of the intermediates, explain why...

Key Concepts

Biphenyl Structure and Regioselectivity

In biphenyl compounds, the interconnection of two benzene rings allows for extended conjugation. When an electrophilic substitution occurs, attack at the ortho or para positions relative to the connecting bond benefits from additional resonance stabilization provided by the adjacent ring. This enhanced stabilization makes these positions more reactive compared to the meta position, where such resonance contribution is minimal.

Electrophilic Aromatic Substitution (EAS)

EAS is a fundamental reaction mechanism in aromatic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring, leading to the formation of a positively charged arenium ion intermediate. The stability of this intermediate through resonance is a key factor in determining both the rate and regioselectivity of the reaction.

Resonance Stabilization of Intermediates

Resonance stabilization involves the delocalization of charge across a conjugated system, which helps distribute and thus lower the energy of a reaction intermediate. In the context of aromatic substitution, the formation of multiple resonance structures for the arenium ion significantly stabilizes the complex, making certain positions on the ring more favorable for substitution.

Substituent Directing Effects

Substituents on aromatic rings can influence the site of electrophilic attack through resonance and inductive effects. Electron-donating groups, for example, can stabilize the positive charge via resonance when the electrophile attacks at the ortho and para positions, while meta positions do not benefit from such stabilization, leading to a lower reactivity at those sites.

Transcript

00:02 Okay, this question is, i feel like it's mainly here so you can practice with some interesting resonance arrow pushing, but you may be able to understand the answer to this question pretty quickly, just based off of what you already know about directing groups, right? so why does brumination have biphenal, i still called it e here, just out of habit for making these videos? why is it orthoparadirecting that at meta? right.

00:27 So you know from the chart.

00:31 So, okay, the most basic answer minus resonance is that you know that from the chart in the chapter that talks that introduces directing groups, you know that when you have a benzene ring with some aliphatic or aromatic group, i'll call it r.

00:46 You know that it's orthopar directing because of residence that they showed you in the chapter.

00:54 Because if you have a carbon atom here, right, it doesn't matter what else is on here.

01:01 It could be a ring like in biphenal.

01:04 It could just be two methyl groups and an h.

01:06 It could be whatever, right? but you know that once you add something else, you're able to resonate.

01:13 Let's put an e here.

01:14 You're able to resonate that charge so you could put a carbocatione at that position and make it a tertiary allilic carboccalion, right? i say allelic because it's also one carbon away from a pie system.

01:27 So this is the most stable energy form of that kind of carbocadion.

01:33 So you could just look at biphenyl and be like, all right, i'll just treat it as phenol benzene, for example.

01:39 And then i'll draw the residents accordingly.

01:41 But there's a reason for why, in terms of benzene, the whole system is in conjugation.

01:47 I guess is what i'm trying to say.

01:48 So you have 12 pie bonds.

01:51 You have 12 pilotrons in this system.

01:53 So the problem is mainly having you draw the resident structures that shows the stabilization from orthopara.

02:04 So just for shorthand, i'm going to abbreviate the phenogroof as r until it comes into play.

02:14 So you've done this a few times with some of these problems already.

02:17 I could put it down here, that i can push this guy there.

02:23 So you're going to get to a point where you are allowed to, you arrive at this.

02:32 I forgot the e.

02:33 Sorry, i knew i was forgetting something.

02:35 Really, it's bromine.

02:36 The e is bromine.

02:37 E equals bromine.

02:39 We're talking about bromination here...

Please give Ace some feedback