[Chump0]

Hello again,

I am now trying to resolve the following question:

Treatment of pentan-2-one with sodium hydroxide and iodine in aqueous solution causes a reaction that results in a yellow precipitate with an "antiseptic" odor. (hmm what would that be ) However, treatment of pentan-3-one does not give the same precipitate. Show the mechanisms of the reactions and suggest what the precipitate is.

Obviously (for me) the precipitate is Iodoform CHI₃ and it's know as an disinfectant with an "antiseptic" odor.
So let's move on, I know the mechanism of pentan-2-one, and can be found underneath:


The point where I am stuck is that I'm not sure what the mechanism of pentan-3-one is.
I have deduced the mechanism of it from the mechanism of pentan-2-one. In the question is says that pentan-3-one does not form Iodoform so obviously the end product has to be something slightly different. Here's what I came up with:


My question is; Is my previous deduction correct? Or is my mechanism wrong?

Thank you

[Doc Oc]

You won't see hydrolysis of the 3-pentanone, even if you iodinate both alpha carbons.  What is the key difference and what explanation can you make?

[Chump0]

You won't see hydrolysis of the 3-pentanone, even if you iodinate both alpha carbons.  What is the key difference and what explanation can you make?

From what I understood is that you say the mechanism ends with eventually this product, because 3-pentanone does not hydrolyse.


The key difference that I can make is the fact that both alpha carbons are connected to a methyl group. While in 2-pentanone the alpha carbon is not connected to a methyl group.
Are you suggesting that the methyl group is hindering the hydroylisis?

[Doc Oc]

Yes, the alkyl group is the key.  What properties are different between the methyl group and an iodide?

[Chump0]

Yes, the alkyl group is the key.  What properties are different between the methyl group and an iodide?

I don't want to say it, but I really don't know....

[Doc Oc]

That's okay, this is how you learn things.

Think in terms of sterics or electronics.  In this situation, is it more likely that sterics are affecting the hydrolysis or the electronics?

[Chump0]

That's okay, this is how you learn things.

Think in terms of sterics or electronics.  In this situation, is it more likely that sterics are affecting the hydrolysis or the electronics?

I don't think sterics have any effect, I thinks it's due the electronics. If both groups left and right to the middle carbon atom break the bond you will get a 2+ charge on the carbon.
And that's not favorable.