jonny1985_2
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#1
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Hi,

I understand that HO and OH are cations and anions of the same molecule but I keep seeing them used interchangeably.

For example, in an old CGP Biology coursebook, the group on the left-hand side of a glucose molecule was shown as OH, but in the latest revision guide, it is HO.

I see the same thing in the revision guide, on the same page, talking about the same molecule.

Why is this? How do I know which one to use? Does it matter when it comes to being awarded marks in a question?

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Jpw1097
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#2
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(Original post by jonny1985_2)
Hi,

I understand that HO and OH are cations and anions of the same molecule but I keep seeing them used interchangeably.

For example, in an old CGP Biology coursebook, the group on the left-hand side of a glucose molecule was shown as OH, but in the latest revision guide, it is HO.

I see the same thing in the revision guide, on the same page, talking about the same molecule.

Why is this? How do I know which one to use? Does it matter when it comes to being awarded marks in a question?

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It depends on which side you draw the OH group relative to the bond. If you draw it on the right side, write -OH, if you draw it on the left side, write HO-. It’s because the covalent bond is with the oxygen atom, not the hydrogen atom.
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Felynalanine
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Also the first picture is technically wrong but frequently people aren't rigorous about it. It's the same idea as when you write a radical, e.g., the hydroxyl radical is •OH because the unpaired electron is on the O so if you write OH• it's technically wrong but people would still understand what it meant
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andrealocquiao
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They are the same thing it's just that some textbooks take into account the actual structure of the molecule when portraying it. Personally, I think that you should get into a habit of putting the oxygen next to the carbon chain rather than the hydrogen for exams. If you're doing a-level chemistry this is really important to remember because the hydroxy (OH) group is often involved in reaction mechanisms.

For example, you can write ethanol as OHCH2CH3 but it's better to write it as HOCH2CH3 to show the covalent bond between the oxygen and the carbon chain. The electrons are shared between the carbon and the oxygen in the covalent bond, which is why the oxygen goes next to the carbon.

That being said, the first picture could be seen as wrong so avoid it in exams but it is widely accepted.
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jonny1985_2
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#5
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Excellent, thank you so much for all the responses!
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