Warning: Hydrobromic and sulfuric acids are corrosive.
Bromoethane and bromohexane are carcinogenic.
Bromoethane is volatile.
Wear gloves when handling them and work outside or in a fume hood.
Greetings fellow nerds.
I need some alkyl halides for some upcoming videos.
For those of you new to chemistry, alkyl halides are generally any compound containing an alkane carbon bonded to a halogen like fluorine, chlorine, bromine, and iodine.
Now alkyl halides have tremendous application in chemistry.
They can be found in pesticides, flame retardants, drugs, lubricants, solvents and refrigerants.
I want them because i want to make grignard reagents.
Now there are lots of ways to make alkyl halides and I actually already covered a few special reactions involving them.
We made chloroform by the reaction of a ketone with bleach in the haloform reaction.
In another video we made chlorobenzyl chloride by photochemical halogenation with chlorine gas.
In this video, i'm going convert an alcohol into an alkyl halide by reacting it with a halogen based mineral acid.
In this case, hydrobromic acid.
So let's get started with a simpler one, bromoethane.
First we setup a distillation apparatus since we'll be distilling the products later.
The boiling flask is 1L in size while the receiving flask is 500mL in size.
Now we pack the boiling flask in ice water for now since the bromoethane we produce is volatile and we don't want it to boil out until we're ready.
Once it's packed we add in our bromide ion source, in this case about 350g of 48% hydrobromic acid.
We actually showed how to make hydrobromic acid in a previous video.
You can check the video description for the links.
Once it's all added give it ten minutes or so to cool down.
Then we add in 55mL of concentrated sulfuric acid.
Go slowly and give it time to cool.
Sulfuric acid tends to heat up a lot when added to water and solutions that contain it.
Once that's all added and allowed to cool, we add in 120mL of ethanol.
I'm using water free ethanol since i had a lot, but you can use easier to get 95% ethanol.
Once everything is added, give the mixture 20 minutes to cool.
Now we remove the ice water bath and pack our receiver flask in ice water.
Looking back now this this step was totally redundant.
We just need to pack the receiver in ice water, but the boiling flask can remain as is since we'll be heating it up later.
Anyway, we now attach an addition funnel to the boiling flask and drip in another 90mL of concentrated sulfuric acid.
You might be asking if all the 55mL portion of sulfuric acid from earlier could have been combined with this one and added all at once.
The answer is yes and if you want to save time you could do that.
I added them separately since i wanted to give the mixture time to cool.
But in looking at how well behaved it is,
I think all the sulfuric acid can be added at the end as along as you have an ice bath and are careful to go slowly.
The bromoethane is forming right now and if the acid is added too fast the self-heating will boil it off.
As you can see we're already getting some condensation as the bromoethane vaporizes.
Once all the sulfuric acid is added, we place the boiling flask in a water bath and start heating.
The water doesn't need to boil, and in fact it would be best to adjust the temperature so you slowly condense the bromoethane
and don't overshoot the cooling ability of your condenser.
It would also be helpful to put a thermometer in your water bath and start at 40 celsius,
gradually ramping up the temperature to a maximum of 70 celsius as the dripping rate slows.
So what's happening.
The hydrobromic acid reacts with the ethanol to make bromoethane.
This is one of the classic Sn2 nucleophilic substitution reactions taught in basic organic chemistry courses.
The alcohol group gets protonated by the highly acidic conditions of being in a mixture of hydrobromic and sulfuric acids.
This makes it an exceptionally good leaving group and highly susceptible to nucleophilic attack.
The bromide ions in solution then attack the carbon and this pops off the water.
In the end we get bromoethane and water.
Now since this reaction generates water the sulfuric acid in the mixture helps drive it forward by absorbing water and removing it from the reaction.
The fact that we're also distilling out our desired bromoethane helps tremendously as well.
For those of you familiar with making diethyl ether you might be asking if the mixture of ethanol and sulfuric acid also makes diethyl ether as well.
It does actually and this represents an unwanted side product.
The quantity produced is small though and for most purposes like in making grignard reagents the diethyl ether won't be a problem.
Another side product is bromine itself.
Hydrobromic acid reacts with sulfuric acid to create sulfur dioxide and small amounts of bromine.
This explains the orange color of our distillate.
Anyway, keep running the distillation until nothing distills over even when you reach a water bath temperature of 70 degrees celsius.
And there we have it, crude bromoethane.
It should be clear but we have bromine contamination as well.
To purify it, we first get about 300 mL of water and directly add our crude bromoethane.
Bromoethane is denser than water so it sinks to the bottom.
Stir the mixture for ten minutes to absorb any dissolved hydrogen bromide and ethanol that may have distilled over.
Now decant off the supernatant.
Add in a 5% solution of sodium bicarbonate.
Basically 5g of sodium bicarbonate and 95g of water.
I'm using 350mL worth.
Stir the solution for ten minutes or so until it clears.
What's happening is the bromine is reacting with the alkaline conditions of the sodium bicarbonate to make sodium hypobromite.
This dissolves in the aqueous layer and clears up the bromoethane.
Now decant that off and add in water to wash out the sodium bicarbonate.
Finally decant off the water and use a separatory funnel to get the purified bromoethane.
And here is our bromoethane.
It has trace water dissolved in it so you can add in anhydrous magnesium sulfate if you wish to remove it.
It also contains small amounts of diethyl ether.
I'm going to leave that in but if you wish to purify it further then you can use fractional distillation
and collect the fraction that boils at 38 degrees celsius.
Anyway, my total yield was 116g or about 50%.
While this is bad it's not representative of this reaction in general.
Other amateur chemist have easily gotten 90% or better yield using the exact same procedure.
I think i simply didn't heat it long enough or high enough and i likely lost a lot due to evaporation.
Anyway I'll be using my bromoethane for making grignard reagents.
Now this process works great for volatile alkyl halides that can be directly distilled out.
But what do you do when we need an alkyl halide that boils at a higher temperature than water?
We can't simply distill out the alkyl halide in that case.
And what about the situation where we don't have hydrobromic acid but instead just have sodium bromide.
Can we avoid having to go through the time consuming process of making hydrobromic acid?
I'm going to address both issues by making bromohexane.
First we get 150g of sodium bromide.
This will be our bromide source rather than hydrobromic acid.
Then we add in 150mL of water and stir until most of it is dissolved.
Depending on temperature it may not completely dissolve but this is acceptable.
Now we add in 102g of 1-hexanol.
This is the alcohol we'll be converting into an alkyl halide.
To add our sulfuric acid we set up a pressure equalized addition funnel filled with 100mL of concentrated sulfuric acid.
Mine is brown because i'm using low grade drain cleaner acid.
We slowly drip in the sulfuric acid careful not to boil the mixture as it heats up.
Once it's all in, we replace the addition funnel with a reflux condenser.
With the cooling water turned on we turn on heating and stirring and gently reflux the mixture for six hours.
What's happening is first the sodium bromide reacts with the sulfuric acid to produce hydrobromic acid and sodium bisulfate.
Then the mechanism proceeds similarly to bromoethane.
The alcohol group on the hexanol is protonated by the acidic conditions and bromide comes in and attacks the carbon, forming water and bromohexane.
In general, using hydrobromic acid seems to be higher yielding than using bromide salts and sulfuric acid.
So chemists prefer to use hydrobromic acid based processes if hydrobromic acid is available.
Now you might be wondering why i'm refluxing and not distilling.
Unlike bromoethane, bromohexane has a high boiling point, similar to that of hexanol,
so we can't just distill it out or we'd lose our hexanol as well.
It's therefore preferable to just reflux everything and let the reaction go to completion.
Anyway, once the six hours are over, we stop heating and let the mixture cool.
Now using a separatory funnel we separate out our bromohexane layer.
Even though it's denser than water it actually floats in this mixture because the brominating mixture is full of salts,
hydrobromic acid and sulfuric acid, so it's actually denser than bromohexane.
Therefore, keep just the top layer.
And there it is, crude bromohexane.
Now our bromohexane is somewhat impure since it wasn't automatically distilled like in bromoethane, so we'll need to distill it now.
Set up a distillation apparatus.
Turn on the cooling water and start heating.
You may need to enclose the boiling flask in an aluminum foil shroud to keep the heat in.
What comes off first is an azeotrope of bromohexane as well as water and hydrobromic acids.
It'll actually phase separate when it cools so you can save this and separate off the aqueous products for increased yield.
Now bromohexane boils at 154 celsius so when the distillate temperature reaches 150 celsius change out the receiver and collect the bromohexane.
And there it is, bromohexane.
Similar to bromoethane we wash it with an equal volume of water, then with 5% sodium bicarbonate solution and then with water again.
This time the bromohexane will be heavier than our water or sodium bicarbonate solutions
so remember to keep the bottom layer this time rather than the top.
All too often i've blundered and discarded the wrong layer.
Anyway, our bromohexane is relatively pure but with water impurities.
Since i intend to use this for grignard reactions i'll have to remove the extra water.
We can use magnesium sulfate like with bromoethane.
But bromohexane has such a high boiling point we can also use fractional distillation.
You don't actually need to distill everything.
You just need to distill all the volatiles below 150 Celsius.
As soon as we hit 154 celsius we stop heating and let the bromohexane cool.
Since we already distilled this before, anything above 150 celsius will be relatively pure bromohexane.
And there it is 1-bromohexane.
Our yield was 155g or about 93%.
And that is how we make bromoethane and bromohexane from their corresponding alcohols.
I'll be using these in upcoming videos to make grignard reagents.
Thanks for watching.
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