020200
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Hi,

Can someone explain how and why aldehydes, primary and secondary alcohols can be oxidised. But tertiary alcohols and ketones cannot. I've tried researching - my text book says tertiary alcohols cannot be oxidised as they would need a C-C bond to break rather than a C-H bond. I really don't understand this concept... Can someone please explain?

Thanks!
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charco
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(Original post by 020200)
Hi,

Can someone explain how and why aldehydes, primary and secondary alcohols can be oxidised. But tertiary alcohols and ketones cannot. I've tried researching - my text book says tertiary alcohols cannot be oxidised as they would need a C-C bond to break rather than a C-H bond. I really don't understand this concept... Can someone please explain?

Thanks!
The statement says it all. If there is no C-H bond then a C-C bond must be broken which is too difficult.
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020200
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But tertiary alcohols and ketones do have C-H bonds?
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charco
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(Original post by 020200)
But tertiary alcohols and ketones do have C-H bonds?
Not on the carbon attached to the oxygen.
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Anonymouspsych
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(Original post by 020200)
Hi,

Can someone explain how and why aldehydes, primary and secondary alcohols can be oxidised. But tertiary alcohols and ketones cannot. I've tried researching - my text book says tertiary alcohols cannot be oxidised as they would need a C-C bond to break rather than a C-H bond. I really don't understand this concept... Can someone please explain?

Thanks!
C-H bonds are much much easier to break than C-C bonds as this takes less energy. If you think about it for tertiary alcohols, you will have the hydroxyl group attached to a carbon atom which in turn will have 3 other R groups attached to it. When an aldehyde or ketone forms, a C double bond O forms with the carbon atom to which the hydroxyl group was attached to initially before oxidation. Carbon can only form 4 bonds overall so in order for the carbonyl group to form on the aldehyde/ketone , two pairs of electrons are needed (so 4 overall) to be shared between the carbon and oxygen atom so a double bond is formed between them. In a tertiary alcohol, one of those bonds will come from the initial single bond between the carbon and the oxygen in the hydroxyl group but the second bond can only come from breaking one of the other C-C bonds (because there are three R groups and one hydroxyl group). But since C-C bonds are too strong this doesn't happen and hence tertiary aldehydes are not oxidised to ketones and aldehydes.
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charco
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(Original post by Anonymouspsych)
C-H bonds are much much easier to break than C-C bonds as this takes less energy..
Paradoxically this is not correct.

C-H bond energy is about 412 kJ/mol
C-C bond energy is about 346 kJ/mol

So it can't be the explanation.
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Anonymouspsych
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(Original post by charco)
Paradoxically this is not correct.

C-H bond energy is about 412 kJ/mol
C-C bond energy is about 346 kJ/mol

So it can't be the explanation.
oh that's weird. why is is harder to break C-C bonds then?
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020200
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I think I am confused by this:

When you (try to) oxidise a tertiary alcohol, you would remove the -OH group attached to the carbon. Why does the other hydrogen removed from the alcohol have to be a hydrogen attached to the carbon which is attached to the -OH group. Why can't the other hydrogen be removed from a carbon atom somewhere else along the chain?
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charco
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(Original post by 020200)
I think I am confused by this:

When you (try to) oxidise a tertiary alcohol, you would remove the -OH group attached to the carbon. Why does the other hydrogen removed from the alcohol have to be a hydrogen attached to the carbon which is attached to the -OH group. Why can't the other hydrogen be removed from a carbon atom somewhere else along the chain?
Because a pi bond needs to be formed between adjacent carbon atoms.
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charco
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(Original post by Anonymouspsych)
oh that's weird. why is is harder to break C-C bonds then?
As I said, it is not.

The answer must lie in the mechanism, which must not allow oxidation of tertiary carbonium ions for some other reason.

Oxidation mechanisms are often complex, although I have not looked into this one. When I do, I'll get back to you.
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charco
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(Original post by Anonymouspsych)
oh that's weird. why is is harder to break C-C bonds then?
The mechanism takes place in several stages, as I thought.

The first stage is binding the oxygen of the hydroxyl group to the dicromate ion.

Then the hydrogen from the hydroxyl group is lost as a proton leaving the electron pair to the dichromate.

At this point the hydrogen attached to the carbon of the C-O atoms is abstracted by a species with a lone pair, probably water.

Water cannot do this to the alkyl groups, which is the reason why the tertiary structures cannot be oxidised.

The previous mechanistic steps are reversible and can proceed.

In summary, it has nothing to do with the strength of the C-H bond, and everything to do with the ability of water to abstract protons during the mechanism.
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Anonymouspsych
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(Original post by charco)
The mechanism takes place in several stages, as I thought.

The first stage is binding the oxygen of the hydroxyl group to the dicromate ion.

Then the hydrogen from the hydroxyl group is lost as a proton leaving the electron pair to the dichromate.

At this point the hydrogen attached to the carbon of the C-O atoms is abstracted by a species with a lone pair, probably water.

Water cannot do this to the alkyl groups, which is the reason why the tertiary structures cannot be oxidised.

The previous mechanistic steps are reversible and can proceed.

In summary, it has nothing to do with the strength of the C-H bond, and everything to do with the ability of water to abstract protons during the mechanism.
oh wow that's quite interesting and more complex than I thought! Thanks for the insight
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