[Hanlon]

So I got this question:

Nucleophilic substitutions can be subject to solvent effects.

Based on your results, did you observe a solvent effect in this experiment?
(In the second part, choose the appropriate reason for your conclusion.)

based on Gas Chromography I have 25% 2-chlorobutane and 75% 2-bromobutane made from 2-butanol in a substitution reaction.

my nucleophilic medium contains 100g ice, 76ml conc. H2SO4, 19.0g NH4Cl and 35g NH4Br

based on the fact that I got more 2-bromobutane then 2-chlorobutane am I correct to assume that there was "no solvent effect" because:

H2SO4 is a polar protic solvent, which will cause hydrogen bonding around the reactant compound therefore inversing the relationship between nucleophilicity and basicity. However, Br- being a weaker base and thus a better nucleophile still created more product then Cl- being the worse of the two nucleophiles, then there was no solvent effect in this experiement.

And am I also right to assume that this is a SN1 reaction, because SN1 reactions favor protic solvents?

[orgopete]

Let us assume the reaction were SN1. If that were the case, what should the product ratio be?

Let us assume the reaction were SN2, what would the product ratio be, or how would it differ from the SN1 ratio?

[discodermolide]

do not use grams, use Moles for any calculation, you may get on better

[rackye]

i think you are in the right way if you want to know if the reaction is SN1 or SN2 think about what kind of reaction is possible between the H₂SO₄ and the alcohol group.

[Hanlon]

I think I got confused somewhere:

o I just want to know if there is a solvent effect in the experiment.

whether its a SN1 or SN2 reaction is extra.

I believe there is a solvent effect because we are using a polar protic solvent (H2SO4) and in a polar protic solvent the nucleophilicity is inversed so weak bases are better nucleophile then strong bases. But in the experiment Br 75% of the product and and it is the weaker base compared with Cl which has 25% product, so there was an inversion of nucleophilicity so there is a solvent effect.

I just need someone to make sure my reasoning is correct because this is worth quite a bit of my organic chem lab mark.

[orgopete]

I thought everything was just mixed together. If so, how would the solvent affect chloride formation and not the bromide or vice versa? I really thought my first hint should have led you to the answer. If it does not, then I suggest you review SN1 and SN2 reactions because that is the key.

[Hanlon]

I believe this is a SN1 reaction due to the fact that SN1 reactions favor poor nucleophiles, and since Br- was 75% of the product and Cl- was 25% even though Cl- is the better nucleophile then it should be a SN1 reaction.

Now since its a SN1 reaction in a polar protic solvent it would still favor the weak base because the solvated molecules would want weaker bases since they can more easily break the H-bonds so it should be no solvent effect?

[rackye]

you will have a solvent effect if your products are differents of what you expected... with the acid you are only having a deshidratation reaction

[orgopete]

I believe this is a SN1 reaction due to the fact that SN1 reactions favor poor nucleophiles, and since Br- was 75% of the product and Cl- was 25% even though Cl- is the better nucleophile then it should be a SN1 reaction.

Okay, let us assume the reaction were an SN1 reaction. What would the ratio be? (Hint, it isn't rate determining.)

[Hanlon]

well I dont know the exact ratio but wouldnt the weaker base produce more product therefore Br- would have a higher amount of product then Cl-

[orgopete]

If you have a carbocation, won't the chloride and bromide attack equally well? Won't the ratio simply depend on the number of chloride and bromide ions?