quick ochem question...any help appreciated

[goldngrl1611]

Hey guys-
Quick question. If you had only HCN in solution with a ketone, I know that one possible mechanism is for the carbonyl to be protonated and then have the CN- attack to give a nucleophilic addition reaction. However, would it be possible for CN- to attack an unprotonated carbonyl in this acidic solution? I guess I ask this for two reasons: 1) would the HCN automatically protonate the carbonyl regardless and 2) if not, is it strong enough to attack without the O protonated. Thanks!

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[orgohacks]

Hey guys-
Quick question. If you had only HCN in solution with a ketone, I know that one possible mechanism is for the carbonyl to be protonated and then have the CN- attack to give a nucleophilic addition reaction. However, would it be possible for CN- to attack an unprotonated carbonyl in this acidic solution? I guess I ask this for two reasons: 1) would the HCN automatically protonate the carbonyl regardless and 2) if not, is it strong enough to attack without the O protonated. Thanks!

The rate of reaction of CN with protonated carbonyl would be MUCH faster than the rate of reaction of CN with unprotonated carbonyl.

It *is* possible for the CN to attack the C=O without it being protonated, but it is much slower: and the C=O would become O-, which would immediately be protonated by the H+ .

Acid makes carbonyls more reactive. It weakens the C-O pi bond (double bond) and there is a greater resonance contribution from the resonance form where you have a positive charge on carbon.

hope this helps - James

 

[loveoforganic]

For all practical purposes in this instance, it's indiscernible. Proton transfers take place so rapidly in solution that there's really no difference between if the cyanide attacks and the pi electrons start to rehybridize before the proton pops onto the oxygen or if the proton pops onto the oxygen before the attack.

 

[orgohacks]

For all practical purposes in this instance, it's indiscernible. Proton transfers take place so rapidly in solution that there's really no difference between if the cyanide attacks and the pi electrons start to rehybridize before the proton pops onto the oxygen or if the proton pops onto the oxygen before the attack.

That's true, they both lead to the same product and it would be a fast reaction.

Bottom line holds true - it's important to know that acid acts as a catalyst for these types of reactions and that the acid-catalyzed pathway is faster.

 

[goldngrl1611]

That's true, they both lead to the same product and it would be a fast reaction.

Bottom line holds true - it's important to know that acid acts as a catalyst for these types of reactions and that the acid-catalyzed pathway is faster.

well there was a question i came across (roman numeral) that asked to identify which of the following three reactions were possible. The first was protonation of the oxygen (which I know is correct) the second was most assuredly wrong, and the third showed the CN- attacking without indication of where the proton was lost (but the rest of the reaction was fine). I figured with it being in acidic solution that reaction wouldn't be possible, especially considering they just showed it suddenly as CN- without it protonating the carbonyl molecule in solution with it. Any thoughts?