Which of the following alkyl halides would you expect to...

Which of the following alkyl halides would you expect to undergo FriedelCrafts reaction with rearrangement and which without? Explain. (a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$ (b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$ (c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$ (d) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{Cl}$ (e) Chlorocyclohexane

Which of the following alkyl halides would you expect to undergo FriedelCrafts reaction with rearrangement and which without? Explain.
(a) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$
(d) $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{Cl}$
(e) Chlorocyclohexane

Key Concepts

Friedel-Crafts Alkylation

This is an electrophilic aromatic substitution reaction where an alkyl group is introduced to an aromatic ring via the formation of a carbocation intermediate. A Lewis acid catalyst (such as AlCl3) is used to generate the electrophile by activating the alkyl halide. The overall reaction mechanism relies on the stability and formation of carbocations that can then react with the aromatic system.

Carbocation Formation and Stability

The reaction mechanism of Friedel-Crafts alkylation hinges on the formation of a carbocation intermediate. The stability of the carbocation plays a crucial role in the efficiency of the reaction. Generally, tertiary carbocations are more stable than secondary, and secondary are more stable than primary carbocations. The ability to form a stable carbocation determines whether the reaction can proceed effectively.

Carbocation Rearrangement

When a carbocation intermediate is generated, if it is not in its most stable form, it can undergo rearrangements such as hydride shifts or alkyl shifts. These rearrangements result in a more stable carbocation that can then participate in the electrophilic attack on the aromatic ring. The possibility of rearrangement is a key factor in predicting the product of a Friedel-Crafts alkylation reaction.

Effect of Alkyl Halide Structure

The structure of the alkyl halide directly influences the reaction outcome. Primary alkyl halides may form unstable carbocations and are less likely to be successfully alkylated without rearrangement, whereas secondary and tertiary alkyl halides generate more stable carbocations. Additionally, in some cases, even a primary alkyl halide can rearrange to form a more stable carbocation before the aromatic substitution takes place. This interplay between structure and reactivity plays a central role in determining whether a Friedel-Crafts reaction proceeds with or without rearrangement.

Transcript

0:00 All right.

00:01 So in this question, we're given five alkaliads that would potentially react into frio -crafts alkalation reaction with benzene.

00:09 And we have to determine if the alcohol halide, as they are right now, will rearrange to form some more stable carbocation before electrophilic condition.

00:22 Right.

00:23 So let's take a for a second.

00:26 Let's look at a first.

00:27 So really what you're looking for here is some kind of opportunity for a hydride or an alkalis shift that would give you a primary to a secondary carbocadion or secondary to a tertiary.

00:41 I know primary carbocadions don't really exist.

00:44 So like in the mechanism, for example, for a, your electrophile, let's say aluminum chloride was your example.

00:51 You would arrive at something like this where your chlorine would have two bonds.

00:57 So that would be what your benzene ring would eventually attack the carbon.

01:03 Sorry, attack the carbon here.

01:06 And then the electrons would push back to chlorine to give you alc -o -4 minus because you can't draw a free carbocadion as primary.

01:14 So i'm going to be saying primary carbocation, but just know that it's not possible, right? that or methyl.

01:23 Anyway.

01:24 So in this case, you would arrive at a primary, primary carburecaliant, which isn't possible...

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