Question

I need to prepare 2-bromopentane from H2SO4, NaBr, and 2-pentanol using reflux, a separatory funnel, and possibly distillation. My question is if this is the right procedure and if I will get a good yield. Amount of 2-pentanol cannot be changed in the procedure. The apparatuses listed in the procedure are the only ones available. The only possible washes are water and NaHCO3, and only sodium sulfate is available for drying. I am questioning whether I need to distill before using a separatory funnel but I am not sure and not sure what I would need to do. I need a decent yield and the isolated product decently pure. Please help! My first time running it I followed this procedure using less H2SO4 and my NaBr didn't dissolve after reflux so I gravity filtrated it then my liquid formed a solid in it, so I seperated that out? I finished using the rest of the procedure but my yield was extremely low and I am not sure if it is my product. Any help on what the procedure should be to obtain a high or decent and pure yield of 2-bromopentane would be helpful.

1. Combine 7.00g NaBr and 3.00 g of 2-pentanol into a reaction flask that is in cold water. Slowly add, 12 mL of H2SO4 to the flask.

2. Set up a reflux apparatus and reflux the mixture for 45 minutes. After reflux, let cool to room temperature.

3. Pour mixture into a separatory funnel. Wash with 15mL of H2O. Obtain organic layer.

4. Wash organic layer twice with 2 20mL portions of NaHCO3. Obtain organic layer.

5. Dry with sodium sulfate. Filtrate out sodium sulfate and obtain the product.

6. Possible distillation?

Answer 1

Your procedure looks ok to me, however, there are few step may be you can modify to obtain better yields. This my judgment based upon my experiance in organic lab.

1) Use NaBr anhydrus.

2) Use concentrated H2SO4 (as much concentrated as possible)

3) While adding H2SO4 you need not to maintain very cold temperature, it can be room temperature as well. This will help reducing time in protonating all hydroxyl group present on each alcohol molecule. (Any way you are refluxing reaction mixture later-on). Quantity of H2SO4 (12mL) depends on how much diliuted H2SO4 you are using. It is prefered to have concentrated enough.

4) After reflux when you are performing step 3, add diethyl ether to paored reaction mixture before washing with water. The reason for lower yields are i. no attack of nucleophile (Br-), and ii. if product is formed it is getting hydrolysed back to the alcohol when in contact with water of basic conditions.

5) After obtaining diethyl ether mixture of reaction mixture, water genrated, H2SO4, unreacted NaBr will come out in aqueous layer. Add small amount of water for washing. Our purpose of washing with water is to remove unreacted H2SO4 and NaBr.

6) While giving wash with NaHCO3 solution do not use more then 5 mL each time for two washes. This will reduce contact of your product with water which will reduce the chances of hydrolysis of product.

7) After sepration of ether layer, dry it over anhydrous Na2SO4 and then filter to reomove it. Lastly, evaporate ether to obtain your product. If there is unreacted alcohol present in the reaction mixture then you can again make ether solution give 2-3 wahses of water to remove alchol (no need to use NaHCO3 solution).

By this modification we are achiving to avoide as much as possible the hydrolysis of formed product to increase yield.