Wittig Reaction

[oohpsjin]

1. The Wittig Decomposition reaction is driven by the strength of the phosphorus-oxygen bond that is newly formed. So the P-C bond in ylide is weaker than P-O in the new product. But how? Comparing electronegativities, C=2.55, P=2.19, and O=3.44. Since the difference in eneg is larger in P-O, it is more ionic, and thus weaker i thought?

2. How can you tell whether if a compound is a good oxidizer just by looking at the formula? E.g. How do you know KMnO4 will oxidize things?

3. When oxidizing alcohols, we can oxidize 1 and 2' alcohols to carboxylic acids, but not 3' alcohols. That's because there's no H to detach in 3' alcohol...but why can't the oxidizer just detach a C-C bond? C-C bonds have lower dissociation energies than C-H bonds afterall?

please help!

---

Members don't see this ad.

 

[161927]

1. The Wittig Decomposition reaction is driven by the strength of the phosphorus-oxygen bond that is newly formed. So the P-C bond in ylide is weaker than P-O in the new product. But how? Comparing electronegativities, C=2.55, P=2.19, and O=3.44. Since the difference in eneg is larger in P-O, it is more ionic, and thus weaker i thought?

The Wittig Decomposition involves the destruction of the betaine-like four atom ring, and it is a concerted process. Overall it is an energetically favored process. From a therodynamic perspective (and entropic perspective), opening a constrained 4 member ring and forming multiple products is favored. Don't try to micromanage the reaction by analyzing the electronegativities and polarities of the bonds. If you really wanted to calculate bond dissociation energies and calculate the enthalpy of each step, it's possible, but why would you put yourself through that when you know that overall, it's an energetically favored process?

2. How can you tell whether if a compound is a good oxidizer just by looking at the formula? E.g. How do you know KMnO4 will oxidize things?

There are only so many oxidants you need to know in Organic Chemistry. Peroxides and superperoxides, ozones, chromic acid, etc. If you're looking for a rule of thumb, how about any compound with lots of bonds to oxygen?

3. When oxidizing alcohols, we can oxidize 1 and 2' alcohols to carboxylic acids, but not 3' alcohols. That's because there's no H to detach in 3' alcohol...but why can't the oxidizer just detach a C-C bond? C-C bonds have lower dissociation energies than C-H bonds afterall?

please help!

Cleaving and breaking C-C bonds is serious business. That's the whole point of introductory Organic Chemistry. You can't just decide you want to break a C-C bond. If you did, which one would you break? How would you decide? If you think about a common reaction that DOES break C-C bonds, such as the iodoform reaction, you realize that you end up with a leaving group that is stabilized by the three highly electronegative halogens. I can see why you would want to break a tertiary alcohol into a secondary alcohol, but sometimes you are constrained by the reactions you have available to you. Know your limits, Mr. Wayne.

 

[oohpsjin]

Thanks dcody!

 

[cashmoney805]

Can't you only oxidize primary alcohols to CA, and secondary to ketones?