[mushaboom]

In my ochem lab, we made bromobutane from butanol using sodium bromide and sulfuric acid. After the reaction took place, we put our crude product in distilled water and stirred before pipetting out the bromobutane. All of us either got no product or got terrible yield. Is it possible that some of the bromobutane dissolved while we were stirring it in the water? I know organic compounds are insoluble in water but I don't think that many of us made such big mistakes...

[TheUnassuming]

Can you give a more detailed description of your experimental procedure?

[TheUnassuming]

Oh, and no, bromobutane is not appreciably soluble in water.  So stir all you want and it won't go in any more than a tiny fraction.   

[mushaboom]

We put NaBr, sulfuric acid, and 1-butanol into a round bottom flask and heated under reflux for 40 minutes. Then, we put the crude product into deionized water and stirred for 15 minutes. The product was pipetted out, dried with sodium sulfate, then weighed.

Oh, and no, bromobutane is not appreciably soluble in water.  So stir all you want and it won't go in any more than a tiny fraction.   

That's what I thought, but several TA's were saying that the product dissolved. The flask of water and crude bromobutane was stirred on the same hot plate used for reflux, so would the residual heat make a substantial difference?

Other reasons I've come up with are side reactions and that not all of the reactants actually reacted. I can't imagine that either of those would account for so much product loss though.

[kriggy]

How  much is your loss?
Maybe 40 minutes is not enough time? Or maybe by stirring the mixture for 15 minutes it might mix together with water and take some time to separate.

[mushaboom]

How  much is your loss?
Maybe 40 minutes is not enough time? Or maybe by stirring the mixture for 15 minutes it might mix together with water and take some time to separate.

I got 27% yield, which is the highest out of anyone I've talked to.

[TheUnassuming]

Well if all your starting material didn't react that will kill a yield very quickly.  How did you monitor the reaction?  It is possible that stirring for that long in water hurt the yield, but I dont know if the product is that unstable.  What was your extraction solvent? 
My first instinct is that you didn't get very good conversion, so when you poured the reaction into the water there was substantial alchohol left which could potentially make the product significantly more soluble in water.

[mushaboom]

We actually didn't monitor the reaction in any way, just heated under reflux for 45 minutes. We simply got the product out from the water using a pipette. There was some traces of it left in the water that I couldn't pipette out, but only a few droplets. Anyway, I think I have plenty of things to talk about from all the replies, haha. Thank you both for your help.

[orgopete]

Since this is a reasonably common student preparation, I don't think the problem was due to the reaction with water. However, the sulfuric acid concentration is important. If too dilute, this will reduce the amount of protonated butanol and reduce the substitution rate and yield. If too concentrated, it will catalyze dehydration.

[Dan]

The flask of water and crude bromobutane was stirred on the same hot plate used for reflux, so would the residual heat make a substantial difference?

Bromobutane is quite volatile, so if the flask was stirred open and hot/warm you might lose a significant amount through evaporation. This coupled with butene formation (as Orgopete pointed out) is probably the cause of the low yields. It is also possible that the reaction had not proceeded very far if it was not monitored in any way - but I'd assume the conditions have been somewhat optimised before use as a student practical.

Do you actually know what the expected yield range is (the TA should have this information for grading)? Some student experiments are inherently low yielding - it catches out those who get creative with their numbers in an attempt to get better grades: a lesson in scientific fraud.

[TheUnassuming]

Bromobutane is quite volatile, so if the flask was stirred open and hot/warm you might lose a significant amount through evaporation.

Given the difference in bp's I'm wondering why they didn't have the class set up the reaction to distill the product as it formed.  It would improve the yield, make the work up much simpler, and reenforce the lesson of Le Chatelier's principle. 

[TheUnassuming]

We actually didn't monitor the reaction in any way, just heated under reflux for 45 minutes. We simply got the product out from the water using a pipette. There was some traces of it left in the water that I couldn't pipette out, but only a few droplets. Anyway, I think I have plenty of things to talk about from all the replies, haha. Thank you both for your help.

Glad we could help. Good luck in the class!

[mushaboom]

The professor said good yield would be around 65%, but like I said, 27 is the highest I've heard of so far.

We were supposed to do simple distillation and extraction, but we needed to finish the entire experiment in under 4 hours, so instead we mixed with water to get rid of aqueous impurities.

I decided to talk about side reactions and poor conversion as the main reasons for the low yield since we used very concentrated sulfuric acid and did not monitor the temperature. Thanks again everyone!

[orgopete]

... side reactions and poor conversion as the main reasons for the low yield since we used very concentrated sulfuric acid and did not monitor the temperature...

When I searched for the procedure for thus reaction, the first example I found used 65% sulfuric acid. I had also searched quite a lot for similar procedures as I wanted to avoid using conc sulfuric acid and I wanted to use different alcohols to prevent students from concluding what their product was. I wanted them to analyze their product and then conclude the structure of their product. As a result, I discovered the student prep was fairly optimized for butyl bromide, though it does work reasonably well for other bromides. Therefore, I was suspicious that the sulfuric acid concentration had been varied. The quoted comment suggests that was the case.