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Reaction of cyclooctatetraene with sulfuric acid
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Larry Radford
Reaction of cyclooctatetraene with sulfuric acid
This is a rather unusual and interesting case of aromaticity, which has been given a special name: homoaromaticity.
The Wikipedia page does a quite nice job of explaining what's going on. As you state, protonation of cyclooctatetraene generates a $\ce{C8H9^{+}}$ cation containing an $\mathrm{sp^3}$-hybridised carbon atom between all the other $\mathrm{sp^2}$-hybridised carbons. However, what's surprising is that this does not imply aromaticity must be broken. If you comfortably understand that the cyclooctatetraenide anion $\ce{C8H8^{2-}}$ is aromatic, you probably also know that the tropylium cation $\ce{C7H7^{+}}$ is also aromatic. As it turns out, this aromaticity is sufficiently strong that if you add a $\mathrm{sp^3}$-hybridised $\mathrm{CH_2}$ linkage, the cation pushes the$\mathrm{sp^3}$carbon out of the way to allow through-space conjugation and maintain aromaticity.
The homotropylium cation is exactly what cyclooctatetraene rearranges into upon protonation. The evidence for the aromatic nature of the homotropylium cation is ironclad. NMR spectrometry is one of the best techniques to determine the presence and strength of aromaticity in a compound, due to aromatic ring currents. In the homotropylium cation, the hydrogen atoms in the $\mathrm{CH_2}$ linkage are magnetically inequivalent, and indeed, starkly so - their $\mathrm{^1H}$ chemical shifts differ by over 6 ppm. In particular, one hydrogen atom lies above the ring and is heavily shielded by the aromatic ring current, generating a signal at -0.73 ppm which is very unusual for a carbocation.
It would be excellent to study the crystal structure of the homotropylium cation, but there do not seem to be any experimental data for examples containing the parent ion, only derivatives such as a hydroxylated homotropylium cation (in fact, I'm surprised the parent ion hasn't been crystallised as a carborane superacid salt). Nevertheless, structural evidence for aromaticity can also be found in the derivatives, in the approximate coplanarity of the $\mathrm{sp^2}$-hybridised carbons and the unusual distance between the $\mathrm{sp^2}$ carbons directly attached to the $\mathrm{sp^3}$ carbon (longer than almost any C-C single bond, but far too short for isolated carbon atoms in the absence of aggressive steric constraints).
For some more information, check out this reference: The homotropylium ion and homoaromaticity, Ronald F. Childs, Accounts of Chemical Research, 1984, 17 (10), 347-352 DOI: 10.1021/ar00106a001
This is a rather unusual and interesting case of aromaticity, which has been given a special name: homoaromaticity.
The Wikipedia page does a quite nice job of explaining what's going on. As you state, protonation of cyclooctatetraene generates a $\ce{C8H9^{+}}$ cation containing an $\mathrm{sp^3}$-hybridised carbon atom between all the other $\mathrm{sp^2}$-hybridised carbons. However, what's surprising is that this does not imply aromaticity must be broken. If you comfortably understand that the cyclooctatetraenide anion $\ce{C8H8^{2-}}$ is aromatic, you probably also know that the tropylium cation $\ce{C7H7^{+}}$ is also aromatic. As it turns out, this aromaticity is sufficiently strong that if you add a $\mathrm{sp^3}$-hybridised $\mathrm{CH_2}$ linkage, the cation pushes the$\mathrm{sp^3}$carbon out of the way to allow through-space conjugation and maintain aromaticity.
The homotropylium cation is exactly what cyclooctatetraene rearranges into upon protonation. The evidence for the aromatic nature of the homotropylium cation is ironclad. NMR spectrometry is one of the best techniques to determine the presence and strength of aromaticity in a compound, due to aromatic ring currents. In the homotropylium cation, the hydrogen atoms in the $\mathrm{CH_2}$ linkage are magnetically inequivalent, and indeed, starkly so - their $\mathrm{^1H}$ chemical shifts differ by over 6 ppm. In particular, one hydrogen atom lies above the ring and is heavily shielded by the aromatic ring current, generating a signal at -0.73 ppm which is very unusual for a carbocation.
It would be excellent to study the crystal structure of the homotropylium cation, but there do not seem to be any experimental data for examples containing the parent ion, only derivatives such as a hydroxylated homotropylium cation (in fact, I'm surprised the parent ion hasn't been crystallised as a carborane superacid salt). Nevertheless, structural evidence for aromaticity can also be found in the derivatives, in the approximate coplanarity of the $\mathrm{sp^2}$-hybridised carbons and the unusual distance between the $\mathrm{sp^2}$ carbons directly attached to the $\mathrm{sp^3}$ carbon (longer than almost any C-C single bond, but far too short for isolated carbon atoms in the absence of aggressive steric constraints).
For some more information, check out this reference: The homotropylium ion and homoaromaticity, Ronald F. Childs, Accounts of Chemical Research, 1984, 17 (10), 347-352 DOI: 10.1021/ar00106a001
Thanks a lot. The structures and Wikipedia links clarified it for me (I dont know anything about NMR except its full form, so I couldnt make sense of that paragraph. But of course more learnt users will certainly find that part useful too).More
I guess its a tiny leap for extended pi-systems becoming aromatic, its a bit funky that this description works best for cations. The wonderful $\ce{C7H8}$, which should possibly be the best representative is mentioned in the footnotes. I guess it just sounds funky enough. Apart from this, one point that should always be mentioned when written down: There are no such thing as hybridised atoms. They might be atoms which can be described with hybrid orbitals, but that is just it.More
This is a rather unusual and interesting case of aromaticity, which has been given a special name: homoaromaticity.
The Wikipedia page does a quite nice job of explaining what's going on. As you state, protonation of cyclooctatetraene generates a $\ce{C8H9^{+}}$ cation containing an $\mathrm{sp^3}$-hybridised carbon atom between all the other $\mathrm{sp^2}$-hybridised carbons. However, what's surprising is that this does not imply aromaticity must be broken. If you comfortably understand that the cyclooctatetraenide anion $\ce{C8H8^{2-}}$ is aromatic, you probably also know that the tropylium cation $\ce{C7H7^{+}}$ is also aromatic. As it turns out, this aromaticity is sufficiently strong that if you add a $\mathrm{sp^3}$-hybridised $\mathrm{CH_2}$ linkage, the cation pushes the $\mathrm{sp^3}$ carbon out of the way to allow through-space conjugation and maintain aromaticity.
The homotropylium cation is exactly what cyclooctatetraene rearranges into upon protonation. The evidence for the aromatic nature of the homotropylium cation is ironclad. NMR spectrometry is one of the best techniques to determine the presence and strength of aromaticity in a compound, due to aromatic ring currents. In the homotropylium cation, the hydrogen atoms in the $\mathrm{CH_2}$ linkage are magnetically inequivalent, and indeed, starkly so - their $\mathrm{^1H}$ chemical shifts differ by over 6 ppm. In particular, one hydrogen atom lies above the ring and is heavily shielded by the aromatic ring current, generating a signal at -0.73 ppm which is very unusual for a carbocation.
It would be excellent to study the crystal structure of the homotropylium cation, but there do not seem to be any experimental data for examples containing the parent ion, only derivatives such as a hydroxylated homotropylium cation (in fact, I'm surprised the parent ion hasn't been crystallised as a carborane superacid salt). Nevertheless, structural evidence for aromaticity can also be found in the derivatives, in the approximate coplanarity of the $\mathrm{sp^2}$-hybridised carbons and the unusual distance between the $\mathrm{sp^2}$ carbons directly attached to the $\mathrm{sp^3}$ carbon (longer than almost any C-C single bond, but far too short for isolated carbon atoms in the absence of aggressive steric constraints).
For some more information, check out this reference: The homotropylium ion and homoaromaticity, Ronald F. Childs, Accounts of Chemical Research, 1984, 17 (10), 347-352 DOI: 10.1021/ar00106a001
This is a rather unusual and interesting case of aromaticity, which has been given a special name: homoaromaticity.
The Wikipedia page does a quite nice job of explaining what's going on. As you state, protonation of cyclooctatetraene generates a $\ce{C8H9^{+}}$ cation containing an $\mathrm{sp^3}$-hybridised carbon atom between all the other $\mathrm{sp^2}$-hybridised carbons. However, what's surprising is that this does not imply aromaticity must be broken. If you comfortably understand that the cyclooctatetraenide anion $\ce{C8H8^{2-}}$ is aromatic, you probably also know that the tropylium cation $\ce{C7H7^{+}}$ is also aromatic. As it turns out, this aromaticity is sufficiently strong that if you add a $\mathrm{sp^3}$-hybridised $\mathrm{CH_2}$ linkage, the cation pushes the $\mathrm{sp^3}$ carbon out of the way to allow through-space conjugation and maintain aromaticity.
The homotropylium cation is exactly what cyclooctatetraene rearranges into upon protonation. The evidence for the aromatic nature of the homotropylium cation is ironclad. NMR spectrometry is one of the best techniques to determine the presence and strength of aromaticity in a compound, due to aromatic ring currents. In the homotropylium cation, the hydrogen atoms in the $\mathrm{CH_2}$ linkage are magnetically inequivalent, and indeed, starkly so - their $\mathrm{^1H}$ chemical shifts differ by over 6 ppm. In particular, one hydrogen atom lies above the ring and is heavily shielded by the aromatic ring current, generating a signal at -0.73 ppm which is very unusual for a carbocation.
It would be excellent to study the crystal structure of the homotropylium cation, but there do not seem to be any experimental data for examples containing the parent ion, only derivatives such as a hydroxylated homotropylium cation (in fact, I'm surprised the parent ion hasn't been crystallised as a carborane superacid salt). Nevertheless, structural evidence for aromaticity can also be found in the derivatives, in the approximate coplanarity of the $\mathrm{sp^2}$-hybridised carbons and the unusual distance between the $\mathrm{sp^2}$ carbons directly attached to the $\mathrm{sp^3}$ carbon (longer than almost any C-C single bond, but far too short for isolated carbon atoms in the absence of aggressive steric constraints).
For some more information, check out this reference: The homotropylium ion and homoaromaticity, Ronald F. Childs, Accounts of Chemical Research, 1984, 17 (10), 347-352 DOI: 10.1021/ar00106a001
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