You know the mechanism of HBr addition to alkenes, and you know the effects of various substitue

You know the mechanism of HBr addition to alkenes, and you...

Key Concepts

Substituent Effects

Substituent effects refer to the influence of attached groups on the reactivity of the reactive center, such as in carbocation formation. Electron-donating groups stabilize carbocations by providing electron density through resonance or induction, while electron-withdrawing groups destabilize them. Similar concepts that are applied in aromatic substitution help predict how different substituents on an alkene will affect the rate of reaction with electrophiles like HBr.

Electrophilic Addition Mechanism

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile adds to a double bond. In the addition of HBr to an alkene, the ? electrons attack the acid, forming a carbocation intermediate followed by nucleophilic attack by the bromide. This stepwise mechanism is key to understanding the overall reaction kinetics and regioselectivity observed in these reactions.

Carbocation Stability

The reactivity of the alkene toward HBr is largely determined by the stability of the carbocation intermediate formed during the reaction. More stable carbocations, which result from greater substitution or resonance stabilization, lower the activation energy for the reaction and lead to a faster reaction rate. Understanding the factors that enhance carbocation stability is essential for predicting the rate and outcome of electrophilic addition reactions.

Resonance Stabilization

Resonance stabilization plays a significant role in the stabilization of carbocation intermediates. When a positive charge can be delocalized over an extended ?-system or through adjacent substituents, the stability of the carbocation increases. Drawing resonance structures can illustrate how electron donation from substituents or aromatic systems can help disperse the positive charge, thereby influencing the reaction rate.

Transcript

00:01 All right.

00:02 So which of these alkenes do you think is going to be faster to react with a via electrophilic substitution with hbr? so you have methoxy on one that's para to an alken and you have a nitro on the other that's power to an alkeen.

00:20 So you might be thinking, i don't understand why this is a problem if i have an electron donating group that's attached to a pie system.

00:28 The pie system will have more electron density.

00:30 It'll make the alkyne more electron rich, so it'll react with protons faster.

00:37 Yeah, that's very valid, right? that's super valid.

00:40 So by extension, if you have an electron withdrawal group on the benzene ring, it's going to be sucking electron density out of the pie system.

00:47 So therefore, the alkyene at the ends will be a weaker, will have less electron density, so it won't be as basic for that proton.

00:58 So that's a super good conclusion, but how do you prove it with resonance? right? so let's just say that i don't like how i'm going to write this.

01:10 I'll put the h's on the left of the c because we show accurate connectivity here.

01:24 What if i tell you, let's physically donate those electrons into the ring and let's push the electrons around? so i'm going to push that pie bonds to the other side, to that carbon carbon bonds.

01:35 To push this onto the alkene.

01:38 I'm going to push this double bond onto that terminal carbon.

01:42 So by doing that, i will arrive at a structure that is electrons stabilized by this positive charge here.

01:56 And now you'll notice that i've pushed the electrons onto this carbon that now gives me this carbonyon structure.

02:06 So you'll notice that by resonance, this, this carbon is electron rich, right? which means it'll be more bronze -dad basic.

02:18 It'll be more basic in general for the proton in hbr, which means you will arrive at a faster benzilic carbocadion in that intermediate.

02:29 Also, having that group para means that even before i get to this resident structure where i have the oxygen.

02:42 On the methoxy already in a double bond.

02:45 Let's just draw a mechanism where i've already taken the proton.

02:53 And you'll be like, matt, what are you going to show us? right.

02:57 So let's say i decided to do this.

03:02 That will give me a ring that has the benzodia carbonateon.

03:08 Here's the h we added.

03:10 Right.

03:11 And then i could totally, i'm not going to draw all of them because that's overkill right now, but i could totally push my electrons like this...

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