Oxidinzing and Reducing agents, need some help? Really confused.

[EECStoMed]

Oh, so now that I've gone through 900/1001 of the EK 1001 organic chemistry questions I'm both better and at the same time confused on certain subjects, especially on reducing and oxidizing agents. Here is what I'm confused about:

According to TPR and EK book, H2 and a metal catalyst is used to hydrogenate alkenes and alkynes. Now apparently as I can see from question #890, that it can also reduce the open-chain fructose into an alcohol? I mean WTF? I didn't see that in the book anywhere? Along the same lines, I also discovered that K2CrO7 and KMno4 are both oxidizing agents but when it comes to primary alcohols one converts them into an aldehyde and the other into a COOH. I was like WHOA! Could someone shed some light onto this matter? And also, why is it that a dicarboxylic acid separated by an alkene in the middle when reacted with excess LiAlH4 forms a diol and not a tetraol? Someone please help? I'm diene here.

---

Members don't see this ad.

 

[Foghorn]

Oh, so now that I've gone through 900/1001 of the EK 1001 organic chemistry questions I'm both better and at the same time confused on certain subjects, especially on reducing and oxidizing agents. Here is what I'm confused about:

According to TPR and EK book, H2 and a metal catalyst is used to hydrogenate alkenes and alkynes. Now apparently as I can see from question #890, that it can also reduce the open-chain fructose into an alcohol? I mean WTF? I didn't see that in the book anywhere? Along the same lines, I also discovered that K2CrO7 and KMno4 are both oxidizing agents but when it comes to primary alcohols one converts them into an aldehyde and the other into a COOH. I was like WHOA! Could someone shed some light onto this matter? And also, why is it that a dicarboxylic acid separated by an alkene in the middle when reacted with excess LiAlH4 forms a diol and not a tetraol? Someone please help? I'm diene here.

LiAlH4 reduces carbonyl functional groups to alcohols. You're thinking of OsO4 (osmium tetraoxide) that catalyzes formation of diols from alkenes. The mechanisms on how LiAlH4 and OsO4 work are different.

The difference between K2Cr2O7 and KMnO4 is that one is a milder oxidizing agent compared to the other when reacting with an alcohol molecule. Similarly, LiAlH4 acts a stronger reducing agent compared to NaBH4 when reacting with carbonyl containing compounds.

The open chain form of fructose has a tautomer i.e. an enol form of the molecule at equilibrium that can react with H2 and metal catalyst.

Wow you ask a lot of questions 😀

Edit: oops!

 

[jochi1543]

LiAlH4 oxidizes carbonyl functional groups to alcohols.

Reduces.

 

[Foghorn]

Reduces.

I stand corrected 🙂

 

[BerkReviewTeach]

Here is what I'm confused about:

I also discovered that K2CrO7 and KMno4 are both oxidizing agents but when it comes to primary alcohols one converts them into an aldehyde and the other into a COOH.

The biggest difference is not in the oxidizing agent, as both are strong, but in the conditions. The permangante is used in aqueous, basic conditions. It's selectivity is controlled by temperature. It can pretty much oxidize anything, including small children and housepets.

Chromate is also strong, but it works best under acidic conditions. It can convert a primary alcohol into either an aldehyde (in pyridine with HCl-known as PCC) or into a carboxylic acid if there is strong acid (i.e., sulfuric) and water present. So what you should have discovered is that KMnO4 oxidizes completely, while CrO3 oxidation can be poisoned (stopped at incomplete oxidation).

My suggestion is that you relax a bit on the details and see the bigger, more general picture. Organic oxidizing agents are rich in O and/or poor in H. As such, they increase the bonds to oxygen or decrease the bond to hydrogen in the organic molecule. This approach should be able to eliminate two to three incorrect choices on a given question. In the cases where it's not enough information, trust your instincts to recognize a "best" answer.

In the case of oxidizing a sugar into an aladaric or aldonic acid, bonds to oxygen are gained, so the reagent must be rich in O and/or poor in H (NAD+, CrO3, MNO4-, HNO3, etc...). See the big picture and you'll be fine.

 

[EECStoMed]

The biggest difference is not in the oxidizing agent, as both are strong, but in the conditions. The permangante is used in aqueous, basic conditions. It's selectivity is controlled by temperature. It can pretty much oxidize anything, including small children and housepets.

Chromate is also strong, but it works best under acidic conditions. It can convert a primary alcohol into either an aldehyde (in pyridine with HCl-known as PCC) or into a carboxylic acid if there is strong acid (i.e., sulfuric) and water present. So what you should have discovered is that KMnO4 oxidizes completely, while CrO3 oxidation can be poisoned (stopped at incomplete oxidation).

My suggestion is that you relax a bit on the details and see the bigger, more general picture. Organic oxidizing agents are rich in O and/or poor in H. As such, they increase the bonds to oxygen or decrease the bond to hydrogen in the organic molecule. This approach should be able to eliminate two to three incorrect choices on a given question. In the cases where it's not enough information, trust your instincts to recognize a "best" answer.

In the case of oxidizing a sugar into an aladaric or aldonic acid, bonds to oxygen are gained, so the reagent must be rich in O and/or poor in H (NAD+, CrO3, MNO4-, HNO3, etc...). See the big picture and you'll be fine.

Thanks a lot everyone. Whenever I see a compound I can always see if it is an oxidizing or reducing agent but I just can't seem to pick which one to use. In the case of EK 1001 I think they really get to the nitty gritty parts of ochem. I guess I know the basic details already but whenever I miss a question, I'm like WHY oh WHY?