Preparation of n-Butyl Bromide from Sodium Bromide and n-Butyl Alcohol

[chrisf]

This is the reaction for our lab. We added the NaBr, obviously, to brominate the 1-butanol. OH^- is a stronger nucleophile than Br^- so the reaction wouldn't be favorable. An easy way to push the equilibrium to the products is to make the OH^- a leaving group by protonating it with the H₂SO₄. -H₂O is an excellent leaving group and the bromination proceeds. At least, this is my understanding of the reaction.

We have a couple of post-lab questions that I'm working on and I'd appreciate any insight anyone might have.

An ether and an alkene are formed as by-products in this reaction. Draw the structure of these by-products and give mechanisms for their formation.

I've got the answer for this, but not on me. I'll create the image and get it up here.

Aqueous sodium bicarbonate was used to wash the crude n-butyl bromide. What was the purpose of this wash? Give equations. Why would it be undesirable to wash the crude halide with aqueous sodium hydroxide?


The answer to the first question is because the sodium carbonate neutralizes any excess acid that may be present from the previous separation process.

I believe the answer to the second question is because the introduction of a stronger nucleophile would favor a reverse reaction and de-brominate the product.

[chrisf]

1-butoxybutane:

1-butanol reacts with H⁺ from acid to create an H₂O leaving group. The H₂O group leaves creating a... and I see a mistake I made and the professor didn't catch.

I have this leaving a primary carbocation (!!!) which reacts with another 1-butanol, which then has an H⁺ abstracted by another 1-butanol or possibly the H₂O (far less likely) to leave no formal charge.

However, this wouldn't work.. the primary carbocation would rearrange.

Hmmmmm, I should look into this.

[ssdallas]

Hello, I was wondering if I could see the mechanism drawing for the ether and alkene that are formed. I'm REALLY interested in seeing this! Thank you!
  Shae