Use Figure 16.11 to explain why Friedel-Crafts alkylations often give polysubstitution but Fried

Use Figure 16.11 to explain why Friedel-Crafts alkylations...

Key Concepts

Substituent Effects in Aromatic Substitution

Substituent effects play a crucial role in determining both the rate and orientation of electrophilic aromatic substitution reactions. Electron-donating groups, such as alkyl groups, activate the aromatic ring and direct incoming electrophiles to the ortho and para positions, enhancing the likelihood of multiple substitutions. Conversely, electron-withdrawing groups, like the acyl functional group, deactivate the ring and direct electrophiles to the meta position, thereby reducing reactivity and limiting further substitution.

Friedel-Crafts Acylation

In Friedel-Crafts acylation, an acyl group (RCO-) is introduced onto an aromatic ring using an acyl halide and a Lewis acid. The resulting acylium ion is stabilized by resonance with the carbonyl group, and when it attaches to the aromatic ring, the carbonyl group acts as an electron-withdrawing, deactivating substituent. This deactivation lowers the electron density of the aromatic ring, thus impeding further electrophilic substitutions and preventing polysubstitution.

Electrophilic Aromatic Substitution

This is a reaction mechanism in which an electrophile reacts with an aromatic ring, displacing a proton. The process involves the formation of a carbocation intermediate (or a sigma complex) that is stabilized by resonance. It is a key reaction in organic chemistry, and its outcome is largely dictated by the nature of substituents already present on the aromatic ring.

Friedel-Crafts Alkylation

Friedel-Crafts alkylation is a type of electrophilic aromatic substitution where an alkyl group is introduced onto an aromatic ring using an alkyl halide and a Lewis acid catalyst. The alkyl substituent that is added is typically an activating group, which increases the electron density of the aromatic ring and makes it more susceptible to further electrophilic attack. This often leads to polysubstitution, as the ring becomes progressively more reactive after the initial alkylation.

Transcript

00:01 Okay, so the question's asking a pretty good one, right? so why does friedelcraft's alkylation lack control in terms of its ability to have mono and dye substituted products, polysubstitution as it's called, but friedel alkylation, sorry, friedel craft's acillation pretty much yields only one product.

00:25 And the reason for that is in your, think about the kind of group you're adding here, right? so in alkalation, you're adding alkyl groups.

00:37 So you're adding a methyl group here, which means that your ring as a result of having those extra alcohol groups on there are going to be more activated compared to benzene.

00:50 So benzene's pretty much your reference with respect to activation when you add a group to it.

00:56 So you add a methyl group.

00:58 Now you have an alkalog group, which is able to donate electron density into the ring and make it more nucleophilic towards electrophiles.

01:08 So once you end up at tullulene after methylating your benzene ring, your ring is going to be more activated compared to benzene for a second methyl group addition.

01:19 So that's why with alkalation, one of the issues with alkylation is the lack of control.

01:26 With your reaction here.

01:28 Compared to acillation, now the opposite is probably true for acillation, right? so you're still doing the same kind of chemistry.

01:34 You have aluminum chloride and you're activating, you're making an electrophile, and it's adding to the ring.

01:39 The mechanisms are pretty much the same.

01:42 The only thing that's different now is that instead of an activating group, you've added a pretty strong deactivating group in the form of this acyl group here...

Please give Ace some feedback