[supercodplayer1995]

Hi. I am currently working on an assignment and would like some help. However, I would appreciate if you refrained from giving me absolute answers, just some help regarding my current confusion:

During a nucleophilic subsitution lab, three different alcohols react with a nucleophilic medium to yield alkyl halides.

The alcohols:

1-butanol
2-butanol
2-methyl-2-propanol

The same medium is used in the reaction of these alcohols, and this medium consists of:

100g water
76mL conc. sulfuric acid
19.0g NH4Cl
35.0g NH4Br

The results (I am going to round these answers significantly, just to get a general sense)

1-butanol reaction yields:

10% 1-chlorobutane
90% 1-bromobutane

2-butanol reaction yields:

20% 2-chlorobutane
80% 2-bromobutane

t-butanol reaction yields

60% 2-chloro-2-methylpropane
40% 2-bromo-2-methylpropane

Since, nucleophilic subsitutions can be subject to solvent effects, a question is asked:

Did you observe a solvent effect in this experiment?

And if so, why?

So here are my thoughts, and the my reasons for my confusion.

The composition of NH4Cl and NH4Br appears to be equimolar in the nucleophilic medium. While there appears to be slightly more bromine (0.357 moles) than chlorine (0.355 moles), I will assume this difference is negligible.

First looking at the reaction of 1-butanol, a primary alcohol, since there is significantly more chlorine products than bromine products, I assume that there is a solvent effect and that the reaction is an SN2 reaction.

The reason for this is because in polar PROTIC solvents, the nucleophilic anions can be solvated, making them less nucleophilic. Since the smaller the anion, the more solvated the anion will be, chlorine is more solvated than bromine, and thus this matches with the results, in which there are more bromine products.

Similar results are seen in the reaction of 2-butanol, a secondary alcohol.

However, the results are reversed in the reaction of t-butanol, a tertiary alcohol. This made me wonder whether this is due to perhaps t-butanol undergoing a SN1 reaction, while the rest undergo SN2 reactions.

My chemistry notes state that SN1 is a "non-selective" process and that the rate is dependant on the concentration of the elelectrophile, and it is independent of the concentration of the nucleophile.

However, in my mind, this would mean that the products of the t-butanol reaction would be approximately 50/50 chlorine and bromine, not 60% chlorine.

To conclude, I have already stated, right or wrong, in this assignment, that I DID observe a solvent effect in this experiment and that the reason is that there is "competition from bromine and chlorine products".

Am I mistaken? Can you provide hints as to why, if I am?

[mjc123]

Did you observe a solvent effect in this experiment?

You can't have observed a solvent effect in this experiment, because you were reacting different substrates in the same solvent. To observe a solvent effect, you need to react the same substrate in different solvents, and see whether you can relate any differences in the results to the different character of the solvents.
I'm not saying there isn't a solvent effect, or that the points you make are wrong, but you can't observe it unless you compare the same reaction in different solvents.
In this experiment, the differences are primarily due to the different mechanisms of S_N2 and S_N1 in the same solvent.

[orgopete]

I think you are on the right track (except the word solvent vs reactant). I think you should think about the t-butyl product. There are two reasons the ratio is not 1:1. You gave a predicted result, what could be another? (Hint, could a product also be a reactant?)

[zsinger]

MJC hit the nail on the head.