Triphenylmethane can be prepared by reaction of benzene and chloroform in the presence of AlCl.

Triphenylmethane can be prepared by reaction of benzene and...

Key Concepts

Friedel?Crafts Electrophilic Aromatic Substitution

This reaction type involves an aromatic ring acting as a nucleophile that attacks an electrophilic species. In the overall mechanism, benzene serves as the nucleophile and undergoes substitution reactions with electrophilic intermediates generated throughout the process, a hallmark of Friedel?Crafts chemistry.

Lewis Acid Activation

A key step in the mechanism is the activation of the halogenated methane by a Lewis acid, in this case aluminum chloride (AlCl?). The Lewis acid coordinates to the chlorines in chloroform (CHCl?), polarizing the C–Cl bonds and facilitating the formation of a highly electrophilic species that can be attacked by benzene.

Carbocation Intermediate Formation and Stabilization

The interaction of chloroform with AlCl? leads to the generation of a carbocationic intermediate (such as a trichloromethyl cation or a related species). This carbocation is stabilized by the electron-donating effects of the aromatic rings as they are progressively added, which is critical for the subsequent nucleophilic aromatic substitution steps.

Sequential Aromatic Substitution and Dehalogenation

The mechanism proceeds through multiple rounds of electrophilic aromatic substitution events. In each step, a benzene ring attacks the electrophilic carbon, replacing a chlorine atom with a phenyl group. This sequential process, combined with dehalogenation steps facilitated by the Lewis acid, ultimately builds up the triphenylmethane structure.

Transcript

00:01 All right.

00:01 So this is an interesting frito -crafts alkylation example, right? so chloroform plus benzene will give you trifenomethine in the presence of aluminum chloride.

00:13 And why is that? so generally, if you're thinking about a mechanism, you would want to relate it to what you know about fridotrath's alkylation as closely as you can.

00:24 Right.

00:24 So generally, in all of the examples that you've done so far, there's only been one chlorine carbon bond that you're able to explain.

00:31 Exploit with aluminum chloride.

00:33 In this case, you have three.

00:35 So you might be able to see the connection between the three carbon chlorine bonds in chloroform and the three phenol groups on the methane.

00:47 Right.

00:48 So if you, let's just do the first one together.

00:54 So ignore whatever else is on the chlorine, on the chloroform, right? so if you're going to activate, so in the first step, right, so you have this cl2, you have this carbon chlorine bond that you're activating with aluminum chloride.

01:15 And remember, you can't make methyl carboxadion.

01:20 So this is going to look weird.

01:22 And you don't quite, you might not quite understand why it's possible for chlorine to have two bonds here.

01:28 But just hang with me.

01:30 So you have two chlorines.

01:33 Here's your h.

01:35 There's the chlorine that attacked the aluminum, and then you have aluminum trichloride.

01:41 So you have to write it like this because you can't, in a textbook, the mechanism uses a, i think it's a secondary alkaliad.

01:49 So you're able to put a secondary carbocadion free in solution as your electrophile for benzene to then attack.

01:54 In this case, you can't just put, you can't have this.

02:02 Floating in solution that carbocata? because it's technically a methylcarbacadone.

02:07 And i'm sure those chlorines don't really help the stability of that carbocadion at all.

02:13 But in this case, this is your electrophile right here.

02:17 So i have to put a negative charge on aluminum because it has four bonds.

02:21 Aluminum likes to have three bonds.

02:23 It's also in group 13.

02:24 So four minus three in terms of the formal charge calculation is negative one.

02:28 Chlorine likes to have one bond in three long pairs right now.

02:31 It has two bonds and two loan pairs.

02:34 So it's going to be seven minus six is going to give you positive one.

02:39 So this is actually a coordination bonds, right? so it's a it's a bonds between the loan pair on chlorine and a metal aluminum.

02:49 So that's the kind of bond that chlorine is allowed to have those coordination bonds.

02:54 If you're going to draw chlorine with two bonds, it also has de -orbidals.

02:57 We can pretty much do whatever it wants, but you don't really know too much of that hour pushing.

03:00 And you won't until you take inorganic chemistry.

03:03 But anyway, forget that.

03:05 Right, so that's your electrophile.

03:06 So now this is where benzene is going to come in.

03:11 I'm actually going to put it over here just to save space.

03:17 Benzine comes in, and with this long arrow, there is your electrophile.

03:24 So the positive charge is on chlorine, but it actually makes the carbon more electrophilic through induction.

03:30 So it's going to use the electron density from that carbon atom to, support the positive charge on chlorine so it actually makes the carbon more electrophilic and you might have seen that in in a previous question as to why when you have some kind of tetraalcohol ammonium on a benzene ring with that formal positive charge why it's a stronger electron withdrawing group than a nitro group for example it's because you need all that electron density from the benzene ring to support the positive charge and stabilize it so with that you're going to have the first addition here.

04:09 So you're going to end up with this positive charge.

04:13 There's the h...

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