Carbocations are reactive intermediates having a charge of +1 on the central carbon atom, with three — Organic Chemistry Chemistry Question
Carbocations and Aromaticity
Carbocations are reactive intermediates having a charge of +1 on the central carbon atom, with three groups attached. The carbocation center is electron deficient and lies in the same plane of the three attached atoms. Proton NMR is one of the primary instrumental methods applied to determine the structure and properties of carbocations. In a superacid media, such as SbF5, stable carbocations can be generated and directly observed by NMR. SbF5 is a strong Lewis acid that complexes with a weak base such as F– to form SbF6–.
What is the product A in the following reaction?
[VISUAL]
Model Answer
(CH3)3C+ SbF6–
Two proton NMR spectra of (CH3)3CF were obtained using neat (CH3)3CF and (CH3)3CF in SbF5, respectively. One spectrum, denoted as spectrum I, showed a singlet at 4.35, and the other, spectrum II, revealed a doublet at 1.30 with a coupling constant J = 20 Hz. Which spectrum was obtained from (CH3)3CF in SbF5?
Model Answer
Spectrum I: (CH3)3CF in SbF5
Tropylium ion B is one of the most stable carbocations. How many π electrons are there in the tropylium ion?
[VISUAL]
Model Answer
6 π electrons
Is tropylium ion B an aromatic structure? Explain.
Model Answer
yes
The chemical shift of benzene in the 1H NMR spectrum is = 7.27. What does the proton NMR spectrum of B look like?
(a) A singlet at 9.17.
(b) A singlet at 5.37
(c) A triplet at 9.17.
(d) A Triplet at 5.37.
Model Answer
(a) A singlet at 9.17.
4–Isopropyltropolone C was the first example of a non–benzenoid aromatic compound. It was isolated from cypress trees in Taiwan by Professor T. Nozoe at the National Taiwan University in 1938. Draw resonance structure(s) to illustrate the aromaticity of C.
[VISUAL]
Model Answer
[VISUAL] (Resonance structures showing the carbonyl oxygen carrying a negative charge and the seven-membered ring carrying a positive charge, demonstrating a tropylium-like aromatic ring with 6 pi electrons)
The proton of the OH group in tropolone is acidic. Three moles of tropolone C can react with one mole of tris(2,4–pentanedionato)iron(III) (Fe(acac)3) to form a red complex D. What is the structure of D?
Model Answer
[VISUAL] (Red complex D of Fe(III) coordinated with three bidentate deprotonated tropolonate ligands)