When 3,3-dimethyl-1-butene is treated with HBr alone, the...

Key Concepts

Anti-Markovnikov Addition

Anti-Markovnikov addition refers to the phenomenon where the addition of reagents to an alkene leads to the substituent (for example, bromine) attaching to the less substituted carbon of the double bond. This outcome is characteristic of the free radical mechanism, where the radical intermediate forms preferentially at the less hindered position, contrary to the trend observed in ionic mechanisms driven by carbocation stability.

Free Radical Mechanism

When peroxides are present, the reaction pathway shifts from a carbocation (ionic) mechanism to a free radical mechanism. Free radicals are highly reactive species that form through the homolytic cleavage of bonds, often initiated by peroxides. In this mechanism, a radical chain reaction is established via initiation, propagation, and termination steps. The outcome is a shift in regioselectivity of the addition, leading to products that differ from those formed under the electrophilic pathway.

Peroxide Effect

The peroxide effect describes how the presence of peroxides can change the pathway of addition reactions with hydrogen halides. In the presence of peroxides, the reaction mechanism shifts to a radical pathway, initiating free radical reactions that result in anti-Markovnikov product formation. This effect is a key example of how reaction conditions can drastically alter the course of an organic reaction by stabilizing different intermediate species.

Electrophilic Addition Mechanism

This mechanism describes how electrophiles add to alkenes. In the reaction with HBr alone, the alkene’s ? electrons interact with the hydrogen of HBr to first form a carbocation intermediate. The subsequent attack by bromide on this carbocation leads to product formation following Markovnikov's rule. This mechanism is central to many addition reactions in organic chemistry, where the stability of intermediate species determines the outcome of the reaction.

Carbocation Stability

During electrophilic addition, the formation of a carbocation intermediate occurs, and its stability is crucial in determining which product will predominate. More substituted carbocations are generally more stabilized by hyperconjugation and inductive effects. In reactions following this pathway, the major product arises through the formation of the most stable carbocation intermediate, which then captures a nucleophile to yield the product in accordance with Markovnikov's rule.

Transcript

00:03 This is the answer to chapter 15, problem number 59, from the smith organic chemistry textbook.

00:11 And this problem says, when 3 -3 -dymethyl -1 butene is treated with hbr alone, the major product is 2 -bromo -2 -3 -dymethylbutane.

00:21 When the same alkene is treated with hbr and peroxide, the sole product is 1 -bromo -3 -dymethylbutane.

00:28 Explain these results by referring to the mechanisms.

00:31 Okay, so let's do that.

00:36 So the molecule that we're talking about, 3 -3 -dymethyl 1 -butene looks like this.

00:55 There we go.

00:56 So 3 -3 -dymethyl 1 -butene.

00:59 And so when we treat this with hbr under non -radical conditions, the mechanism is going to be this.

01:10 So the electrons of the double bond will grab this proton.

01:17 The hbr bond breaks.

01:20 And so we have this after the first step with a carbocadion located here on this secondary carbon.

01:35 And what's going to happen here is we will get a 1 -2 methyl shift.

01:49 So a 1 -2 methyl shift to get this intermediate.

01:59 And so now our carbocation is on a tertiary carbon.

02:06 And so actually, i guess, not that it matters since the molecule is symmetrical, but it would be more correct to say that the carbocation is located here.

02:18 Again, it doesn't matter the molecule symmetrical.

02:23 And so now we have a tertiary carbocadion.

02:26 And so now the bromine ion, the bromide, that we formed in our first step of this mechanism can come and attack and add there.

02:43 And so we get to our product, which is this.

02:54 Okay, so 2 -bromo -2 -3 dimethylbutane.

03:01 Okay...

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