Organic Chem Q

[MedHopeful234]

Two quick Orgo questions:
If you have a secondary alkyl halide (think bromo-cyclohexane)... why is it that adding a strong nucleophile/base (C2H5O-Na+, EtOH) goes E2 and not SN2? SN2 still works with secondary halides so why is E2 preferred?

Also, am I wrong to think that CH3OH is a WEAK nucleophile/base but CH3O- is a strong nucleophile/base? A lot of times when I assume this, I get the question wrong because I assume it would go SN1/E1 (since I consider the protonated version to be weak).
Example: 2-butanol + PBr3 ---> 2-bromobutane + CH3SH, DMSO ---> SN2 (But why? CH3SH is a weak nucleophile, SR- or SH- is a strong nucleophile)

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[Soleus715]

SN2 prefers Methyl > primary > secondary
E2 prefers Tertiary > Secondary > primary

Since secondary is the least preferred halide for SN2, E2 would be a more preferred method.

When distinguishing between SN2 vs E2 vs E1/SN1, it is important to look at the nucleophile AND its basicity.

-Weak nucleophile/weak base goes through both E1 AND SN1.
-Strong nucleophile/strong base will go E2 when Tertiary or Secondary.
-Strong nucleophile/strong base will go SN2 when Primary or Methyl.
-Strong nucleophile/weak base will go SN2
-Weak nucleophile/ Strong base will go E2 (in the case of bulky bases)

It is easy to tell if a nucleophile is a strong base or not since strong bases will have a negative charge on the oxygen. Some examples of strong nuc/weak base are CN-. N3-, and Cl-. (EDIT: Also the SH-, SR-, I-, Br-, Cl- just to cover the bases).

It's also important to factor in the solvent (aprotic vs protic) but don't go to this first to get your answer because methods such as E2 can use protic solvents, too.

 

[MedHopeful234]

Thanks so much Soleus, but I'm still confused on why CH3SH acts SN2... I thought the protonated version of CH3SH was a WEAK nucleophile.

I guess what I'm having trouble with is what is a WEAK nucleophile/base? I can easily find the strong base/nucleophile.... it's the weak that's getting me confused

 

[Dotoday]

Any molecule that has an SH is a good nucleophile. It want to donate electrons because it is not very electronegative. Therefore it wants to do Sn2. It can donate electrons with enough force to displace a leaving group. Hope this helps.

 

[MedHopeful234]

So RSH, SH-, and SR- are all good nucleophiles?

Also, just to make sure... what exactly is a weak nucleophile then? Are the protonated bases not weak nucleophiles (ex. CH3OH)?

 

[Dotoday]

Yes. You are correct. Sh- is a good nuc. Water and alcohols are weak nucleophiles.