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Author: Subject: Orange liquid from Grignard reaction: what is it?
Stibnut
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[*] posted on 13-12-2016 at 12:47
Orange liquid from Grignard reaction: what is it?


I got a strange result from my second attempt at a Grignard reaction. I used bromopropane and butyraldehyde in an attempt to make 4-heptanol. It seemed to go well, but what I ended up getting was an orange liquid that smells like a mixture of an alcohol and one of the banana-smelling esters. The orange stuff is not bromine: I took out a mL of it and added it to several mL of concentrated sodium thiosulfate solution, and nothing happened. Does anyone have any good guesses for what I ended up with, and what tests might I use to figure it out?

Here's what I did in this experiment:

I oven-dried all glassware and magnesium turnings for a couple of hours at 120 C. THF was used as the solvent, and it had been drying over molecular sieves for several weeks. I did a quick KI/acetic acid test for peroxides and found none. Molecular sieves were also used to dry the bromopropane and butyraldehyde, although they had only been drying for a day each.

On a two-necked flask I placed a Liebig condenser topped with a drying tube containing CaCl2. The condenser was chilled with room-temperature water. I added 5.5 g (0.226 mol) of Mg turnings. Then I placed an addition funnel on the other neck and added 50 mL of THF. I then added 27.8 g (20.5 mL, 0.226 mol) bromopropane and waited. The reaction initiated in less than 10 minutes, quickly enough to surprise me, and got hot enough to start boiling the THF until I cooled it down with a cup of ice water on the bottom of the flask. Eventually it cooled down enough to put aside the ice water, but I waited about 90 minutes to be sure all the Grignard had been made before starting with the butyraldehyde.

Through the addition funnel, I added 17.2 g of butyraldehyde (21.5 mL, 0.238 mol) slowly in small portions over the course of the next hour. The reaction was predictably vigorous and exothermic. It was late, so I left it overnight. The next morning I discovered I was out of distilled water, but settled for tap water and used 20 mL of it to start quenching the reaction. Then I added a total of 25 mL of 31.45% HCl mixed with 25 mL of ice-cold water slowly over the next couple of hours.

I then gravity-filtered the solution and poured it into a separatory funnel. Two layers formed, and the organic layer was yellowish-orange. I drained off the aqueous layer and then washed the organic layer with water containing a mix of sodium carbonate and sodium thiosulfate, shook, and drained the aqueous layer off again. I placed in a bottle with MgSO4 to dry it for several hours.

Then I set up for simple distillation, using a boiling water bath as a heat source. 4-heptanol boils at 156 C while THF, bromopropane, and butyraldehyde all boil at 66-72 C. I distilled off all the stuff with bp < 100 C for a couple of hours, leaving my product, which turned out to be orange and smell partly like an ester and partly like an alcohol.

Any pointers on what this might be, what might have gone wrong, and what tests I can do to figure out what it is would be greatly appreciated!



[Edited on 13-12-2016 by Stibnut]
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[*] posted on 13-12-2016 at 13:00


Is there some reason you did not distill off your product? If you do you might leave some of the garbage behind. Also, you will see if you get the right boiling point.



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[*] posted on 13-12-2016 at 13:05


Well, it's certainly possible that most of the orange stuff is your product.
I'm guessing that you have mainly the right stuff and a small amount of very darkly coloured impurity.
When I was a student we used to joke about the fact that all the syntheses we did produced YellowCrapEne as a by-product.
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[*] posted on 13-12-2016 at 13:34


Doesn't seem to mater what the OC synth is, there's a pretty high chance of getting something Orange that forms IMHO, or some brown tarry stuff.

Here's some Orange i got from a benzene synth.

crudite.JPG - 64kB

I think it's called 1,3,5 benzocrudite, or cis-shitrate.

Early 19th century chemists called it <i>damngunk</i> possibly derived from the German term 'votisit'.




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[*] posted on 13-12-2016 at 13:37


I don't have a heating mantle and didn't want to use an open flame to distill something flammable. I do have a hotplate though - the plate itself reaches temperatures of ~250 C. I'll see if it can heat an sand bath to the 156 C I would need. Alternately, I guess the stove would work.

I just weighed a volumetric flask of it and got 0.894 g/mL; the accepted value for 4-heptanol is 0.815. So there's definitely some dense thing in there.

About colored crud, generally speaking: what is it? What sorts of side reactions produce crud? Does anyone know what the makeup of different varieties of YellowCrapEne is? ;)
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[*] posted on 13-12-2016 at 15:38


You mention that your mixture has an fruity ester odor. Taking this as qualitative evidence for esters, you may have formed propyl propionate, propyl butyrate, n-butyl butyrate, or n-butyl propionate. The odors are described as follows:

"distinctly fruity odor which may be described as a chemically tinged pineapple."
https://en.wikipedia.org/wiki/Propyl_propanoate

(propyl butyrate) "fruity sweet apricot pineapple rancid sweaty"
http://www.thegoodscentscompany.com/data/rw1009231.html

"similar to that of pineapple. It occurs naturally in many kinds of fruit including apple, banana, berries, pear, plum, and strawberry."
https://en.wikipedia.org/wiki/Butyl_butyrate

(couldn't find anything on n-butyl propionate's odor)

So how the hell do you get esters from your reaction mixture? It sounds as if you successfully formed the Grignard reagent based on the vigorous reaction of Mg with bromopropane. So one culprit is atmospheric CO2 which may have reacted with your Grignard reagent. This makes esters of propanoic acid the suspect, particularly propyl propionate.

Aldehydes oxidize to acids in air. So depending on how your butyraldehyde was stored and how long you've had it, it may have partially converted to butyric acid. Also, storing it over molecular sieves (which are acidic) may have led to some aldol condensation products. Try distilling it before use.




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[*] posted on 13-12-2016 at 16:23


Quote: Originally posted by DDTea  

So how the hell do you get esters from your reaction mixture? It sounds as if you successfully formed the Grignard reagent based on the vigorous reaction of Mg with bromopropane. So one culprit is atmospheric CO2 which may have reacted with your Grignard reagent. This makes esters of propanoic acid the suspect, particularly propyl propionate.


Good observation, but wouldn't the esters be of butanoic acid? Adding carbon dioxide to n-propylmagnesium bromide would produce butanoic acid.




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[*] posted on 13-12-2016 at 18:24


Quote: Originally posted by Metacelsus  


Good observation, but wouldn't the esters be of butanoic acid? Adding carbon dioxide to n-propylmagnesium bromide would produce butanoic acid.


Derp. Brain fart.

You're absolutely right: if esters are present, they should be esters of butanoic acid.




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[*] posted on 13-12-2016 at 20:29


Two sources of side products are possible after reading the details:

1. You did not apparently distill your THF. In my experience, sieve dried THF which has at some point contained peroxides contains some high molecular weight materials which never seem to go away in normal reaction work ups. THF with peroxides will eventually become peroxide-free if stored long enough over sieves.

2. Your initial Grignard formation got pretty hot. THF is not inert to organometallic reagents under vigorous reaction conditions. The ring can opened and side reactions occur. This is documented but I have not got the references immediately at hand.

Repeat the reaction with distilled THF and generate the Grignard under more carefully controlled conditions. It looks as if you added the bromopropane in one shot - not good. Let the reaction commence with a small amount of bromopropane then add the remainder dropwise to maintain a reasonable temperature. It's all about technique.

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[*] posted on 14-12-2016 at 13:16


I used a sand bath to distill it past 100 C. At first I used my hotplate, but even at max heat and with aluminum foil covering the apparatus, it couldn't seem to get past 100 C even after a couple of hours. A few drops of something clear with an etherial odor came over; not sure if this is THF or bromopropane that failed to distill off or what it is. I then used the stove, which worked much better.

A faintly yellow liquid boiled at 137-141 C. It does have the rather unpleasant combination of fruity/rancid/sweaty that propyl butyrate is described as having, and the bp of propyl butyrate is 142 C. The thermometer I used was a cheap alcohol one, and it might have been reading low. There are two possible ways I could have gotten butyric acid into the reaction - oxidation of butyraldehyde and addition of CO2 to propylmagnesium bromide, and the bromopropane would provide the propyl group, so if for some reason esters were forming, that would be the one I'd most expect to see.

Another yellow liquid came over at about 160 C. My thermometer only read to 150 C, so this could be off. The bp of 4-heptanol is 156 C, so this may have been my product. However, after at least half an hour of sluggish dripping, the temperature rose while I wasn't paying attention, broke the bulb off my thermometer, and yellow liquid started dripping more vigorously. Presumably this is something else with a bp > 170 C, which contaminated my product when it distilled over. I only got maybe 4 mL in total anyway, so I'm not going to bother distilling again to separate the two. The mixture smells like a combination of an alcohol and sweaty socks left to fester for a long time.

The remaining stuff in the flask is dark brown and looks like medium-viscosity crude oil. It still smells like an ester, and it's much more pleasant-smelling than the smell of the contaminant in the fraction that should contain the product. Not sure what it is.

I saved all three fractions and the leftover pleasant-smelling sludge. Given that I only have a few mL of each, what simple tests could I perform to get some idea as to their composition? I think I will try the KMnO4 test to see if there are non-tertiary alcohols or anything else that can be oxidized. Are there any tests for esters? I didn't seem to find anything when I Googled it.

Also, what causes things that are supposed to be clear to come over yellow instead? There must be something that absorbs blue light, but my impression is that it takes a pretty high degree of delocalization for an organic compound to do that.

Thanks for the help everyone! When I get a chance, I will try another Grignard using acetone instead of butyraldehyde, since 2-methyl-2-pentanol has a bp of only 122 C and my remaining cheap thermometers can actually handle that. I'll distill the acetone and THF beforehand, and add the bromopropane dropwise this time.

[Edited on 14-12-2016 by Stibnut]
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[*] posted on 14-12-2016 at 16:08


Quote: Originally posted by Stibnut  


Another yellow liquid came over at about 160 C. My thermometer only read to 150 C, so this could be off. The bp of 4-heptanol is 156 C, so this may have been my product. However, after at least half an hour of sluggish dripping, the temperature rose while I wasn't paying attention, broke the bulb off my thermometer, and yellow liquid started dripping more vigorously. Presumably this is something else with a bp > 170 C, which contaminated my product when it distilled over. I only got maybe 4 mL in total anyway, so I'm not going to bother distilling again to separate the two. The mixture smells like a combination of an alcohol and sweaty socks left to fester for a long time.


Based on the smell and boiling point, I believe the odorous fraction is butyric acid (bp 163.5 C).

https://en.m.wikipedia.org/wiki/Butyric_acid




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[*] posted on 14-12-2016 at 18:29


Quote: Originally posted by DDTea  
Quote: Originally posted by Stibnut  


Another yellow liquid came over at about 160 C. My thermometer only read to 150 C, so this could be off. The bp of 4-heptanol is 156 C, so this may have been my product. However, after at least half an hour of sluggish dripping, the temperature rose while I wasn't paying attention, broke the bulb off my thermometer, and yellow liquid started dripping more vigorously. Presumably this is something else with a bp > 170 C, which contaminated my product when it distilled over. I only got maybe 4 mL in total anyway, so I'm not going to bother distilling again to separate the two. The mixture smells like a combination of an alcohol and sweaty socks left to fester for a long time.


Based on the smell and boiling point, I believe the odorous fraction is butyric acid (bp 163.5 C).

https://en.m.wikipedia.org/wiki/Butyric_acid


I just did the KMnO4 test, and without seeing your post I happened to use butyraldehyde as my positive control. Butyraldehyde itself is stinky, but after it was oxidized by the KMnO4, I could detect a sharp extra-bad smell that matches the smell of my third fraction, minus the alcoholic part of the odor. So I think that's right: the third fraction is likely a mix of 4-heptanol and butyric acid.

The KMnO4 test on the three fractions plus the sludge, using MEK as the solvent, were as follows. All of them reacted eventually, but the second and third fractions reacted quickly while the first fraction and the sludge were both fairly slow (several minutes). I'm tentatively willing to say that I made a little 4-heptanol, which got mixed with propyl butyrate (?) in the second fraction and butyric acid in the third fraction. No idea what the first fraction (100 C for a couple of hours) or the components of the sludge are, but both seem to have a low but nonzero concentration of things that are oxidizable by KMnO4.
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