[fredodgers]

Hello everyone,

I have a few questions about substitution and elimination reactions.
I have attached the pictures of two different reactants used in two separate reactions.

In "1a" the reactant is treated with EtONa in DCM.  In my opinion, this is an SN2 reaction.  We have a strong base/ strong nucleophile, so we go on to see that the solvent is moderately polar aprotic (favorable for SN2).  Further, the reaction is at room temperatue, so would that not favor the SN2 as well?  With all of these clues, shouldn't SN2 be favored overall?

In "1b," the reactant is treated with HCl.  Looking at the reactant, the OH is on a primary carbon, therefore SN1 and E1 reactions are out of the question.  Looking at HCl, it is a very poor base, but a relatively good nucleophile.  An SN2 reaction should take place, but the reaction should not be very favorable. (because HCl is not a great nucleophile).

Any thoughts on these?

Sorry for the poorly drawn pictures!

[AlphaScent]

If a base is strong it is also a strong nucleophile?

[orgopete]

It can be difficult to predict the product for the first reaction. A rule of thumb for secondary halides is elimination is favored if the pKa of the base (conjugate acid) is greater than 12. That would predict an elimination reaction. However, this reaction may be predicted to give or rather, you may be asked to give the substitution product for this reaction. In that case, they may be emphasizing the mechanistic aspect of the reaction more than the identity of the final product.

I agree with the rationale given for the second reaction, it should give substitution, but the reaction is likely to be slow. You will probably find other reagents are commonly used for the preparation of a chloride from an alcohol.

[fredodgers]

I appreciate the response!  My confusion lies in the following:  In class, we looked at the reaction "1c" I have attached and said it was SN2.  The reaction "1a" I have posted was said to be E2, however.  I'm confused where the two reactions are different.  Is this an inconsistency?

[fredodgers]

If a base is strong it is also a strong nucleophile?

Not necessarily, so does that mean the minus charge on the oxygen automatically makes it a "stronger" strong base than a strong nucleophile?  Is the oxygen system too bulky to be a nucleophile?

[orgopete]

I appreciate the response!  My confusion lies in the following:  In class, we looked at the reaction I have attached and said it was SN2.  The reaction "1a" I have posted was said to be E2, however.  I'm confused where the two reactions are different.  Is this an inconsistency?

I think the reason for the confusion is that ethoxide is a simple potential nucleophile and is therefore indicated as an example when discussing substitution reactions. It will give some substitution product, with inversion in this example. However, this same reaction will also result in elimination, actually probably as the major product, when elimination reactions are discussed.

I gave an ambiguous answer as I didn't know if the class was on substitution, elimination, or both. If you were to answer what the major product is, it will be elimination. The rule of thumb given in Brown, Foote, and Iverson is a good predictor of the difference.

I too am baffled as to why very strong bases should prefer to result in an elimination. The "rules" of chemistry are often empirical rather than follow a strict law of physics. I consider this among those cases.