[LuckyDude35]

My teacher told us that DMF and THF are both polar aprotic, does that mean the equation done in these will always be SN2 reactions? Thank you for your help, it is GREATLY appreciated!

[huoshiyong]

they are just solvents,not decisive.

[LuckyDude35]

What role would it play in the problem then? I know the SN2 reactions are supposed to be aprotic and polar and thats what the DMF is. I'm getting a little confused, thanks for the help

[movies]

Polar aprotic solvents like DMF and DMSO greatly accelerate S_N2 reactions, but that doesn't mean that other types of reactions can't occur in these solvents.

[LuckyDude35]

Ok, then I haev another question. I have a 5-chloro-1-pentoxide ion in DMF and I have to decide whether it is proton transfer, substitution, elimination, or no reaction and write it out. I'm not sure how I woudl tell with this one. Any help would be greatly appreciated, thanks!

[movies]

Well, think about the possibilities and think about how likely each one is.

[LuckyDude35]

I'm leaning towards elimination. I'm not sure what would be eliminated though, would it be the Cl, and it just goes off as an ion?

[movies]

Well, yeah that's a possibility.  What would make that more or less likely?  What are the other poissibilities and their pros/cons?

Do you know anything about the structure of DMF and how it accelerates S_N2 reactions?

[LuckyDude35]

I really don't know about DMF, my teacher covered SN1, SN2, elimination, and proton transfer all in about 45 minutes and everyone was lost. I know DMF is aprotic and polar but thats about it. Would the Cl react with the DMF? I would think that it wouldn't and maybe the compound comes eliminates the Cl- from it?

[LuckyDude35]

I think that there is a better chance of the Cl being eliminated then the O. He didn't really explain this to throughly and I'm trying to piece it together on my own in order to get these problems done

[movies]

Alright, I won't confuse you with the DMF explanation then.

The Cl- won't react with DMF.

Okay, think about it this way:

1) What kind of nucleophiles favor elimination reactions?  What kind favor substitution reactions?  Which one do you have here?

2) What kind of electrophiles favor elimination reactions?  What kind favor substitution reactions?  Which kind do you have here?

Then account for what the solvent favors.

[LuckyDude35]

I'm sorry but I am so lost. I know substitution reactions are favored but strong nucleophiles and I think weak electrophiles? I'm sorry but I am getting so confused

[movies]

Alright, here is a quick primer:

Substitution reactions:

- typically with strong nucleophiles
- nucleophile is usually small
- electrophile (with the leaving group) must also be small
- favored by polar aprotic solvents

Elimination reactions:

- strong bases that are not good nucleophiles (e.g. t-butoxide)
- large bases
- sterically hindered leaving groups

So based on those primary factors, what would you expect in this case?

[LuckyDude35]

substitution?

[movies]

Sounds good to me.

By the way, this is an intramolecular version of the Williamson ether synthesis.

http://www.chemhelper.com/williamsonether.html