Reducing and oxidizing reagents

[sherry225]

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Hi all,

I have been studying a few Ochem reactions. However, I'm really confused by the use of reducing and oxidizing reagents. How can you predict the degree of reduction/oxidation by each reagents? (ex. LAH, NaBH4, KMnO4, CrO3, MCPBA, PCC,ect) For example, I've ran into questions on LAH that reduces a ketone to alcohol. However, another question on LAH reduces another ketone to alkane........can someone help me on this? I'm very frustrated! 😕

 

[JavadiCavity]

Of the reducting agents you mentioned, LAH is the most powerful--it'll reduce carboxylic acids, esters, aldehydes and ketones to alcohols. I forget under what circumstances it will reduce reactants all the way to an alkane. NaBH4 is also powerful but less so than LAH. It is used to reduce aldehydes and ketones to alchols but not carboxylic acids. It can be used with esters, but the rate is slow. I know that NaBH is used in preference to LAH in some reactions (especially when H20 is involved) because LAH would produce a violent reaction (i.e. explosive). The best way to sort those out is by memorizing or, at least, reviewing some aldehyde, ketone, ester, and carboxylic acid reactions. It's pretty straight forward and I don't think it would take any longer than an hour or two.

As for oxidizing reagents, PCC is used to oxidize alchols to aldehydes or ketones. KMNO4/CrO3 are such good oxidizers that they will oxidize alcohols all the way to carboxylic acids. Thus, PCC or PDC is used when you want the reaction to halt at the aldehyde/ketone product. Essentially, KMNO4 and CR03 are identical--they are just two different ways to skin the same cat. My college text uses Cr03 exclusively, and only briefly mentions KMN04 in the margin. Lastly, MCPBA, I think (and I would definitely check me on all of this) is used to make expoxides--but I'm not 100% sure.

I know their must be some exceptions, but they are few. You can definitely master these in a few minutes.

 

[Sprgrover]

MCPBA is used to create epoxides from alkenes.

 

[sherry225]

Thanks! I'll make sure I memorize those.

 

[wimmcs]

Of the reducting agents you mentioned, LAH is the most powerful--it'll reduce carboxylic acids, esters, aldehydes and ketones to alcohols. I forget under what circumstances it will reduce reactants all the way to an alkane. NaBH4 is also powerful but less so than LAH. It is used to reduce aldehydes and ketones to alchols but not carboxylic acids. It can be used with esters, but the rate is slow. I know that NaBH is used in preference to LAH in some reactions (especially when H20 is involved) because LAH would produce a violent reaction (i.e. explosive). The best way to sort those out is by memorizing or, at least, reviewing some aldehyde, ketone, ester, and carboxylic acid reactions. It's pretty straight forward and I don't think it would take any longer than an hour or two.

As for oxidizing reagents, PCC is used to oxidize alchols to aldehydes or ketones. KMNO4/CrO3 are such good oxidizers that they will oxidize alcohols all the way to carboxylic acids. Thus, PCC or PDC is used when you want the reaction to halt at the aldehyde/ketone product. Essentially, KMNO4 and CR03 are identical--they are just two different ways to skin the same cat. My college text uses Cr03 exclusively, and only briefly mentions KMN04 in the margin. Lastly, MCPBA, I think (and I would definitely check me on all of this) is used to make expoxides--but I'm not 100% sure.

I know their must be some exceptions, but they are few. You can definitely master these in a few minutes.

Nice explanation java!! PCC is a milder oxidizer than KMnO4, however be on the watchout when the KMnO4 is cold (milder) than when it is reacted with heat (much stronger). Good luck!!

 

[jk5177]

To get MCPBA as part of your synthesis. You need to go walk up to the front counter and ask the cashier.

"Can I order a McPBA? I also like fries and coke. Supersize it please!"

 

[Kahr]

Just want to add another function of MCPBA, it can easily make an ester from a ketone, and a carboxylic acid from an aldehyde.

 

[rmm30]

Hi all,

I have been studying a few Ochem reactions. However, I'm really confused by the use of reducing and oxidizing reagents. How can you predict the degree of reduction/oxidation by each reagents? (ex. LAH, NaBH4, KMnO4, CrO3, MCPBA, PCC,ect) For example, I've ran into questions on LAH that reduces a ketone to alcohol. However, another question on LAH reduces another ketone to alkane........can someone help me on this? I'm very frustrated! 😕

Watch Chad Videos, take notes, go through destroyer, take notes: This will clear up any confusion on this topic.