[Kuzilevich]

I  need to determine which substance of the following is the weakest nucleophile:

1. (CH₃CH₂)₃N
2. BF₃
3. CH₃SH
4. (CH₃)₃C OH

Can someone please help me understand how can I compare these four substances  ? Who is the weakest ? Please help me !

[TwistedConf]

First, make sure they're all nucleophiles (one of them isn't).  After that, you have probably learned about ranking nucleophiles, but it may be useful to rank them in terms of basicity....  which is often parallel to nucleophilicity. Start there.

[Kuzilevich]

OK. I will follow the steps you suggested:

1. There is no electronegativity difference between the carbon and nitrogen, but there are non-bonding pairs on the nitrogen... it is a weak nucleophile.


2. BF₃ has non-bonding pairs only on the Fluorine . Since the F atoms are more electronegative than the B atom, it is a pretty good nucleophile.

3. CH₃SF has four non-bonding pairs on the Sulfor atom . Since there is no difference in electronegativity, I guess it is a pretty weak nucleophile.

4. (CH₃)₃ COH - There is a big difference between the electroneativity of carbon and oxygen and two non-bonding pairs on the oxygen, so it is indeed a nucleophile.


So, how can I determine between 1 and 3?

THanks !

[discodermolide]

I can understand your reasoning however BF₃ is a strong Lewis acid and not nucleophillic at all.
So what about the others, which are nucleophillic or not?

[Babcock_Hall]

I would look at the steric bulk of the potential nucleophile.

[Kuzilevich]

discodermolide : I agree...Thanks

Babcock_Hall: As for the steric bulk- do you mean that A will be a much weaker nucleophile than C because of the streic bulk?


Thanks

[Babcock_Hall]

The question that you were asked is made more difficult by the fact that the classes of molecules are very different from each other.  Let me make some very general points to illustrate.  I would expect that (CH₃)₃COH is a much worse nucleophile than methanol on strictly steric grounds.  How about triethylamine (1) versus ammonia?  I also think that you are too focused on electronegativity.  Instead, I think you should be looking at the basicity of the lone pairs, sterics, and polarizability to find a good nucleophile.  Therefore I think that 1 versus 3 can be decided mainly on two factors, one of which is steric bulk.

[Kuzilevich]

Babcock_Hall:
Thanks. Got it !