[mps]

Hello,
I'm reading a description of reaction conditions which will favor E2 over SN2.  It says:

"We can use conditions in which steric hindrance blocks the SN2 reaction but leaves the E2 reaction unaffected.  One way to create hindrance is by selecting a bulky substrate for the reaction, such as t-chlorobutane with a OH- nucleophile [base to abstract methyl H+].  In this reaction, the nucleophile would not be able to access the tertiary carbon to create an SN2 reaction, but the protons on the neighboring [primary] carbons would still be plenty available and open, so the E2 reaction would proceed." 

That's fine-- but it says nothing about competition with SN1.  t-chlorobutane seems like a fine substrate candidate for SN1: Cl-, as the conj base of a strong acid, would readily leave; and alkyl substituents on tertiary C will inductively stabilize the carbocation. 

Do SN1 and E2 not compete?  Is it just assumed that SN1 only occurs in protic solvents (based on carbocation stabilization), and in aprotic solvents SN1 will be unfavorable-- so E2, in aprotics, will always occur over SN1?  In other words, are solvent properties the reason these two don't compete?  Or some other factor?

2) Then, with regard to E1, can 1-chloromethyl-1-methylcyclohexane even undergo E1 rxn??  The only alpha-carbon with respect to the chloromethyl carbon is a quartenary ring carbon-- which doesn't have a hydrogen to lose, as req'd for E1.

Thanks for helping clarify.   

[movies]

Solvent can have a big effect on the relative rates of these reactions.  Suppose you tried to run your reaction in a non-polar solvent like hexane.  The S_N1 pathway would be very slow because you would form two ions in a very non-polar medium.

You are ultimately correct though.  All of these pathways are competing, and yes you probably would see some S_N1 products if you were in a polar solvent.  However, I think in general these reactions are slower because of the activation energy involved in breaking a C-Cl bond (bond dissociation energy = 80 kcal/mol).  You really only see S_N1 when you have a non-basic nucleophile or a Lewis acid to aid in the bond dissociation.

As for 2, you are right.  With no H next to the leaving group you can't get E1.

[organic help]

I agree with that too...solvent effects can be huge.

I would also say that there is a big difference between laboratory chemistry and paper chemistry in an undergraduate organic chemistry class.  In the undergrad class, as long as you have good reasoning behind your argument, you will most likely get points on the exam.  However, real life chemistry in the lab is a different story.

[english]

I don't prefer to go by most textbook definitions for matters like these.

Instead, the first thing I try to recongize is the conditions of the reaction.  Is the reaction under polar acidic or polar basic conditions?

Then I try and recognize what sort of tranformation is occuring.  In this case, you're positive it's a substitution or elimination reaction.

Those are always my first two steps.

-----

(1) If it's polar basic, chances are it's either an SN2- or E2-like mechanism.  To figure out which one it is, I look at the structure and the attacking atom. 

Is the structure (substrate) hindered?  Most of the time, if it has three groups attached it's E2. 
Is the attacking atom a good nucleophile?  Is it a good base?  Weak nucleophile?  Weak base?  If it's a good nucleophile but a poor base, it's most certainly SN2.  (Look at the chart I've provided as a .doc file)

(2) If it's polar acidic, chances are it's either SN1 or E1. 

Of course in this case it's not a matter of substrate hindrance but substrate stabilization. 

----

Solvent effects, as movies said, has a large effect on kinetics.  There are many factors that influence what mechanism the reaction will undergo.  Substrate dependence, nucleophilicity vs. basicity, kinetics, as well as thermodynamics all affect substitution and elimination mechanisms.

If you're looking at two starting materials, one is obviously your alkyl halide and the other is a potential nucleophile/base, ask yourself the same questions I did.  Solvent effects stabilize transition states and/or starting materials, but are not necessarily the most important thing in determining which mechanism will prevail. 

Let me end with an example.  Let's say we have a reaction between t-BuBr and MeOH.  These are polar acidic conditions, and the substrate is fairly hindered.  We can rule out SN2.  We can also rule out E2 because MeOH is not a strong base (or even remotely basic for that matter). 

So we've narrowed it down to either SN1 or E1.  This is a 3° substrate, so a resulting alkene (after E1) would be thermodynamicallly favorable. 

In this case solvent effects are of very little importance because it just effects kinetics.  Because the possible mechanisms are E1 or SN1, the stabilization effects are the same.

We can conclude that the overal mechanism will be SN1, because the poor basicity of MeOH overrules the thermodynamic favorability of the resulting alkene after E1.  This does not mean E1 will not occur.  But given the circumstances, the amounts of alkene obtained will probably be really small.


As you can see, I've cirumvented the issue of E2 vs. SN1 based on the argument of basicity of MeOH, or lack thereof.

Remember, solvent effects influence TSs and energies of activation more than anything else.  Don't get too bogged down by these effects, as they're not always the most important thing in determining how a reaction proceeds.