K2Cr2O7 - Potassium Dichromate Question

[CedarZ4]

So I have a question with K2Cr2O7.
I understand it's a oxidizing agent, so it creates more bonds to oxygen (or anything more electronegative)

How come then, does it sometimes turns OHs into carbonyl, while other times, you get a carboxylic acid?
Example: Methanol + K2Cr2O7 gives you methyl carboxylic acid.
but, when you do Phenol without the double bonds (so just a 6 membered ring, not Benzene, not sure what this is called? ) You just get a ketone, not a carboxylic acid like with methanol

How do I know if it just oxidizes to form a carbonyl, or if I get a carboxylic acid.

Does it have to do with the alcohol being primary or secondary?
Only primary get carboxylic acid?

Thanks

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[mario0923]

methanol is a primary alcohol. So with a strong oxidizing agent like H2CrO4, H2Cr2O7, H2MnO4, etc... you will get a carboxylic acid. but if you have a primary alcohol with a mild oxidizing agent like PCC, you will get an aldehyde.

cyclohexanol is the compound you meant. phenol is a benzene with a hydroxyl group. cyclohexanol is a secondary alcohol so a mild oxidizing agent will not be able to oxidize the alcohol. only the strong ones will oxidize it to ketone, no aldehyde, no carboxylic acid.

tertiary alcohols are not oxidized.

hope it helped

ps: make sure you know how to name all the compounds

 

[CedarZ4]

Thank you. That makes a lot of sense. So it does have to do with whether the alcohol is primary or secondary.

So then a follow-up question is, how come then when (Benzene connected to ethane <-- what's this one called I might have skipped the nomenclature chapter, oops) reacts with Potassium Dichromate, you get a carboxylic acid? I guess it's for the same reason - Like primary alcohols, they will react with just mild oxidizing agents to form carboxylic acid?

Thank you

 

[mario0923]

Thank you. That makes a lot of sense. So it does have to do with whether the alcohol is primary or secondary.

So then a follow-up question is, how come then when (Benzene connected to ethane <-- what's this one called I might have skipped the nomenclature chapter, oops) reacts with Potassium Dichromate, you get a carboxylic acid? I guess it's for the same reason - Like primary alcohols, they will react with just mild oxidizing agents to form carboxylic acid?

Thank you

I am not sure of the name....lol
I guess it is benzyl ethanol
benzyl alcohols and benzyl alkanes get oxidized to carboxylic acids with strong oxidizing agents no matter what.
For those reactions refer to Benzene Reactions, EAS of Chad's Video
For reactions of alcohols go to Chad's Video and watch Alcohols. He talks about oxidation of alcohols.

But again strong oxidizing agent converts primary alcohol to carboxylic acid
mild oxidizing agent converts primary alcohol to aldehyde
strong oxidizing agent converts secondary alcohol to ketone
mild oxidizing agents doesn't do anything on secondary alcohol
strong/mild oxidizing agent doesn't do anything on tertiary alcohol

 

[coda4451]

Can the strong oxidizing agents oxidize an alkane to an alcohol?

 

[DAT Destroyer]

So I have a question with K2Cr2O7.
I understand it's a oxidizing agent, so it creates more bonds to oxygen (or anything more electronegative)

How come then, does it sometimes turns OHs into carbonyl, while other times, you get a carboxylic acid?
Example: Methanol + K2Cr2O7 gives you methyl carboxylic acid.
but, when you do Phenol without the double bonds (so just a 6 membered ring, not Benzene, not sure what this is called? ) You just get a ketone, not a carboxylic acid like with methanol

How do I know if it just oxidizes to form a carbonyl, or if I get a carboxylic acid.

Does it have to do with the alcohol being primary or secondary?
Only primary get carboxylic acid?

Thanks

I have been using this reagent for over 30 years, thus I can help you. Your answer involves what is called a MECHANISM. For undergraduate students, we do NOT usually present the mechanisms because they involve chemistry that is beyond the scope of what you need. If your want to pursue oxidation with me a bit, here goes....... The active ingredient with this reagent is CrO3....which is attacked by the nucleophilic alcohol to form a chromate ester. Water removes an H from this intermediate which collapses into the carbonyl compound . . Tertiary alcohols lack a hydrogen in the correct position , thus do not oxidize. For the DAT exam...this is NOT needed.....Here is the bottom line...... Methanol and primary alcohols oxidize into aldehydes.......secondary alcohols oxidize into ketones.......tertiary do NOT normally oxidize without blowing the molecule into a messy pulp.....and phenols oxidize into benzoquinones. If you have a burning interest in this fascinating area however, a very simple mechanism is proposed in the Wade and Klein text book. I have read texts and literature on oxidizing agents that actually go hundreds of pages. It is a very popular research area in our field.

I hope this helps.

Dr. Romano