What is enol racemization?

[510586]

In this compound, how is it racemizing? Is it because it is going between two enol structures or is it turning something from R to S or S to R?

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[DAT Destroyer]

In this compound, how is it racemizing? Is it because it is going between two enol structures or is it turning something from R to S or S to R?View attachment 193984

Racemization is possible if the ketone has 2 necessary components..... An alpha hydrogen 2. The alpha carbon is chiral.

Why ?

Upon removal of the alpha hydrogen either under acidic or basic conditions, we form the enol. The enol is ACHIRAL.....and reverts back to the more stable carbonyl compound. Upon reverting back, the Hydrogen can come in from either side. This produces the racemic mixture. If you need further clarity, the Francis Carey book does a splendid job on more examples.

Hope this helps.

Dr. Jim Romano

 

[510586]

Racemization is possible if the ketone has 2 necessary components..... An alpha hydrogen 2. The alpha carbon is chiral.

Why ?

Upon removal of the alpha hydrogen either under acidic or basic conditions, we form the enol. The enol is ACHIRAL.....and reverts back to the more stable carbonyl compound. Upon reverting back, the Hydrogen can come in from either side. This produces the racemic mixture. If you need further clarity, the Francis Carey book does a splendid job on more examples.

Hope this helps.

Dr. Jim Romano

Oh OK so it works for any enol not just enols that can make two different enols on both sides. Got it thank you. Also I posted a question earlier about HBr. How come HBr can replace primary alcohols but not HCl? Is it because they are large?

 

[RuffDay]

Oh OK so it works for any enol not just enols that can make two different enols on both sides. Got it thank you. Also I posted a question earlier about HBr. How come HBr can replace primary alcohols but not HCl? Is it because they are large?

Hmm. I am not sure where you are getting the idea that HCl cannot react with primary alcohol, but if it is from a resource, then it is probably right. I will try my best to explain though. The reactivity of alcohol depends on where the OH is attached. With tertiary being the most reactive, and a primary one least reactive. The reactivity of hydrogen halide would be HI > HBr > HCl > HF. The reactivity is like that because you have to remember that when you are in a protic environment, then nucleophile's reactivity increases as you are going down. Since we are dealing with a not very reactive alcohol, then we would need a hydrogen halide that is really reactive in order for the reaction to work. In this case, it will be HBr. In cases with HCl, it is not very reactive so we might need to add proably 2 moles of HCl or excess HCl for the reaction to proceed. I hope that helps!

 

[510586]

Hmm. I am not sure where you are getting the idea that HCl cannot react with primary alcohol, but if it is from a resource, then it is probably right. I will try my best to explain though. The reactivity of alcohol depends on where the OH is attached. With tertiary being the most reactive, and a primary one least reactive. The reactivity of hydrogen halide would be HI > HBr > HCl > HF. The reactivity is like that because you have to remember that when you are in a protic environment, then nucleophile's reactivity increases as you are going down. Since we are dealing with a not very reactive alcohol, then we would need a hydrogen halide that is really reactive in order for the reaction to work. In this case, it will be HBr. In cases with HCl, it is not very reactive so we might need to add proably 2 moles of HCl or excess HCl for the reaction to proceed. I hope that helps!

I think it was answered this morning by Dr. Romano. http://forums.studentdoctor.net/threads/hcl-replacing-oh.1149724/#post-16691825
However, the link I posted showed it as an sn2. It might be wrong though I have no idea

 

[DAT Destroyer]

Oh OK so it works for any enol not just enols that can make two different enols on both sides. Got it thank you. Also I posted a question earlier about HBr. How come HBr can replace primary alcohols but not HCl? Is it because they are large?

No....Br- is simply a better nucleophile.....it is less electronegative than Cl- and the electrons are more loosely held. HCl can work, but we use ZnCl2 catalyst to accelerate the reaction rate.

Chlorine works poorly with Primary halides....reaction rate is way too slow.

For over 30 years, I have always used SOCl2 in pyridine or PCl3 on primary alcohols.

Hope this helps.

Dr. Romano

 

[510586]

No....Br- is simply a better nucleophile.....it is less electronegative than Cl- and the electrons are more loosely held. HCl can work, but we use ZnCl2 catalyst to accelerate the reaction rate.

Chlorine works poorly with Primary halides....reaction rate is way too slow.

For over 30 years, I have always used SOCl2 in pyridine or PCl3 on primary alcohols.

Hope this helps.

Dr. Romano

Oh OK so the cl doesn't do any sn2 style because we consider it a weaker nucleophile so it does sn1 like ch3oh while the Br is like a strong nuc that does sn2 like naoch3 . I get it now

 

[DAT Destroyer]

Oh OK so the cl doesn't do any sn2 style because we consider it a weaker nucleophile so it does sn1 like ch3oh while the Br is like a strong nuc that does sn2 like naoch3 . I get it now

Chlorine will not do the SN2 reaction on a primary alcohol since it is a rather poor nucleophile. Chlorine can work on a secondary or tertiary alcohol because the Sn1 mechanism takes over.