Creatine - creatinine conversion

[id86]

Hi,

I wonder if someone could help. Im currently an undergraduate student and my organic chemistry is not good and I would like some advice with a reaction mechanism.

I am trying to work out the reaction for the cyclisation of creatine to creatinine in acidic conditions. All I have been able to find online is the first image.

My apologies of there is nothing else too this and that is all there is too it.

I have been trying for a while to figure this out, one pathway I have attached below but I can see this is wrong but its the closest I can seem to figure out the would depend on the H⁺ ions in acidic solution.

I know i'm probably completely wrong and sorry if this is a stupid question.

I believe the first reaction probably is all there is as I cant seem to make sense of anything further, and my problem is I do seem to overthink things and over complicate matters.

But what I am struggling to understand is why the reaction occurs faster under acidic conditions if the free H⁺ in solution do not react, why does a decrease in pH make this reaction occur faster. Also under acidic conditions, would the oxygen on the carbonyl group not also react? why would the reaction stop there?

Any help/advice would be appreciated

Thank you

Edited March 17, 2019 by id86

[hypervalent_iodine]

The second step you have drawn out is incorrect. It should look like this:

As to why it occurs faster, think about what happens to the carbonyl carbon when the oxygen is attatched to when it becomes protonated. There are two resonance structures of the intermediate that forms, and it has a very big impact on the carbonyl carbon’s reactivity. The H+ you’ll notice is regenerated in the course of the reaction, which makes it a catalyst and is why we say it doesn’t formally participate in the reaction (since it isn’t consumed). 

[id86]

Thank you so much. That makes sense. 

I think I may have made a mistake with the question regarding why it occurs faster. I understand that the addition of the proton to the carbonyl oxygen creates a dipole allowing the amine group to bond. What I've read says that a lower pH acidic solution will make the reaction occur faster. so pH 3 will be faster than pH 4. If only the one H⁺is being used from the acid solution, why will an increase in free H⁺ ions make that occur faster, unless it is down to the fact more molecules have the potential to be protonated faster or dehydration step can occur faster?

Again I do apologise if this seems obvious. and thank you once again for your help.

 

[hypervalent_iodine]

It’s just basic kinetics. Reactions occur when molecules collide at particular trajectories and with sufficient energy. If you have more molecules of a reactant around, you are more likely to get collisions more frequently and thus, the reaction occurs faster.