Are the following substituents ortho-para directors or meta...

Key Concepts

Electrophilic Aromatic Substitution (EAS)

This is a reaction where an aromatic ring, such as benzene, reacts with an electrophile, replacing one of its hydrogen atoms. The reactivity and regioselectivity (i.e., where the substitution occurs) are highly influenced by the substituents already present on the ring.

Ortho-para Directing Groups

These groups typically activate the benzene ring toward electrophilic attack and direct incoming electrophiles to the ortho and para positions. They generally achieve this by donating electron density to the ring through resonance or inductive effects, thus stabilizing the intermediate formed during the reaction.

Meta Directing Groups

Meta directing groups usually deactivate the aromatic ring and direct electrophilic substitution to the meta position. These groups withdraw electron density through inductive or resonance effects, destabilizing the transition state for substitutions at the ortho and para positions relative to their placement.

Electron Donating and Withdrawing Effects

The effect of a substituent on the aromatic ring is largely determined by whether it donates or withdraws electrons. Electron donating groups increase the electron density of the ring, facilitating electrophilic attack and typically resulting in ortho/para direction, whereas electron withdrawing groups reduce electron density, making the ring less reactive and favoring substitution at the meta position.

Resonance and Inductive Effects

These are two important mechanisms by which substituents influence the reactivity of aromatic compounds. Resonance effects involve the delocalization of electrons through overlapping p-orbitals, which can either stabilize or destabilize intermediates, while inductive effects involve the transmission of electron density through sigma bonds, playing a critical role in directing electrophilic attack on substituted benzene rings.

Transcript

00:05 Let's determine if different substituents could be orthopara or meta -directors.

00:13 And let's try the first one.

00:15 Well, we have an alkene and a nitral group.

00:19 We know that an alkyne is like a lot of alkyl groups, it will be orthopar directing.

00:25 However, nitral groups are very, very strong metadirecting groups.

00:33 In fact, if this was a substituent on a benzene ring, it will try to go for resonance towards the nitrogen, which would make it metadirecting.

00:53 No2 is going to be our strongest deactivating group.

01:00 And if it's a very strong deactivating group, what does that say about its director? or what does that say about what it directs? it would be metadirecting.

01:20 What about methanol? if this was bonded to a benzene rube, ring.

01:30 Well, methanol would function a lot like just regular alcohol.

01:34 In fact, if this was connected to a benzene, it would probably be like a methoxy, which is an ether, which functions a lot like alcohols or a lot like a phenol in terms of directing and activating.

01:54 It is an activator.

01:56 So that should tell us that it is orthopera, because these loan, these loan pairs on the oxygen would be involved in resonance and would donate those electrons to the benzene ring.

02:17 What about carboxylic acid? if we had a carboxylic acid bonded to a benzene ring, well, we know that there's a double bond.

02:37 And whenever there's a double bond or a triple bond, it would like to take those electrons, and it will pull those electrons through induction and resonance.

02:50 In fact, this would be a meta -directing group...

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