OChem #80

[NOORALLAH]

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Hey guys... try this Q frm destroyer and let me know what you think:

Ar-COCH3 ( ketone with an aromatic ring and a methyl) + ex D2O in DCl would yield....
Answer: replace all alpha H (meaning all methyl H)

when exactly do you replace all your alpha H and when do you yield the hydration product?

Isn't it true that when you are in an acidic solution you only replace one alpha H and when in basic replace all... but what about hydration then?

 

[needzmoar]

Hey guys... try this Q frm destroyer and let me know what you think:

Ar-COCH3 ( ketone with an aromatic ring and a methyl) + ex D2O in DCl would yield....
Answer: replace all alpha H (meaning all methyl H)

when exactly do you replace all your alpha H and when do you yield the hydration product?

Isn't it true that when you are in an acidic solution you only replace one alpha H and when in basic replace all... but what about hydration then?

Rarely will you get the hydration product. The Keq of the hydration product of a ketone is very small.

Isn't it true that when you are in an acidic solution you only replace one alpha H and when in basic replace all... but what about hydration then?

I've never heard of that rule before. Usually, if it's mechanistically reasonable, isotopic exchange will occur in both acidic and basic conditions. Since there is an excess of D2O, we'd expect the fully deuterated product.

 

[NOORALLAH]

So basically look for if there is an excess then replace all, if no excess then replace one and it doesn't matter if in acidic or basic solution

 

[needzmoar]

So basically look for if there is an excess then replace all, if no excess then replace one and it doesn't matter if in acidic or basic solution

Right. Usually you can figure it out by looking at the answer choices.

 

[heartSD]

Rarely will you get the hydration product. The Keq of the hydration product of a ketone is very small.

I've never heard of that rule before. Usually, if it's mechanistically reasonable, isotopic exchange will occur in both acidic and basic conditions. Since there is an excess of D2O, we'd expect the fully deuterated product.

I also thought this was a hydration product. When you say the Keq of the hydration product of a ketone is very small are you only referring to the reaction with D2O or also with H20?

 

[needzmoar]

I also thought this was a hydration product. When you say the Keq of the hydration product of a ketone is very small are you only referring to the reaction with D2O or also with H20?

I haven't looked at the destroyer in months but I believe this problem was over isotopic exchange. The hydration product that I was referring to is when an H2O molecule serves as a nucleophile and attacks the carbonyl.