Question

Lab Question?????

Reactants: 4.00g 2- pentanol, 7.00g NaBr, 5ml 9M H2SO4 (cant use more than 4 or 3 g of 2-pentanol or change reactants)        Want Product: 2-bromopentane

What procedure in the lab would you do to create from reactants and isolate 2-bromopentane from left over 2-pentanol, NaBr, H2SO4, and water?

I have:

1. Combine Reagants (NaBr is in excess) and reflux for 30 minutes. Problem: NaBr did not dissolve.

2. To get rid of NaBr that did not dissolve, I gravity filtrated (liquid formed a solid at the bottom?). I took only the liquid without solid that formed (liquid was transferred out carefully) and washed with 15 mL DI water. (aqueous layer on top and organic on the bottom which was cloudy?)

3. I then washed the organic layer with NaHCO3 twice with 10 ml portions. (aqueous layer on top and organic on bottom)

4. I had very little liquid from the organic layer. I dried with sodium sulfate to get rid of H2O.

5. I had very little product yield at the end and am not sure it's the product. (only like a milliliter of product)

6. I did not distill since I didn't have enough product.

What changes can I make to my procedure to increase product yield? What did I do wrong in my procedure? How can I get my product, 2-bromopentane, in high yield?

Answer 1

NaBr heated with sulphuric acid generates hydrobromic acid in situ, which brominates your alcohol to yield bromoalkanes. First, combine sodium bromide and sulphuric acid in aqueous condition, avoiding excess sodium bromide. If excess salt is used, it tends to stay unreacted and also supresses the ionization of bromide ionization from the HBr formed. Next, addition of 2-pentanol causes a steep drop in the dielectric constant of the solution, causing instability of excess sodium bromide in the solution, giving it a tendency to precipitate. All of these factors ensure that less than the required equivalents of sodium bromide reacts to yield the proper quantity of HBr necessary to yield 2-bromopentane. Note that this reaction is usually high-yielding. So, to increase your yield, first combine sodium bromide and sulphuric acid and warm gently without refluxing. This will initiate HBr generation. After a few minutes of warming the mixture, start refluxing the reaction and when it reaches refluxing temperatures, add 2-pentanol slowly, using a pressure-equalizing dropping funnel, fitted with the same ground-joint as the reaction flask to curb leakage and fumes.

Most importantly, never disturb the reaction by filtering the excess NaBr, etc. as this causes a sudden change in the reaction rate and will instigate the unfavourable decomposition of transition states formed, decreasing your yield. Moreover, ensure that there is enough solvent to maintain good solubility of NaBr even after addition of all 2-pentanol. Literature shows good solubility of sodium bromide even in slightly non-polar solvents, showing its good tendency to accomodate change in solvent polarity, But, if sudden change is brought by actions such as single-batch, complete addition of the alcohol, it might disturb the HBr generation. Hence, keep all thse in mind and prepare out sulphuric acid and your substrate weights.

For solvent polarity, the dielectric constant of a solvent-system is determined by the formula:

where 1 and 2 show the constituent solvents, epsilon represents the dielectric constant of the pure solvents and v stands for the fraction of each solvent in the mixture (eg. for a binary solvent mixture, v₁ = (1 - v₂) and vice versa).