[iScience]

http://i.imgur.com/Kyl9AoS.jpg

in this first pic is one of the questions from the test i just got back today. the question was to draw out a possible reaction pathway to get to the final product. can someone explain to me why i got partial credit? i don't get what was wrong.

http://i.imgur.com/Rm7npal.jpg

this is the same problem type. he explained why it was wrong, "because CH₃OH is neutral". does this mean that it stays in tact, as opposed to what i proposed (where the H falls off)?

if it does stay in tact, i understand why i got it wrong, but is what i proposed even possible? just wondering.

[discodermolide]

Presumably methanol attacks to give an intermediate where the oxygen carries a positive charge and the proton is still attached to the oxygen. The I^- floating about then removes the proton to regenerate the neutral species.

[iScience]

well i redid the mechanism before your response and my new pathway doesn't agree with what you stated. can you tell me where i went wrong?

http://i.imgur.com/zY9tMMb.jpg

(second part in the top line on the methanol, that's an electronegativeity arrow (i forget the name of the arrow if it even has one), showing the polarity due to electron density) in case you were wondering what that was

what you stated sounds similar to what i have except i don't get a positive formal charge on my oxygen anywhere in my pathway.

[discodermolide]

This is what I was thinking of:

[zsinger]

I agree with what Disco said (as usual).

[iScience]

if the 'I' falls off because it's interacting with the polar H (partial +), how do i know whether it will stick to that H (as i have it in my pathway), or whether it will interact with the H just long enough for it to fall off and then not stick (as you have it in your pathway)?

I have seen similar reactions where the leaving group sticks to the electrophile (in this case), though i can't recall any specific ones off the top of my head, i don't know when a leaving group will fall off (due to some interaction with another species(electrophilic or nucleophilic)) and then stick to that species and when it won't. how do i tell what happens?

[zsinger]

Isci-
Pay particular attention to how disco has pushed his arrows, and remember a golden rule of electron pushing:  never kick off H- or C- (haloform reactions the exception with carbon).  He kicks the extra electrons in that Hydrogen to the Oxygen (which is electronegative and can happily take it), and I- has taken the electrons from the H (therefore removing it instead of sticking to it)
         Hope this helps.
                 -Zack

[discodermolide]

if the 'I' falls off because it's interacting with the polar H (partial +), how do i know whether it will stick to that H (as i have it in my pathway), or whether it will interact with the H just long enough for it to fall off and then not stick (as you have it in your pathway)?

I have seen similar reactions where the leaving group sticks to the electrophile (in this case), though i can't recall any specific ones off the top of my head, i don't know when a leaving group will fall off (due to some interaction with another species(electrophilic or nucleophilic)) and then stick to that species and when it won't. how do i tell what happens?

If I understand you correctly:
That is difficult to answer without complicated experiments involving labelling etc.

[iScience]

If I understand you correctly:
That is difficult to answer without complicated experiments involving labelling etc.

in other words, the reaction i gave and the reaction you gave are both perfectly valid and therefore we can say that both are occurring in solution?

[discodermolide]

Surely if the reaction is carried out in methanol then the pathway I suggested will predominate.

[orgopete]

if the 'I' falls off because it's interacting with the polar H (partial +), how do i know whether it will stick to that H (as i have it in my pathway), or whether it will interact with the H just long enough for it to fall off and then not stick (as you have it in your pathway)?

I have seen similar reactions where the leaving group sticks to the electrophile (in this case), though i can't recall any specific ones off the top of my head, i don't know when a leaving group will fall off (due to some interaction with another species(electrophilic or nucleophilic)) and then stick to that species and when it won't. how do i tell what happens?

I agree with this premise. An SN1 reaction occurs well in the presence of water. It seems very plausible for an interaction between the protons of water and a halogen. However, I don't think this alters overall basicity. Although it seems inviting to think a negative charge results in basic properties, recall that neutral ammonia is more basic than all halides. Water and methanol are more basic than chloride, bromide, or iodide. I would have used a neural methanol to abstract the proton. I would consider this consistent with the equilibrium between HI and H2O favoring iodide and hydronium ions.

I think the difficulty is understanding weaker interactions. Even though HI ionizes in water, I don't think the free iodide will exist without interactions with bulk water. While we may write hydronium ions as though it exits as an isolated species, we should recognize that water has extensive hydrogen bonds and similar hydrogen bonds will be present for hydronium ions. It is messy to write H⁺(H₂O)_n, but not including them does not mean they are not present. Although we may refer to relatively strong hydrogen bonds, weaker interactions probably also exist. The solvolysis of the iodine is likely an example.