What is the major product of a Friedel-Crafts alkylation...

What is the major product of a Friedel-Crafts alkylation using the following alkyl chlorides? a. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$ b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$ c. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$ d. $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}$ e. $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{Cl}$ f. $\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{Cl}$

What is the major product of a Friedel-Crafts alkylation using the following alkyl chlorides?
a. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}$
b. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}$
c. $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}_{3}$
d. $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl}$
e. $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCH}_{2} \mathrm{Cl}$
f. $\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{Cl}$

Key Concepts

Friedel?Crafts Alkylation

This is an electrophilic aromatic substitution reaction where an alkyl group is introduced onto an aromatic ring using an alkyl halide in the presence of a Lewis acid catalyst such as aluminum chloride. The reaction typically involves the formation of a carbocation intermediate or a related electrophilic complex that attacks the ?-system of the aromatic ring.

Carbocation Stability

The formation and stability of a carbocation intermediate is central to the mechanism of Friedel?Crafts alkylation. Carbocations are more stable when they are tertiary > secondary > primary, due to factors such as hyperconjugation and inductive effects. This stability influences which alkyl halides are effective under the reaction conditions and determines the likelihood of competing side reactions such as rearrangements.

Lewis Acid Catalysis

Lewis acids such as AlCl3 or FeCl3 play a critical role by coordinating to the alkyl halide, thereby polarizing the carbon-halogen bond and facilitating the formation of the carbocation. The strength of the Lewis acid and the nature of the alkyl halide both affect the efficiency of the electrophile generation and, consequently, the overall reaction outcome.

Carbocation Rearrangement

Because the formation of a carbocation intermediate is a key step, rearrangements can occur to yield a more stable carbocation. Hydride shifts or alkyl shifts may take place during the reaction, altering the structure of the electrophile and leading to unexpected reaction outcomes. This is particularly important when starting with primary or secondary alkyl halides that can rearrange to form more stable secondary or tertiary carbocations.

Substrate Limitations

Not all alkyl halides are suitable for Friedel?Crafts alkylation. Primary alkyl halides may lead to unstable carbocations and side reactions, while substrates such as vinyl or aryl halides do not form carbocations readily under these conditions. The success of the reaction is highly dependent on the substrate structure, determining both the feasibility of carbocation formation and the selectivity of the alkylation process.

Transcript

00:03 Suppose we want to predict different reactions or different acylations or, sorry, alkylations of benzene rings.

00:11 So, well, the best way to approach this is to first identify what we're adding.

00:18 And we're adding an alkan.

00:20 And we know that this is always going to be electrophilic, a substitution.

00:27 Electrophilic.

00:30 So what is our electrophile? well, it's going to be a carbocat.

00:37 Ion.

00:39 So we take our alkan group and we ask ourselves which one will be the, where would the ion be? so when i drew this molecule, it's just an ethane group and just an ethane group.

00:59 And where would the carbocadion reside? well, it could be either of them.

01:07 Both of them are equally substituted.

01:09 So now we just know that it'll make this aromatic because our carbonation.

01:18 Carbocatine would not substitute.

01:20 So we know that it will react in the same mechanism that, for example, bromine would react or chlorine would react or acyl compounds would react.

01:35 So there we have it.

01:38 What about this one? whoops.

01:40 Our second one.

01:43 So our electrophile is this.

01:45 So that is a propane or my apologies, a propyl group.

01:51 And the chlorine is attached to the end.

01:53 So we know that we have a carbocadone right here.

01:57 Would that rearrange? as a matter of fact, it would, and it would rearrange to the middle carbon.

02:06 So that will be our reacting carbon...