inaccensa

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- During the hemiketal/ acetal formation, why doesn't the carbonyl group reform like it does in ester synthesis by abstracting the proton.

I'm a little confused because when oxygen when carries a positive charge during an intermediate stage, it gets rid of the proton, so why doesn't the carbon do the same? I do know the distinction that in ester synthesis, water is the leaving group but why can't the hydroxyl group stay bonded to carbom and oxygen becomes protonated to form a hydroxyl group.


 

sleepy425

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Mar 6, 2008
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- During the hemiketal/ acetal formation, why doesn't the carbonyl group reform like it does in ester synthesis by abstracting the proton.

I'm a little confused because when oxygen when carries a positive charge during an intermediate stage, it gets rid of the proton, so why doesn't the carbon do the same? I do know the distinction that in ester synthesis, water is the leaving group but why can't the hydroxyl group stay bonded to carbom and oxygen becomes protonated to form a hydroxyl group.


may I suggest rephrasing your question. I'm having a hard time understanding what you're asking (also, your image isn't working), and I think it will be difficult for us to help you without clarification.
 
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inaccensa

inaccensa

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I'm wondering why during ester synthesis, we get the carbonyl group at the end, whereas when hemiacetals are formed we end up with an alkoxyl group.

Normal if oxygen carries a positive charge and has a hydrogen bound to it, it simply gets rid of the hydrogen. However, when hemiacetal is formed, why doesn't carbon follow suit?
 

sleepy425

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Mar 6, 2008
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I'm wondering why during ester synthesis, we get the carbonyl group at the end, whereas when hemiacetals are formed we end up with an alkoxyl group.

Normal if oxygen carries a positive charge and has a hydrogen bound to it, it simply gets rid of the hydrogen. However, when hemiacetal is formed, why doesn't carbon follow suit?
So you're saying, after the alcohol attacks the carbonyl carbon of the aldehyde and kicks the electrons up onto the carbonyl oxygen, why don't those electrons come back down and kick of H- (hydride) as a leaving group? Well, because hydride is not a good leaving group at all. And if you deprotonate at the carbon, that won't help you reform the carbonyl because there will still be too many electrons at the carbon for the electrons to come down from the oxygen and reform the carbonyl (and also, the proton attached to the carbon is not acidic at all).

Just to clarify, there is a big difference between a proton and a hydride. A hydride is H-. It has two electrons and one proton (no neutrons). Hydrides are bases. They do not act as nucleophiles, and are horrible leaving groups. A proton is, well, a proton, and is H+. It has no electrons. Protons cannot be leaving groups, because a leaving group accepts an electron pair as it leaves. When you pull off a proton, you're donating an electron pair, not accepting one. But always remember, when you pull off a proton, you leave an electron pair behind. Since you leave the electron pair behind, the carbon still has a full octet so the electrons from the oxygen cannot come down.

I don't know if that answers your question, I hope it helps.
 
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inaccensa

inaccensa

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Sep 5, 2008
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So you're saying, after the alcohol attacks the carbonyl carbon of the aldehyde and kicks the electrons up onto the carbonyl oxygen, why don't those electrons come back down and kick of H- (hydride) as a leaving group? Well, because hydride is not a good leaving group at all. And if you deprotonate at the carbon, that won't help you reform the carbonyl because there will still be too many electrons at the carbon for the electrons to come down from the oxygen and reform the carbonyl (and also, the proton attached to the carbon is not acidic at all).

Just to clarify, there is a big difference between a proton and a hydride. A hydride is H-. It has two electrons and one proton (no neutrons). Hydrides are bases. They do not act as nucleophiles, and are horrible leaving groups. A proton is, well, a proton, and is H+. It has no electrons. Protons cannot be leaving groups, because a leaving group accepts an electron pair as it leaves. When you pull off a proton, you're donating an electron pair, not accepting one. But always remember, when you pull off a proton, you leave an electron pair behind. Since you leave the electron pair behind, the carbon still has a full octet so the electrons from the oxygen cannot come down.

I don't know if that answers your question, I hope it helps.

So when groups leave, they carry electrons with them, but when a proton is abstracted with a base, it doesn't take electrons.