Quick question on SN1 reactions

[Lazerous]

Do SN1 reactions favor protic or aprotic solvents?

---

Members don't see this ad.

 

[PreDent2009]

SN1 tend to favor polar protic solvents which help to stabalize the carbocation intermediate. SN2 favors aprotic solvents because a protic solvent can essentially "clog" up the base and since SN2 is second order the base/nucleophile is part of the rate law.

Do SN1 reactions favor protic or aprotic solvents?

 

[osimsDDS]

SN1 favor protic polar solvents because it solvates the carbocation while an SN2 reaction favors polar aprotic solvents so it wont reaction with the nucleophile...

ex/ of polar aprotic solvents can be anything that doesnt hydrogen bond but is polar...CH3SH

 

[Lazerous]

Thanks for the responses.

Can someone clarify this: SN2 is favored over E2 by weak lewis base (strong nucleophile)

How is a weak base a strong nucleophile? I thought they were one in the same.

 

[osimsDDS]

i am wondering exactly the same thing, you got this from kaplan...i dont understand this either maybe someone can explain...

why do SN2 reactions favor weak lewis base, strong nucleophiles over E2???? this is like a contradiction to me...how can a weak lewis base be a strong nucleophile???? thanks

 

[PreDent2009]

A perfect example of a weak base that is a great nucleophile is Sulfur, for example KSCH2CH3. This molecule will not undergoe E2 reactions from my understanding unless they are given extreme help through rxn conditions. To understand why it is a weak base, we look at the conjugate acid which is HSCH2CH3. This is a very acidic proton due to the orbital mismatch that occurs between the 3rd shell Sulfur and the 1st shell hydrogen. The reason why this weak base is a great nucleophile is because it is an extremely polarizable molecule that is able to "deform" it's electrons in such a way that enables it to undergoe SN2 backside attack very efficiently. This is the only case I know of that is a weak weak base that does not do E2 and mainly does SN2. Other reactants such as KOCH2CH3 (potassium ethoxide) is both a strong base and strong nuclephile. FOr reactions with this compound you must first consider the E2 product, determine if it is stable, if it is not, then it will be a SN2 reaction,etc. Hope this helps.

i am wondering exactly the same thing, you got this from kaplan...i dont understand this either maybe someone can explain...

why do SN2 reactions favor weak lewis base, strong nucleophiles over E2???? this is like a contradiction to me...how can a weak lewis base be a strong nucleophile???? thanks

 

[niktacal82]

As I learned; we have two types of STRONG nucleophiles: 1) strong bases such as OH- 2)weak lewis bases ; In SN2 you need strong nucleophile to attack from behind ( not necessarily strong base)...For E2 you need strong base good nucleophile to remove Hydrogen from carbon next to leaving group. so when you have weak lewis base,strong nucleophile ( it can't remove hydrogen really as well as attacking), so SN2 favors E2.

Just to add, if you have bulky nucleophile such as tert-butoxide, then E2 favors, because in SN2, nucleophile needs to reach (attack)!

let me know if I am wrong or way off🙂