[Moz29386]

First of all hi, I am new to this forum  

I just want to know what conditions reagents would I change to make SN1 favourable instead of E1 and vice versa.

And the same with SN2 vs E2

Also why does the size of a nucleophile influence wether a tertiary alkyl halide gives a subsitution or an elimination product?

Many thanks.

[orgopete]

In SN1 and E1 reactions, one can exert little control over the fate of the carbocation intermediate. If one wished to suppress elimination, then a strategy that can reform a carbocation will ensure that the final product is a substitution product. That usually means acidic conditions. Since an SN1 reaction is most successful if a tertiary or other stabilized carbocation intermediate forms, then an acid can more easily regenerate a tertiary carbocation.

If one wished to effect an elimination, I would suggest researching an E2 elimination in ethanol or methanol. Those eliminations usually result in the Saytzev elimination product. The only difference one might experience is where the Hoffmann elimination product prevailed. Carefully choosing reaction conditions can increase one or the other.

[bromidewind]

I found this great <a href="http://74.125.113.132/search?q=cache:j0s5eWVCp88J:www.highlands.edu/academics/divisions/scipe/chemistry/Site/OHandouts_files/Summary%2520of%2520SN1-SN2-E1-E2%2520Reactions.doc+Summary+of+SN1-SN2-E1-E2+Reactions.doc&cd=1&hl=en&ct=clnk&gl=us">"cheat sheet"</a> back when we were covering SN1, SN2, E1, and E2 reactions. It really helped clear up a lot of confusion that I had and it is a great reference sheet when working out reactions. I still use it from time to time when my memory fails me.

[orgopete]

Let me add one additional comment, while "Since the SN1 and E1 reactions proceed via the same carbocation intermediate, the product ratios are difficult to control and both substitution and elimination usually take place." (http://www.cem.msu.edu/~reusch/VirtualText/alhalrx3.htm#hal8), increasing the temperature increases elimination.

[Moz29386]

Hey guys thanks for the input.

so:

To favour SN1 over E1 I would need to use a small nucleophile and the precursor most obviously be a Tertiary alkyl halide. The small nucleophile finds it easier to attack the polarised (alpha carbon) carbon bearing the electronegative group due to less steric hinderance. If I wanted E1 to dominate I would use a bulky nucleophile such as (CH3CH2OH) because it creates a steric hinderance with the other large R groups sorrounding that carbon, and this would raise the activation energy of the SN1 pathway making it unfavourable. So it must subtract the proton from the C-H group in the b-carbon.

This would also be the same scenario with SN2 and E2 except that you will need Primary alky halide.


I could also raise the temperature to lower the activation energy of the E1 pathway to make it more likely. Using that analogy, would a lower temperature (i.e 10 degrees centigrade) favour SN1?

Is my logic correct? if not correct me please.

Ps: I understand Secondary alkyl halides can undergo all 4 reaction pathways so I didnt use them because it would just be too messy.

[orgopete]

I do not have anything to add over the Reusch quote about it being difficult to control, that is, once the carbocation is formed, it is difficult to predict whether a substitution or elimination reaction will take place. If small, large, strongly basic, weakly basic, diffused, polarizable, or any other restriction to the nucleophile applied, I assumed this comment covered those possibilities (unless someone can explain further).

Let me try to explain my point with an  example. If you wished to prepare t-butyl chloride from t-butanol, protonation with HCl will result in protonation followed by the rate determining loss of water to form the carbocation. Water can re-attack, an elimination can occur, or chloride can attack. If elimination occurred, HCl can reform the tertiary carbocation. By reforming the carbocation, the competition between elimination can be overcome.

If you wished to convert t-butyl chloride to isobutylene, solvolysis in an organic/water mixture would generate the carbocation. It could be attacked by solvent or water or the elimination reaction would take place. If water attacked, that reaction will be essentially irreversible. If you wished to prepare isobutylene, then change the mechanism so it will be an E2 elimination. No substitution will occur.

Speaking generally, elimination reactions can give a Zaitsev (E1 like) or Hoffmann product. If the question was directed to this possibility, then a good elimination base like KOtBu increase the amount of Hoffmann product that forms. How can that be overcome? Do an E2 elimination is a more polar solvent, like ethanol, to increase the E1 nature of the reaction.

While it is simple to suggest that reactions occur via SN1 or SN2 mechanisms, I prefer to think that they describe the extremes of these reactions. You will find that the leaving group may not completely diffuse away before a nucleophile attacks in an SN1 reaction and some inversion may result. In an SN2 reaction, bond lengthening or breakage may begin before the nucleophile attacks.

In elimination reactions, an additional pair of electrons are involved. The notion of concerted is further stretched and we may even have additional descriptions, such as an E1cb elimination. My comments have been made that recognize these more subtle effects.

Because the rate determining effect of an E1 elimination would be the loss of the leaving group, I do not know how the temperature affects the thermodynamics of a competition v elimination reaction. I simply read that somewhere (probably the Reusch site).