Phenylboronic acid, C6 H5 B(OH)2, is nitrated to give 15 % ortho-substitution product and 85 % m

Phenylboronic acid, C6 H5 B(OH)2, is nitrated to give 15 %...

Key Concepts

Electrophilic Aromatic Substitution

This is a fundamental mechanism in aromatic chemistry where an electrophile attacks the electron-rich benzene ring, forming an arenium ion intermediate before deprotonation restores aromaticity. The nature of substituents present on the ring determines both the reactivity of the ring and the position where the electrophile will attack.

Substituent Effects in Aromatic Chemistry

Substituents on an aromatic ring can either donate or withdraw electron density, affecting the stability of the arenium ion intermediate during electrophilic aromatic substitution. Electron-donating groups typically activate the ring and direct incoming electrophiles to the ortho and para positions, whereas electron-withdrawing groups deactivate the ring and direct substitutions to the meta position.

Electron-Withdrawing Groups

Groups that withdraw electron density via inductive or resonance effects reduce the electron density of the benzene ring and destabilize the positive charge in the intermediate, particularly at positions where positive charge is developed extensively (often at the ortho and para positions). This makes these positions less favorable for electrophilic attack, resulting in a meta-directing effect.

Resonance and Inductive Effects

Electron-withdrawing substituents can influence the reactivity of aromatic rings through resonance (mesomeric) effects, where delocalization of electrons can occur, or inductive effects, where electron density is pulled away through sigma bonds. In the case of certain substituents, the lack of a lone pair or the presence of an electron-deficient atom prevents donation into the ring, reinforcing the meta-directing influence.

Vacant p-Orbital in Electron-Deficient Centers

Some substituents contain atoms, such as boron in boronic acids, that possess an empty p-orbital. This feature renders the substituent electron-deficient and unable to donate electrons into the aromatic ring, thereby withdrawing electron density by resonance. This explains their overall deactivating and meta-directing influence during electrophilic aromatic substitution.

Transcript

00:01 All right.

00:01 So this phenoboronic acid, when you nitrate, you get the major product.

00:08 85 % of your product is the meta and the other 15 % is the ortho, which i'm not going to draw because it's a minor product.

00:18 So we have to explain the meta directing effect of the baronic acid, substituent.

00:24 So chances are if it's meta directing, it's likely deactivating.

00:28 Right? so why do you think boron would be a deactivating species? deactivating means it's withdrawing electron density out of the ring.

00:40 So if you remember why we use aluminum as a lewis acid for freethraps.

00:49 And the reason for that is because aluminum by its own nature is electrophilic.

00:54 Group 13, boron and aluminum are electrophilic in nature.

00:59 Because they have a stable six electron valence shell, which means they are stable only having three bonds, a six electron valence shell, which means that they have a p orbital that's entirely vacant.

01:15 So that's the same reason why when you studied the mechanism for electrophilic, sorry, no, hydroboration oxidation, you're able to draw a bond from a pie bond in alkeen to bromine, right? so because you have that p orbital that you're drawing the electrons from the pie bonds of the alken to the p to the empty p orbital in bromine.

01:41 So bromine is electrophilic because it is stabilized by withdrawing electron density into that mtp orbital.

01:52 So you can think about boron as an electron.

01:57 Electron withdrawing agent.

02:00 Even though it has these hydroxyl groups on it, which will donate electron density, the boron overall is still going to be an electron withdrawing group.

02:08 So it's going to have a destabilizing effect on the ring.

02:12 And when you know that, you can pretty much predict that the meta intermediate of the ring, which you might have drawn a bunch of times now...

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