johnwandering

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May 18, 2009
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I originally thought that the reason for the protic polar solvent in SN1 reactions was to "weaken" the nucleophile to give time for the leaving group to detach from the electrophile.

But as I am reading a passage from BR, they note that "the strength of a nucleophile cannot be determined from an SN1 reaction, because the rate of the reaction only depends on the leaving group."

Which I then learned that the purpose of the solvent was to stabilize the cation intermediate of the SN1.


So I am wondering, in case I encounter such a question,
Is the purpose of the protic quality of the polar solvent to:
A.) Weaken the nucleophile AND stabilize the cation intermediate
or
B,) Just to stabilize the cation intermediate?



I know that the polar solvent serves to weaken the nucleophile for SN2. But am wondering if the protic quality is to enhance this dampening, or for a new purpose.
 

MedPR

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The answer is yes. It does both and there is no rule for which is more important because it will vary.
 
May 3, 2011
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The answer is more "B" because a nucleophile going Sn1 is already fairly weak + the rate limiting step is the formation of the carbocation. The nucleophilic attack on the carbocation is so fast that attenuating or slowing it doesn't do much good.
 

ridethecliche

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The rate limiting step is what is important. If the other stuff doesnt do much if it doesnt affect that step.

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Apr 1, 2012
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A polar protic solvent, such as water, possesses an electronegative atom. This means that there are partial negative charges on the electronegative atom of the solvent and partial positive charges on the other bonded atoms ( hydrogen in the case of water). Sn1 reactions proceed through a Carbocation intermediate, which has a positive charge. The positive charge is stabilized by forming transient bonds with the electronegative oxygen which is partially negative, thus reducing the net charge on the Carbocation in solution to a more neutral state. As we know, neutrality is good. Thus the Carbocation is stabilized by the polar solvent. The protic ability of the solvent favors sn1 reactions due to the Carbocation intermediate. In order for a Carbocation to form, a good leaving group must be present in solution. Often times, the leaving group is formed by protonation. In acidic solution, protonation of a leaving group is inferred. For example, if one combines a primary amine and a strong acid, the amine exists in its protonated state in solution. Protonation of the primary amine yields ammonium, which can leave as ammonia. So the protic nature of a solvent is a direct representation of the state at which a molecule exists in solution. Remember, sn1 proceeds via Carbocation intermediate. In order for the cation to form, a good leaving group must be present in the molecule. In a protic solvent, the leaving group is protonated, and leaves to restore neutrality. The Carbocation then forms as the leaving group leaves. The positively charged cation is stabilized by interactions with the partially negatively charged atoms of the solvent, Similar to the formation of hydration shells formed when a sodium chloride is dissolved In water. Hope this helps!
 

SH3

Oct 4, 2011
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A polar protic solvent, such as water, possesses an electronegative atom. This means that there are partial negative charges on the electronegative atom of the solvent and partial positive charges on the other bonded atoms ( hydrogen in the case of water). Sn1 reactions proceed through a Carbocation intermediate, which has a positive charge. The positive charge is stabilized by forming transient bonds with the electronegative oxygen which is partially negative, thus reducing the net charge on the Carbocation in solution to a more neutral state. As we know, neutrality is good. Thus the Carbocation is stabilized by the polar solvent. The protic ability of the solvent favors sn1 reactions due to the Carbocation intermediate. In order for a Carbocation to form, a good leaving group must be present in solution. Often times, the leaving group is formed by protonation. In acidic solution, protonation of a leaving group is inferred. For example, if one combines a primary amine and a strong acid, the amine exists in its protonated state in solution. Protonation of the primary amine yields ammonium, which can leave as ammonia. So the protic nature of a solvent is a direct representation of the state at which a molecule exists in solution. Remember, sn1 proceeds via Carbocation intermediate. In order for the cation to form, a good leaving group must be present in the molecule. In a protic solvent, the leaving group is protonated, and leaves to restore neutrality. The Carbocation then forms as the leaving group leaves. The positively charged cation is stabilized by interactions with the partially negatively charged atoms of the solvent, Similar to the formation of hydration shells formed when a sodium chloride is dissolved In water. Hope this helps!
Good Summary :p