Electrophilic substitution on 3 -phenylpropanenitrile occurs at the ortho and para positions, bu

Electrophilic substitution on 3 -phenylpropanenitrile occurs...

Key Concepts

Conjugation

Conjugation refers to the overlap of adjacent p orbitals across a molecule, which allows for electron delocalization across multiple atoms. In systems where the aromatic ring is attached to conjugated groups, such as alkenes or alkynes, the conjugative interactions can modify the electron density pattern on the ring, altering both its reactivity and the preferred site of electrophilic attack.

Inductive and Mesomeric (Resonance) Effects

Inductive effects involve the transmission of electron density through sigma bonds due to differences in electronegativity, while mesomeric effects refer to electron delocalization through pi bonds. The balance between these effects in a substituent influences the stability of reaction intermediates in electrophilic aromatic substitution and ultimately dictates the regioselectivity of the reaction.

Directing Effects

Substituents attached to an aromatic ring can influence where an incoming electrophile will add by either donating or withdrawing electron density. These effects are a combination of resonance (mesomeric) and inductive influences, determining whether the substituent directs the electrophile to the ortho/para positions or to the meta position.

Electrophilic Aromatic Substitution

This reaction involves an electrophile attacking an aromatic ring to replace a hydrogen atom. The process typically proceeds through the formation of a carbocation intermediate (arenium ion), where the distribution of positive charge over the ring via resonance determines both the rate and regioselectivity of the substitution.

Resonance Structures

Resonance involves the delocalization of electrons within a molecule and is critical for stabilizing charged intermediates formed during chemical reactions. In electrophilic aromatic substitution, drawing resonance structures helps illustrate how substituents affect the distribution of charge in the intermediate, thereby influencing which positions on the ring are more reactive.

Transcript

00:01 Okay, so you have these similar structures here, three phenyl propane nitrile on the left and three phenyl propene nitrile on the right.

00:12 And according to the book, the propane nitrile is an orpheradirator and the propine nitrile is a meta -director.

00:21 And we have to justify through residents why that is.

00:25 Well you know that in terms of we have to identify the substituent and then dictate based on what we know from what the textbook tells us and when they first introduce directing effects what that range that arrow figure that shows the deactivating groups towards the left and the activating groups towards the right we can make some inferences and assumptions on what these most correspond to right.

00:54 So if the molecule on the left is an ortho -powered director, because they tend to have similar resonance forms, i'm just going to draw the ortho for now.

01:15 So i'll show that.

01:18 Right.

01:18 So i could draw that same resonance.

01:20 So what's most important is when you could put that charge on the same atom as the substituent.

01:29 Right.

01:29 So you know, that ortho, so i'm just going to draw two arrows here just to get that resident structure that i want to talk about right now.

01:38 So that's going to give me a bond here and a bonds there.

01:43 In between those, the charge will be here, just so you know.

01:46 But right now, that's not what i'm drawing.

01:49 So you're going to have this, and the carbocadion is going to be substituents here.

01:57 Carbacallon is going to be at this position.

01:59 So in terms of resonance, that's all we can draw because this is technically an alcohol group.

02:07 So you do have that cyanide group at the end, that nitrile, same thing as it's calling a cyanide group, that is an electron withdrawing group, but you have a decent amount of space between that nitrile group and the ring.

02:22 So because the more carbons you have between the electron on withdrawing group and the thing that it's going to be affecting, in this case the phenal group or the carbocadion, the less of an inductive effect is going to be present.

02:35 I'll say that again.

02:36 So the farther away that nitriol group is from the ring, the less of an inductive effect it's going to have.

02:42 So you can treat this as being really an alkyl group.

02:46 So this has electronics similar to an ethyl group because the nitrials electronics aren't going to be felt by the ring.

02:54 So there's actually an electron stabilization here.

02:59 This is going to be your most stable residence form because you have that electronic support from the from the alcohol group in these two carbons.

03:07 Right.

03:08 So this is the most stable form.

03:10 And with the para, i'll draw it over here.

03:13 With the para product, you will also be able to arrive at a form where you can get that to be in the ring.

03:26 Well.

03:29 So why is the propine nitrile a meta director? so you might be able to, so look structurally at the issue here, right? so why is this a meta director? because let's just, let's just entertain the same resonance structure that i drew that offers some stabilization for the ortho -substituent...

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