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Why is aldehyde C-H bond dissociation energy so much smaller...
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Marcel Müller
Why is aldehyde C-H bond dissociation energy so much smaller...
Of course. You even have a radical depicted in your answer :).
However, the stability of radicals, carb-cations and carb-anions is explained on the basis of the same effects of electron density shift and conjugation (inductive and mesomeric effects, different types of superconjugation, etc.). Therefore, I summarized.
Of course. You even have a radical depicted in your answer :).
However, the stability of radicals, carb-cations and carb-anions is explained on the basis of the same effects of electron density shift and conjugation (inductive and mesomeric effects, different types of superconjugation, etc.). Therefore, I summarized.
This is indeed a beautiful proof. Nevertheless, I prefer the simpler answer Igor and I gave, It is intuitively clear: you stabilize the radical and you lower the De. The C-H bond in the alhedyde group is longer since the same interaction with the lone pair exists in the ground state but now applying Pauli repulsion between the lone-pair on O and the C-H bond.
This is indeed a beautiful proof. Nevertheless, I prefer the simpler answer Igor and I gave, It is intuitively clear: you stabilize the radical and you lower the De. The C-H bond in the alhedyde group is longer since the same interaction with the lone pair exists in the ground state but now applying Pauli repulsion between the lone-pair on O and the C-H bond.
interesting question, I would like to review the paper mentioned by Igor on the 2c,3e-bond on more details, although I think I agree it is the best explanation. Also, would it be consistent with the fact that acylium ion are particularly common and stable in MS (despite ion/radical differenc) ? Therefore, this is a simply a “fact”, clearly a favorable pathway in fragment ion.
interesting question, I would like to review the paper mentioned by Igor on the 2c,3e-bond on more details, although I think I agree it is the best explanation. Also, would it be consistent with the fact that acylium ion are particularly common and stable in MS (despite ion/radical differenc) ? Therefore, this is a simply a “fact”, clearly a favorable pathway in fragment ion.
I also like the proof through radical stabilization. This is a classic of chemistry, as it takes into account the stabilizing factors that are intuitive for chemists.
But, there is one caveat: theoretically, bond breaking can be heterolytic and homolytic, and this strictly depends on the conditions and mechanism of the reactions.
Therefore, the analysis by the structural formula, it seems to me, is more preferable, since we can immediately explain a lot by analyzing only the formula (in fact, X-ray structural analysis). For all mechanisms :).
I think that further development of the theory of chemical bond, for example, the development of electronegativity for organic chemistry similar to inorganics, would clarify a lot depending: properties - structure (biological activity, physical properties, etc.).
I also like the proof through radical stabilization. This is a classic of chemistry, as it takes into account the stabilizing factors that are intuitive for chemists.
But, there is one caveat: theoretically, bond breaking can be heterolytic and homolytic, and this strictly depends on the conditions and mechanism of the reactions.
Therefore, the analysis by the structural formula, it seems to me, is more preferable, since we can immediately explain a lot by analyzing only the formula (in fact, X-ray structural analysis). For all mechanisms :).
I think that further development of the theory of chemical bond, for example, the development of electronegativity for organic chemistry similar to inorganics, would clarify a lot depending: properties - structure (biological activity, physical properties, etc.).
Sason, yes, it is true that chemistry is based on facts and observations. Stable acylium ions and weak C-H bonds in aldehyde are those facts or observations, but what differentiate us from people of 100 years ago is our understanding (called chemistry), which we can rely on to reconcile different observations and even make predictions. sorry I didn’t read the whole threads carefully and I agree with you ;)
Sason, yes, it is true that chemistry is based on facts and observations. Stable acylium ions and weak C-H bonds in aldehyde are those facts or observations, but what differentiate us from people of 100 years ago is our understanding (called chemistry), which we can rely on to reconcile different observations and even make predictions. sorry I didn’t read the whole threads carefully and I agree with you ;)
The short answer is this: the multiplicity of the C-H bond in aldehydes is less than one. To be precise, it is equal to 0.861. This can be shown rigorously and well explained.
There is an equation that uniquely describes the dependence of the energy and the multiplicity of the bond on the length of the chemical bond. Here's the equation:
E = a + b/r + c/r^2
Using the C-H bond data for ethylene, acetylene and ethane, the coefficients can be calculated. Thus, for the C-H bond, we get the equation:
E = a + b/r + c/r^2
a = -112104.88181312
b = 239481.07645903
c = -127352.04548799
If the bond lengths are taken in angstroms, then the bond energy is obtained in KJ/mol.
Further, given that the energy of the C-H bond in the aldehyde group is 364.252 KJ/mol, that is, 87 Kcal/mol, from the corresponding equation we obtain the possible length of the C-H bond. Since this is a quadratic equation, we get two values:
L1 = 1.137787 Å.
L2 = 0.998437 Å.
Considering that the energy of the C-H bond in the aldehyde group is lower than in the C-H bond with a multiplicity of 1 (in ethane), we predict the length of the C-H bond in the aldehyde group to be 1.137787 Å.
The multiplicity of the C-H bond in ethane is taken to be 1.
Then for the aldehyde group:
F = E / L = 364.252 / 1.137787 = 320.14076 KJ/(mol * Å)
Taking into account the multiplicity of the C-H bond in ethane equal to 1 (F = 371.860 KJ / (mol * Å)), we obtain the multiplicity 0.861.
f = 320.14076 / 371.860 = 0.861
The decrease in the multiplicity of bond is easy to explain. Since in the aldehyde group, as in the amide group, there is a three-electron bond between oxygen and carbon (see the structural formula of urea), a part of the electron density of the C-H bond is drawn into the C-O bond. Hence, as a consequence, a decrease in the bond multiplicity, which means a decrease in the bond energy and an increase in the bond length. This is similar to how the basicity of the amine group decreases in amides.
The short answer is this: the multiplicity of the C-H bond in aldehydes is less than one. To be precise, it is equal to 0.861. This can be shown rigorously and well explained.
There is an equation that uniquely describes the dependence of the energy and the multiplicity of the bond on the length of the chemical bond. Here's the equation:
E = a + b/r + c/r^2
Using the C-H bond data for ethylene, acetylene and ethane, the coefficients can be calculated. Thus, for the C-H bond, we get the equation:
E = a + b/r + c/r^2
a = -112104.88181312
b = 239481.07645903
c = -127352.04548799
If the bond lengths are taken in angstroms, then the bond energy is obtained in KJ/mol.
Further, given that the energy of the C-H bond in the aldehyde group is 364.252 KJ/mol, that is, 87 Kcal/mol, from the corresponding equation we obtain the possible length of the C-H bond. Since this is a quadratic equation, we get two values:
L1 = 1.137787 Å.
L2 = 0.998437 Å.
Considering that the energy of the C-H bond in the aldehyde group is lower than in the C-H bond with a multiplicity of 1 (in ethane), we predict the length of the C-H bond in the aldehyde group to be 1.137787 Å.
The multiplicity of the C-H bond in ethane is taken to be 1.
Then for the aldehyde group:
F = E / L = 364.252 / 1.137787 = 320.14076 KJ/(mol * Å)
Taking into account the multiplicity of the C-H bond in ethane equal to 1 (F = 371.860 KJ / (mol * Å)), we obtain the multiplicity 0.861.
f = 320.14076 / 371.860 = 0.861
The decrease in the multiplicity of bond is easy to explain. Since in the aldehyde group, as in the amide group, there is a three-electron bond between oxygen and carbon (see the structural formula of urea), a part of the electron density of the C-H bond is drawn into the C-O bond. Hence, as a consequence, a decrease in the bond multiplicity, which means a decrease in the bond energy and an increase in the bond length. This is similar to how the basicity of the amine group decreases in amides.
Of course. You even have a radical depicted in your answer :).
However, the stability of radicals, carb-cations and carb-anions is explained on the basis of the same effects of electron density shift and conjugation (inductive and mesomeric effects, different types of superconjugation, etc.). Therefore, I summarized.
Of course. You even have a radical depicted in your answer :).
However, the stability of radicals, carb-cations and carb-anions is explained on the basis of the same effects of electron density shift and conjugation (inductive and mesomeric effects, different types of superconjugation, etc.). Therefore, I summarized.
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This is indeed a beautiful proof.
Nevertheless, I prefer the simpler answer Igor and I gave, It is intuitively clear: you stabilize the radical and you lower the De.
The C-H bond in the alhedyde group is longer since the same interaction with the lone pair exists in the ground state but now applying Pauli repulsion between the lone-pair on O and the C-H bond.
This is indeed a beautiful proof.
Nevertheless, I prefer the simpler answer Igor and I gave, It is intuitively clear: you stabilize the radical and you lower the De.
The C-H bond in the alhedyde group is longer since the same interaction with the lone pair exists in the ground state but now applying Pauli repulsion between the lone-pair on O and the C-H bond.
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VOTE
interesting question, I would like to review the paper mentioned by Igor on the 2c,3e-bond on more details, although I think I agree it is the best explanation. Also, would it be consistent with the fact that acylium ion are particularly common and stable in MS (despite ion/radical differenc) ? Therefore, this is a simply a “fact”, clearly a favorable pathway in fragment ion.
interesting question, I would like to review the paper mentioned by Igor on the 2c,3e-bond on more details, although I think I agree it is the best explanation. Also, would it be consistent with the fact that acylium ion are particularly common and stable in MS (despite ion/radical differenc) ? Therefore, this is a simply a “fact”, clearly a favorable pathway in fragment ion.
More
VOTE
I also like the proof through radical stabilization. This is a classic of chemistry, as it takes into account the stabilizing factors that are intuitive for chemists.
But, there is one caveat: theoretically, bond breaking can be heterolytic and homolytic, and this strictly depends on the conditions and mechanism of the reactions.
Therefore, the analysis by the structural formula, it seems to me, is more preferable, since we can immediately explain a lot by analyzing only the formula (in fact, X-ray structural analysis). For all mechanisms :).
I think that further development of the theory of chemical bond, for example, the development of electronegativity for organic chemistry similar to inorganics, would clarify a lot depending: properties - structure (biological activity, physical properties, etc.).
I also like the proof through radical stabilization. This is a classic of chemistry, as it takes into account the stabilizing factors that are intuitive for chemists.
But, there is one caveat: theoretically, bond breaking can be heterolytic and homolytic, and this strictly depends on the conditions and mechanism of the reactions.
Therefore, the analysis by the structural formula, it seems to me, is more preferable, since we can immediately explain a lot by analyzing only the formula (in fact, X-ray structural analysis). For all mechanisms :).
I think that further development of the theory of chemical bond, for example, the development of electronegativity for organic chemistry similar to inorganics, would clarify a lot depending: properties - structure (biological activity, physical properties, etc.).
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I was talking only about the C-H bond dissociation energy. This is by definition a homolyti bond breaking and not heterolytic.
I was talking only about the C-H bond dissociation energy. This is by definition a homolyti bond breaking and not heterolytic.
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Sason, yes, it is true that chemistry is based on facts and observations. Stable acylium ions and weak C-H bonds in aldehyde are those facts or observations, but what differentiate us from people of 100 years ago is our understanding (called chemistry), which we can rely on to reconcile different observations and even make predictions. sorry I didn’t read the whole threads carefully and I agree with you ;)
Sason, yes, it is true that chemistry is based on facts and observations. Stable acylium ions and weak C-H bonds in aldehyde are those facts or observations, but what differentiate us from people of 100 years ago is our understanding (called chemistry), which we can rely on to reconcile different observations and even make predictions. sorry I didn’t read the whole threads carefully and I agree with you ;)
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Thanks Yunlong,
Agreeing with one another is pleasing..
Sason
Thanks Yunlong,
Agreeing with one another is pleasing..
Sason
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the radical R-C(•)=O: is stabilized by 3-electron interaction with the oxygen lone pair.
Sason
the radical R-C(•)=O: is stabilized by 3-electron interaction with the oxygen lone pair.
Sason
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Interest Ask.
The short answer is this: the multiplicity of the C-H bond in aldehydes is less than one. To be precise, it is equal to 0.861. This can be shown rigorously and well explained.
There is an equation that uniquely describes the dependence of the energy and the multiplicity of the bond on the length of the chemical bond. Here's the equation:
E = a + b/r + c/r^2
Using the C-H bond data for ethylene, acetylene and ethane, the coefficients can be calculated. Thus, for the C-H bond, we get the equation:
E = a + b/r + c/r^2
a = -112104.88181312
b = 239481.07645903
c = -127352.04548799
If the bond lengths are taken in angstroms, then the bond energy is obtained in KJ/mol.
The values are taken from the work: What are bond orders? http://dx.doi.org/10.13140/RG.2.2.28092.46726
Further, given that the energy of the C-H bond in the aldehyde group is 364.252 KJ/mol, that is, 87 Kcal/mol, from the corresponding equation we obtain the possible length of the C-H bond. Since this is a quadratic equation, we get two values:
L1 = 1.137787 Å.
L2 = 0.998437 Å.
Considering that the energy of the C-H bond in the aldehyde group is lower than in the C-H bond with a multiplicity of 1 (in ethane), we predict the length of the C-H bond in the aldehyde group to be 1.137787 Å.
CH3 – CH2 – H, Lc-н = 1.0914 Å, Ec-н = 405.848 KJ/mol.
H2C = CH − H, Lc-н = 1.085 Å, Ec-н = 435.136 KJ/mol.
HC≡C − H, Lc-н = 1.059 Å, Ec-н = 476.976 KJ/mol.
That is, the C-H bond in the aldehyde group should have an energy of 364.252 KJ/mol and a length of 1.138 Å.
It is easy to get a fold of 0.861 from here (see http://dx.doi.org/10.13140/RG.2.2.28092.46726).
The multiplicity of the C-H bond in ethane is taken to be 1.
Then for the aldehyde group:
F = E / L = 364.252 / 1.137787 = 320.14076 KJ/(mol * Å)
Taking into account the multiplicity of the C-H bond in ethane equal to 1 (F = 371.860 KJ / (mol * Å)), we obtain the multiplicity 0.861.
f = 320.14076 / 371.860 = 0.861
The decrease in the multiplicity of bond is easy to explain. Since in the aldehyde group, as in the amide group, there is a three-electron bond between oxygen and carbon (see the structural formula of urea), a part of the electron density of the C-H bond is drawn into the C-O bond. Hence, as a consequence, a decrease in the bond multiplicity, which means a decrease in the bond energy and an increase in the bond length. This is similar to how the basicity of the amine group decreases in amides.
Interest Ask.
The short answer is this: the multiplicity of the C-H bond in aldehydes is less than one. To be precise, it is equal to 0.861. This can be shown rigorously and well explained.
There is an equation that uniquely describes the dependence of the energy and the multiplicity of the bond on the length of the chemical bond. Here's the equation:
E = a + b/r + c/r^2
Using the C-H bond data for ethylene, acetylene and ethane, the coefficients can be calculated. Thus, for the C-H bond, we get the equation:
E = a + b/r + c/r^2
a = -112104.88181312
b = 239481.07645903
c = -127352.04548799
If the bond lengths are taken in angstroms, then the bond energy is obtained in KJ/mol.
The values are taken from the work: What are bond orders? http://dx.doi.org/10.13140/RG.2.2.28092.46726
Further, given that the energy of the C-H bond in the aldehyde group is 364.252 KJ/mol, that is, 87 Kcal/mol, from the corresponding equation we obtain the possible length of the C-H bond. Since this is a quadratic equation, we get two values:
L1 = 1.137787 Å.
L2 = 0.998437 Å.
Considering that the energy of the C-H bond in the aldehyde group is lower than in the C-H bond with a multiplicity of 1 (in ethane), we predict the length of the C-H bond in the aldehyde group to be 1.137787 Å.
CH3 – CH2 – H, Lc-н = 1.0914 Å, Ec-н = 405.848 KJ/mol.
H2C = CH − H, Lc-н = 1.085 Å, Ec-н = 435.136 KJ/mol.
HC≡C − H, Lc-н = 1.059 Å, Ec-н = 476.976 KJ/mol.
That is, the C-H bond in the aldehyde group should have an energy of 364.252 KJ/mol and a length of 1.138 Å.
It is easy to get a fold of 0.861 from here (see http://dx.doi.org/10.13140/RG.2.2.28092.46726).
The multiplicity of the C-H bond in ethane is taken to be 1.
Then for the aldehyde group:
F = E / L = 364.252 / 1.137787 = 320.14076 KJ/(mol * Å)
Taking into account the multiplicity of the C-H bond in ethane equal to 1 (F = 371.860 KJ / (mol * Å)), we obtain the multiplicity 0.861.
f = 320.14076 / 371.860 = 0.861
The decrease in the multiplicity of bond is easy to explain. Since in the aldehyde group, as in the amide group, there is a three-electron bond between oxygen and carbon (see the structural formula of urea), a part of the electron density of the C-H bond is drawn into the C-O bond. Hence, as a consequence, a decrease in the bond multiplicity, which means a decrease in the bond energy and an increase in the bond length. This is similar to how the basicity of the amine group decreases in amides.
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You can find a detailed answer here. The 2c,3e-bond in the radical product of C-H bond scission is quite strong:
Article Stereoelectronic power of oxygen in control of chemical reac...
You can find a detailed answer here. The 2c,3e-bond in the radical product of C-H bond scission is quite strong:
Article Stereoelectronic power of oxygen in control of chemical reac...
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Sure, one fact leads to an explanation but generate many more new puzzles. Chemistry advances...
Sason
Sure, one fact leads to an explanation but generate many more new puzzles. Chemistry advances...
Sason
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Glad to read your answer, Igor - so we seem to agree on the stabilizing factor. I just asked for a copy of you paper.
Sason
Glad to read your answer, Igor - so we seem to agree on the stabilizing factor. I just asked for a copy of you paper.
Sason
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