Why Can't you have two alcohol groups (hydroxyl groups) on the same carbon atom?

[Hexon]

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question as in title.
Surely the octet rule is satisfied in this respect?
at least according to the lewis dot structure😛
Thanks guys

 

[Docility]

I'm pretty sure you can have two hydroxyl groups on the same carbon atom...it's called a gem (geminal) diol. 🙂

http://science.uvu.edu/ochem/index.php/alphabetical/g-h/gem-diol/

 

[Hexon]

I'm pretty sure you can have two hydroxyl groups on the same carbon atom...it's called a gem (geminal) diol. 🙂

http://science.uvu.edu/ochem/index.php/alphabetical/g-h/gem-diol/

oh ok, haha i was thinking in terms of alkanes and the such. why you can't have two OH groups on the same carbon atom in the chain😛
two alcohol groups opposite each other on the carbon chain, attached to the same carbon?

 

[LoLCareerGoals]

So what are you asking is why can't there be a C(OH)2 = CH2?
It might be possible, probably just unstable. That carbon would be really el-positive and get easily attacked even by weak nucs. There will prob be reasonance as it draws density away from the double bond to make bonds to each O some sort of 3/2 order bond.

 

[sazerac]

oh ok, haha i was thinking in terms of alkanes and the such. why you can't have two OH groups on the same carbon atom in the chain😛
two alcohol groups opposite each other on the carbon chain, attached to the same carbon?

I'm not sure what you mean by "opposite each other" since the geometry of carbon atoms really doesn't allow two -OH groups to be 180 degrees apart.

But I think the real answer to your question is that gem diols do exist sometimes, but often they are a temporary intermediate of a reaction. Them oxygens really like electrons, so the poor H's are darn near bare protons. Well that's just asking for it, you know, with a horny and negative oxygen so close by. Inevitably three things happen:

One oxygen steals the damn proton from the other -OH group. The jilted oxygen attacks the carbon to make a double bond ketone. The carbon has an extra electron that runs screaming OHH group, which leaves since now it isn't bonded to the carbon anymore. .

End result: a ketone and a water molecule, which is a far more stable arrangement usually (depending on the reaction conditions of course).

 

[Hexon]

So what are you asking is why can't there be a C(OH)2 = CH2?
It might be possible, probably just unstable. That carbon would be really el-positive and get easily attacked even by weak nucs. There will prob be reasonance as it draws density away from the double bond to make bonds to each O some sort of 3/2 order bond.

I'm not sure what you mean by "opposite each other" since the geometry of carbon atoms really doesn't allow two -OH groups to be 180 degrees apart.
).

yeah that's what i was thinking, why they can't be 180 degrees apart😛??
thanks for your previous explanation on gem diols though😉

 

[slz1900]

They can't be 180 degrees apart because of vsepr rules. You could have a very unstable HO=C=OH but the oxygen atoms would be charged. It's the same reason that you can't have a water molecule with the hydrogens 180 degrees apart. The configuration with the least steric repulsion is preferred.

 

[Hexon]

oh ok, thanks
is this phenomenon unique only to OH groups?