The synthesis of ε-caprolactam (hexano-6-lactam) starts with benzene, which is converted to compound — Organic Chemistry Chemistry Question
All roads lead to caprolactam
The synthesis of ε-caprolactam (hexano-6-lactam) starts with benzene, which is converted to compound A by exhaustive catalytic hydrogenation and subsequently oxidized by air in the presence of cobalt(II) salts of a fatty acid. First, the desired product B has to be separated from the side product B' by fractional distillation. Compound C can be obtained by heating B with hydroxylamine sulfate and the desired ε-caprolactam can be obtained by heating C with sulfuric acid.
A modern alternative to this process is the photochemical reaction of compound A with orange-coloured gas E. Compound E contains a chlorine atom and is also formed during the preparation of aqua regia. Compound D immediately rearranges to compound C.
Caprolactam can also be prepared from buta-1,3-diene by sequential catalytic hydrocyanation with two equivalents of hydrogen cyanide. In the first reaction, in addition to the desired compound, F, compound F' is formed and has to be separated first. Compound G, after partial hydrogenation, provides compound H, which is heated in the presence of water in order to give caprolactam.
[VISUAL]
Draw the structures of unknown compounds A–H.
Model Answer
[VISUAL] The structures of the compounds are as follows:
A: cyclohexane
B: cyclohexanone
B': cyclohexanol
C: cyclohexanone oxime
D: nitrosocyclohexane
E: nitrosyl chloride (NOCl)
F: 4-pentenenitrile
F': 3-pentenenitrile
G: adiponitrile (hexanedinitrile)
H: 6-aminohexanenitrile
Under which conditions will the equilibrium be most shifted from benzene to compound A?
a) 300 °C, 1 atm
b) 300 °C, 100 atm
c) 50 °C, 1 atm
d) 50 °C, 100 atm
Model Answer
d)
Write the equation for the formation of compound E in the preparation of aqua regia and suggest at least one other way to prepare E.
Model Answer
Equation for the formation of nitrosyl chloride (E, NOCl) in aqua regia:
HNO3 + 3 HCl → NOCl + H2O + Cl2
Other preparation methods for NOCl:
1) HNO2 + HCl → H2O + NOCl
2) NO2- + 2 HCl → H2O + NOCl + Cl-
3) NO + Cl2 → 2 NOCl
Suggest the mechanism for the photochemical reaction of A with compound E.
Model Answer
[VISUAL] The photochemical reaction proceeds via a radical mechanism:
1) Initiation (photolysis of nitrosyl chloride to form chlorine and nitric oxide radicals):
NOCl --hv--> NO* + Cl*
2) Propagation (hydrogen abstraction from cyclohexane by chlorine radical, followed by coupling of the cyclohexyl radical with nitrosyl chloride to yield nitrosocyclohexane and regenerate the chlorine radical):
Cl* + C6H12 → HCl + C6H11*
C6H11* + NOCl → C6H11NO + Cl*
What is an approximate wavelength suitable to perform the mentioned photochemical reaction? Hint: E is not colourless.
Model Answer
Gas E (NOCl) is orange. Therefore, the optimal wavelength would be below 530 nm (green and blue light).
Suggest a plausible mechanism for the conversion of C to ε-caprolactam. After whom is the reaction named?
Model Answer
[VISUAL] Plausible mechanism of the conversion of cyclohexanone oxime (C) to ε-caprolactam (Beckmann rearrangement):
1) Protonation of the oxime hydroxyl group by sulfuric acid to turn it into a good leaving group (water).
2) Concerted anti-periplanar migration of the alkyl ring-carbon to nitrogen, with concomitant elimination of water, expanding the 6-membered ring to a 7-membered cyclic nitrilium ion.
3) Nucleophilic addition of water to the nitrilium ion followed by deprotonation to yield an imidic acid intermediate.
4) Tautomerization of the imidic acid to form the more stable amide (ε-caprolactam).
The reaction is named after Ernst Otto Beckmann (Beckmann rearrangement).