Propose a mechanism for the electrophilic fluorination of...

Key Concepts

Sigma Complex (Arenium Ion) Formation and Re-aromatization

The reaction mechanism includes the formation of a sigma complex, also known as an arenium ion, resulting from the initial attack of the electrophilic fluorine on the aromatic ring. This intermediate, although non-aromatic, is stabilized by resonance. Its subsequent deprotonation is critical as it re-establishes the aromaticity of the ring, finalizing the substitution process. This two-step process, involving the formation and resolution of a sigma complex, is central to understanding the transformation.

Generation of the Electrophilic Species

The mechanism relies on the formation of an active electrophilic fluorinating species from the reagent. This species is sufficiently electron-deficient to attack the aromatic ring. Understanding the nature of this electrophile, including its polarization and leaving group characteristics, is essential in rationalizing the reactivity and selectivity observed in electrophilic fluorination reactions.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an aromatic compound, like benzene, reacts with an electrophile. In this process, the aromatic ring temporarily loses its aromaticity upon attack by the electrophile, forming a sigma complex (arenium ion), followed by a deprotonation step that restores the aromatic system. This mechanism is key to understanding how electron-deficient species are introduced onto aromatic rings.

Electrophilic Fluorination

Electrophilic fluorination involves the introduction of a fluorine atom into an organic molecule via an electrophilic species, rather than by a nucleophilic fluorine source. Reagents designed for electrophilic fluorination, such as F?TEDA?BF4, generate a positively polarized fluorine atom that can interact with electron-rich aromatic rings, making it possible to form carbon–fluorine bonds under mild conditions.

Transcript

00:01 Okay, so you have to propose a mechanism for the electrophilic fluorination of benzene using the reagent select floor, which was introduced on page 570 in chapter 16.

00:18 So this is a pretty interesting reagent, right? so you're not used to seeing fluorine being electrophilic, but having this massive alkalamonium molecule.

00:32 Here, it actually polarizes this bonds in the direction of nitrogen.

00:38 So if you go strictly based off an electronegativity argument, nitrogen and fluorine are similar, fluorine obviously being more electronegative, but having the alcohol groups on the nitrogen and the positive charge is actually a huge electron withdrawing group.

00:54 You can think about it like that.

00:56 So in this case, the fluorine is actually going to be electrophilic.

01:00 So you can think about fluorine as only having six valence electrons as f plus, right? and then when it gets attacked by a nucleophile, going to form a new two electron bonds to give it eight valence electrons when you form the new bonds, right? so it's just a matter of arrow pushing.

01:22 So i'm actually not going to draw that base the whole time.

01:27 I'm going to substitute it as, i'm going to keep the forming in blue.

01:32 It'll take me, the video will be twice as long if i kept redrawing that base.

01:36 So i'm going to call it, i'll call it r3.

01:44 So realistically, this is a doubly positive charged base, right? so i'll say 2 plus, and then there's two bf4's balancing the charge.

01:56 Bf4 is tetraforoborate anion.

02:00 So first steps, so you write, so you can, think about this as fluorine is partially positive, the nitrogen is partially negative.

02:08 Even though it has a full positive charge, electron density is actually being pulled into that nitrogen.

02:13 So when you break the first bonds, you're going to push electron dense, you're going to push the electrons onto nitrogen.

02:20 So you're going to give that nitrogen back its lone pair.

02:23 So in the process of doing that, you are breaking aromaticity.

02:31 Right.

02:31 So there's your fluorine carbocation.

02:34 And now you have, have a neutral amine base, sorry, that's a big three, a neutral immune base...

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