[xshadow]

If I have this reaction:

1° (primary) Haloalkane + F^-/Cl^-/Br^-/I^- in a Protic solvent

I think I'll have a SN₂ reaction due to the primary haloalkane (more important than the protic solvent)
But with which  halogen the SN₂ will be faster ?? 
I know that a SN₂ is faster if the nucleophilic  is "stronger"
Usually F^- is the strongest one  because is the best base....but in a PROTIC solvent??

Is F^- or I^- the strongest  one in the protic solvent??
(sorry for my bad english)

Thanks

[orthoformate]

xshadow,

I agree with you about the Sn₂ mechanism.

I am wondering, is fluoride the most nucleophilic halogen? Remember that basicity and nucleophilicity are two different things.

[Babcock_Hall]

Which nucleophile will be most affected by being in a protic solvent?

[xshadow]

Thanks for the answers

The Halogen nucleophilicity order I know is:

 Aprotic Solvent:  F^- > Cl^-> Br ^- >I^-
(here follow basicity )

But I think that in a Protic one the order is reversed...because a protic solven can also salvate well anions (an aprotic solvent solvate usually only cations) . So F^- is the anion with the biggest Solvatation energy because is the smallest of halogen ---> bad nucleophilic

I was wrong?

Ps: I know there is difference between basicity and nucleophilicity but I think these two concepts (one kinetic and the other one thermodinamic) follow the same "direction" (basause a good nucleophilic is a species that trands to donate "easily"a lone pair...a bit the same for the lewis base) ....but this common trend  can become different(usually opposite) when ,for example, the overall stochiometry isn't negligible (this thing affects the kinetic, so the nucleophilicity)....I don't know if I'm saying a worth things

[orthoformate]

Thanks for the answers

The Halogen nucleophilicity order I know is:

 Aprotic Solvent:  F^- > Cl^-> Br ^- >I^-
(here follow basicity )

But I think that in a Protic one the order is reversed...because a protic solven can also salvate well anions (an aprotic solvent solvate usually only cations) . So F^- is the anion with the biggest Solvatation energy because is the smallest of halogen ---> bad nucleophilic

I was wrong?

Ps: I know there is difference between basicity and nucleophilicity but I think these two concepts (one kinetic and the other one thermodinamic) follow the same "direction" (basause a good nucleophilic is a species that trands to donate "easily"a lone pair...a bit the same for the lewis base) ....but this common trend  can become different(usually opposite) when ,for example, the overall stochiometry isn't negligible (this thing affects the kinetic, so the nucleophilicity)....I don't know if I'm saying a worth things

http://www.masterorganicchemistry.com/2012/06/18/what-makes-a-good-nucleophile/

look at this webpage, it helped me understand the issue

[xshadow]

So what is the answer for my question??

I^- is better(faster) for my  Sn₂  reaction

[orthoformate]

So what is the answer for my question??

I^- is better(faster) for my  Sn₂  reaction

my understanding is this: In a protic solvent fluorine is the worst nucleophile because it is coordinated to the protons in solution. Iodine is the most nucleophilic halogen in a protic solution because it is coordinating to protons the least of all halogens.

does this sound right?

[OCSaviour]

So what is the answer for my question??

I^- is better(faster) for my  Sn₂  reaction

my understanding is this: In a protic solvent fluorine is the worst nucleophile because it is coordinated to the protons in solution. Iodine is the most nucleophilic halogen in a protic solution because it is coordinating to protons the least of all halogens.

does this sound right?

To add another layer to it, protic solvents tend to stabilize the charge on nucleophiles. The strength of the nucleophile is in inverse proportion to the degree to which the charge on it gets stabilised.

That's why we use aprotic solvents in the case of SN₂ reactions because the concentration of the nucleophile affects the rate of the reaction, even though we have to comprise a little bit on the idea of stabilisation charge on any element throughout the course of the reaction.

Concentration of the nucleophile does not affect the rate of an SN1 reaction, hence we are back on the rule of stabilisation of charge and use protic solvents to improve efficiency.

[Babcock_Hall]

So what is the answer for my question??

I^- is better(faster) for my  Sn₂  reaction

my understanding is this: In a protic solvent fluorine is the worst nucleophile because it is coordinated to the protons in solution. Iodine is the most nucleophilic halogen in a protic solution because it is coordinating to protons the least of all halogens.

does this sound right?

@orthoformate, This sounds very similar to what I found in Lowry and Richardson on page 337 (Mechanism and Theory in Organic Chemistry, 2nd ed.)

@OCSaviour,  One has to be cognizant of the charge or lack of charge on the nucleophile and leaving group to assess the effects of solvent polarity.  A reaction involving a neutral nucleophile and an uncharged leaving group (one that will become negatively charged as it leaves) is predicted to have an increase in rate in a more polar solvent (p. 325).

[xshadow]

So what is the answer for my question??

I^- is better(faster) for my  Sn₂  reaction

my understanding is this: In a protic solvent fluorine is the worst nucleophile because it is coordinated to the protons in solution. Iodine is the most nucleophilic halogen in a protic solution because it is coordinating to protons the least of all halogens.

does this sound right?

yes!!

So SN2 in a Protic solvent  will be slower(as kinetic) If I use F- (more solvatated...weak nucleophilic) rather than I-