ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: PB on April 07, 2013, 08:01:19 pm
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ok so I was looking reading through some organic chem stuff (carboxylic functional groups) and it said that it could participate in hydrogen bonding through the C=O part. I thought that Hydrogen bonding can only occur through N,O or F?
like I understand its Oxygen but doesn't it have to be covalently bonded to Hydrogen to work? Not Carbon? I'm guessing its something to do with the double bond ? :P Thanks
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ok so I was looking reading through some organic chem stuff (carboxylic functional groups) and it said that it could participate in hydrogen bonding through the C=O part. I thought that Hydrogen bonding can only occur through N,O or F?
like I understand its Oxygen but doesn't it have to be covalently bonded to Hydrogen to work? Not Carbon? I'm guessing its something to do with the double bond ? :P Thanks
It can form hydrogen bonds, just think about what would happen if it was to be dissolved in water. It is for this reason that smaller carboxylic acids (methanoic acid and ethanoic acid, etc.) are highly miscible in polar solvents such as water. It can form through Oxygen, when it is dissolved in water the positive dipole of the water molecule (hydrogen) would aline to that double bonded oxygen and the oxygen would aline to the hydrogen of the carboxylic acid. Furthermore, if you look at a carboxylic acid it can form hydrogen bonds with itself because something similar would happen as the aforementioned scenario. I hope that helped! :)
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ok so I was looking reading through some organic chem stuff (carboxylic functional groups) and it said that it could participate in hydrogen bonding through the C=O part. I thought that Hydrogen bonding can only occur through N,O or F?
like I understand its Oxygen but doesn't it have to be covalently bonded to Hydrogen to work? Not Carbon? I'm guessing its something to do with the double bond ? :P Thanks
C=O is a hydrogen bond acceptor.
Hydrogen bonds are formed between a H covalently bonded to N/O/F and an extremely electronegative atom (like N/O/F) covalently bonded to an electropositive atom like H and C.
You'll find that hydrogen bonds are really an arbitrary subcategory of dipole-dipole interactions. Mao will know more about this.
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Ah, thks Rexxy, if even you are slightly unsure the answer then I guess it is not of extreme importance in the VCE course haha :P I think this is just one of the things I would simply accept as true.
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It can form hydrogen bonds, just think about what would happen if it was to be dissolved in water. It is for this reason that smaller carboxylic acids (methanoic acid and ethanoic acid, etc.) are highly miscible in polar solvents such as water. It can form through Oxygen, when it is dissolved in water the positive dipole of the water molecule (hydrogen) would aline to that double bonded oxygen and the oxygen would aline to the hydrogen of the carboxylic acid. Furthermore, if you look at a carboxylic acid it can form hydrogen bonds with itself because something similar would happen as the aforementioned scenario. I hope that helped! :)
Thanks for replying, but I think your answer is an observation-based explanation not really an explanation of why it is that way...but thanks anyway!
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With a C=O bond, the electrons that participate in the bond are closer to the oxygen than the carbon, which gives the oxygen a partial negative charge. As a result, the C has a partial positive charge, because electrons are further away from it. The reason Oxygen attracts these electrons is because it is more electronegative than carbon.
When a Hydrogen is bonded to something like Nitrogen, Oxygen, Chlorine or Fluorine, it will end up with a partial positive charge.
For example, if we had a H-N bond, the nitrogen would pull the electrons in that bond toward itself, resulting in it having a partial negative charge. Since the electrons in that bond spend less time near the Hydrogen and more time near the Nitrogen, the Hydrogen will have a partial positive charge.
Now say we have two separate compounds. One has a C=O bond, and the other has a H-N bond. Say we position these two compounds in such a way that the C=O bond is next to the H-N bond. As explained above, the O in the C=O has a partial negative charge, and the H in the H-N has a partial positive charge. We know that a negative charge is attracted to a positive charge, so we know that the O from the C=O and the H from the H-N will be attracted to each other, because they each carry opposite charges.
As a result, a Hydrogen bond will form between the O and the H of the two separate compounds.
I think what confused you is that you thought a Hydrogen bonded to a Nitrogen, Oxygen of Fluorine is itself a hydrogen bond. A H-F bond is not a Hydrogen bond, it is a covalent bond.
Hydrogen bonds are usually said to be intermolecular (between two separate molecules) while Covalent bonds are intramolecular (within the molecule itself)
Two different molecules or compounds will 'stick' together through a Hydrogen bond, and can be separated relatively easily. Covalent bonds on the other hand are much harder to break.
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As a result, a Hydrogen bond will form between the O and the H of the two separate compounds.
I think what confused you is that you thought a Hydrogen bonded to a Nitrogen, Oxygen of Fluorine is itself a hydrogen bond. A H-F bond is not a Hydrogen bond, it is a covalent bond.
Hey nubs, thanks for replying but I think I am going to have to disagree on this.
Just because two compounds are partially negative and partially positive does not necessarily mean its capable of H bonding...(I apologise if thats not what you meant)
But then again, Rexxy clarified for me that " an extremely electronegative atom (like N/O/F) covalently bonded to an electropositive atom like H and C" is capable of bonding with "between a H covalently bonded to N/O/F". So yeh.
Secondly, no i do not think that a Hydrogen bonded to a Nitrogen, Oxygen of Fluorine is itself a hydrogen bond. I am fully aware that it is a covalent bond :)
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I'll try and clarify. Hydrogen bonding occurs with N, O and F when bonded to hydrogen because they are small atoms and because there is a relatively large electronegativity difference between them and hydrogen. Therefore, when bonded to hydrogen, they acquire a substantial partial negative charge.
When carbon forms a double bond with oxygen, however, we are speaking of a double bond. Here, oxygen is polarized twice as much as a regular C-O bond. Also, the electronegativity difference between oxygen and carbon is around 1.0. Therefore, oxygen here is also significantly polarized, as much, if not more than, as one in an O-H bond. It also still has its two electron pairs to form H-bonds with.
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Hey nubs, thanks for replying but I think I am going to have to disagree on this.
Just because two compounds are partially negative and partially positive does not necessarily mean its capable of H bonding...(I apologise if thats not what you meant)
Hey, yeah I didn't say the compound itself would be partially negative and partially positive, but certain atoms within the compound would be
H-Bonding will only occur between a Hydrogen and any of Nitrogen, Oxygen Fluorine as you've already said.
Other 'bonds' that form between compounds that have a partial positive end and a partial negative end that don't involve H-FON are just dipole-dipole forces.
H-Bonds are just a stronger form of dipole-dipole interactions
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I'll try and clarify. Hydrogen bonding occurs with N, O and F when bonded to hydrogen because they are small atoms and because there is a relatively large electronegativity difference between them and hydrogen. Therefore, when bonded to hydrogen, they acquire a substantial partial negative charge.
When carbon forms a double bond with oxygen, however, we are speaking of a double bond. Here, oxygen is polarized twice as much as a regular C-O bond. Also, the electronegativity difference between oxygen and carbon is around 1.0. Therefore, oxygen here is also significantly polarized, as much, if not more than, as one in an O-H bond. It also still has its two electron pairs to form H-bonds with.
Yeh true, that was what I was thinking too..something to do with the double bond