Notes: Benzene Chemistry

It’s been a while, everyone! How are you? I hope you’re doing well! Today I bring you notes from my organic chemistry class. Sorry it’s out of the blue—I’m doing this as an exercise in note-taking methods. Enjoy!


Electrophilic Substitution on the Ring

Uneven product (ortho-para) distribution on deactivated rings can be attributed to the ability for an incoming electrophile to disrupt resonance between a deactivating group and the aromatic system when coming into the ortho position. Thus, the ortho position is less deactivated. In activated rings, the preference (ortho-para) isn’t so much a steric one—rather, it occurs because the resulting resonance structure is more stable when the electrophile comes into the para position.

“There’s always a ‘but!’ and there’s always an, ‘Oh, and see, this happens.'”

Halogens: Activating or Deactivating?

  • Halogens are electronegative and withdraw electron density from the carbon atoms they are bonded to by induction.
  • However, halogens can also donate lone pairs through resonance.
  • These are opposing characteristics—this makes halogens mild deactivators that direct ortho-para.
    • NOTE: You can still do Friedal-Crafts chemistry on a halogenated ring. The “last hope” for alkylation in terms of deactivators on the ring.

Halogenated rings undergo electrophilic substitution slower than unsubstituted benzene, but still direct ortho-para because “eight is great” (these resonance structures all have an octet).

Multiple Substituent Effects
         Yaaaay, you can do the recitation sheet now.

  • If the two groups complement each other, go with substitution at the spot they both direct to.
  • Even mild activators are stronger than strong deactivators. Give these priority.
  • If the activating groups are opposing, product mixtures result, with the predominant product being the result of the stronger directing group.
  • If different types of groups are on the ring, activators run the show.
    • Don’t believe the book’s crapola—for our purposes, you will never put an electrophile between two existing groups.
    • (Placate the homework, though. If it asks for all products, include the “trace” product.)
  • With multiple rings, substitution will preferentially occur on the less deactivated ring.
    • NOTE: Benzenes are considered donating groups.

Nucleophilic Substitution on the Ring

Two mechanisms: addition/elimination and benzyne. Preference depends on the substituents on the ring.

(A classic example of “It’s gonna do its own chemistry.”)

“You’re trying to send something that’s negative into a sea of negativity.” That’s why normal nucleophilic substitution doesn’t work with aromatic systems.

Addition/Elimination

  • Requirements:
    • A leaving group on the ring (halogen).
    • A withdrawing group (nitro, cyanide, etc.) ortho or para to the leaving group. (Meta elimination takes longer—yay, kinetics~!)
    • A nucleophile to attack with.
  • When the nucleophile attacks, for a brief instant in time, you disrupt the aromaticity of the ring. The resulting structure is called a “Meisenheimer complex.”
  • Nitro is especially good to use for this because it allows for an additional resonance structure that has all the negative charge in the ring on the electronegative oxygens of the nitro group.

Benzyne

  • The Dow process used to be the leading way to make phenol. (“In Midland, Michigan, ‘Dow’ spelled backwards is ‘god’!”)
  • It’s not exactly a friendly reaction, though. (“It’s a bomb waiting to go off.”)
  • The mechanism for this reaction (which, incidentally, wasn’t figured out until a while after it was being used commercially—”No one cares how it works ’till it stops working”) is E2-like.
  • IT PUTS A FREAKING TRIPLE BOND IN A FREAKING RING OKAY
  • HAHA ALKYNE YOU LIKE TO BE LINEAR THAT’S CUTE
  • SUCKS TO BE YOU, MAN
  • [ahem] Then water adds across the triple bond.
    • Don’t worry about substituents. The alkyne is extremely unstable—”It doesn’t give a rip which side you substitute it from.”
  • Issues?
    • STRAIN, MITCHES
    • Orbital overlap
  • We’ve never isolated it, but we’ve trapped it. Turns out it’s a great diene in Diels-Alder.
    • Some hot-stuff chemists with low-temperature spectroscopy have gotten spectra of it.

Which reaction?

  • Nucleophilic of Electrophilic?
    • Electrophilic —> Electrophilic Aromatic Substitution
    • Nucleophilic —> All criteria for substitution?
      • Yes? Addition/elimination. No? Benzyne.

Why have we learned this? We’re doing synthesis.
(Mind blown, yeah?)

Planning a Synthesis

Keep in mind what order you have to do the steps in to get the results you want. You have to oxidize your methyl group to an acid before you add your nitro group if you want it to add ortho. You can’t nitrate until after you do Friedal-Crafts.

Look at example in the book, because she will shamelessly rip test questions from the book. It’s the end of the semester, after all. Also, all the chemistry you’ve ever done in your life is fair game.

Super Easy Problem

Put your acyl group on first. Then add your bromine meta. (The other way, and you’d have gotten ortho-para acylation.)

“If I forget all the science I’ve learned in two semesters, I’m gonna fall on my academic sword.”

Self, what kind of director is this?”

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