[priyanka]

Hi, Wanted to know..
How do polar solvents favour SN1 reactions?

[Smrt guy]

They stabilize the transition state for the rate limiting step as well as the intermediate carbocation.

[srihari]

Well .. will there be a transition intermediate in S_N¹ ?
The rate limiting step is the formation of the carbocation itself why would you
talk of the transition intermediate here ? 

srihari

[srihari]

According to my understanding of SN1 reactions ..

the basic steps are as follows.

the formation of a carbocation , and then attack by the nucleophile on the resulting cation .
The rate limiting step is the formation of carbocation , the moer stable the carbocation more easier it is to form it
hence lesser the rate. and this is now open to attack by the nucleophile.
Since the above reaction is an acid-base reaction ( lewis concept) it is quite fast compared to the
first step in which the carbocation is formed .

Also side reactions are possible like the E1 mechanism type .
So depending on the substrate we need to decide the major product
regards
srihari

[xiaojunzhang]

In an SN1,the rate of reaction depend on the loss of the leaving group, that is to say ,on  the generation of a carbocation.If the carbocation grow quickly, SN1 reaction will be very easy to run.

[Smrt guy]

The rate of all chemical reactions is dependent on the activation energy.  Stabilization of the transition state makes the reaction faster by decreasing the activation energy.  The transition state for the rate limiting step has an elongated C-LG bond.  The increasing polarization (formation of + charge on C and - charge on LG) is stabilized by polar solvents.  Once formed, the intermediate will quickly react with the incoming nucleophile.  Stabilization at this point serves to increase the lifetime of the intermediate and prevent side reactions (like elimination) but does not increase the rate.

[priyanka]

 I agree with smartguy. I think he is right. Thanks a ton.

[srihari]

Okie..
@smrtguy   
thanks ...

messed up