Sciencemadness Discussion Board

Differentiating between amide and acid -- Case of an unknown material

Rich_Insane - 2-2-2013 at 15:49

So for a long time I've been unsuccessfully trying to isomerize oximes into amides using copper salt catalysts. I prepared approximately 10-15 grams of "copper acetate" by heating copper sulfate/NaOH to form the oxide, then reacting that with acetic anhydride/water to form the acetate. Now what I did was I added the oxime in xylene (not fully dissolved) and heated to 110oC with 7.5 mol % of the catalyst. Every 20 minutes, I added a small amount of water. At first I noticed some crystalline material at the bottom (aq. layer), but on the top layer, there was this thick green sludge (very strange, because copper salts aren't quite soluble in aromatic solvents, so I suspect some complexing with the oxime) that was extremely difficult to clean off. Anyways, long story short, I isolated the tarry product and attempted to extract the amide with toluene. But there was no success, so I stuck the flask in the freezer and forgot about it.

A couple of days ago I remembered this flask of waste and thought that maybe I could clean out the flask with boiling hot water and reuse the glassware. Upon boiling, I noticed that some crystalline material had risen to the aq. layer and when I took out a sample, the material formed dense needle-like crystals. Now in my excitement I remembered that amides hydrolyze in strong base upon addition of strong base or strong acid (with heat).

So I added the base (5 M NaOH) and indeed I got a shower of yellowish colored crystals. I knew that this wasn't enough to draw a conclusion, so I took another sample from the boiling water. After taking the sample, I waited for the mixture to cool down (I don't have any reagents to test for ammonia so I planned on heating the material with base and sniffing to detect the ammonia odor). To my surprise, I didn't even need to add a base to get these crystals; they fell right out of solution, some sticking to the side, some forming a layer on the bottom. Upon freezer-cooling followed by filtration by suction and drying, I recovered a small sample of a silky light green-white (almost paper-like) mat of needles that formed flakes.

My question is: How do I know if what I isolated is amide or carboxylic acid without using NMR or other instrument-based techniques? My sense of smell is rather weak unfortunately, especially since my little accident with SO2 gas (that stuff really does a number on your olfactory receptors!) so I'd rather not do the ammonia-sniff test.

The reference for this is attached.



[Edited on 3-2-2013 by Rich_Insane]

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weiming1998 - 3-2-2013 at 03:14

1, Add some of your isolated product to a concentrated solution of NaOH/KOH. Heat if necessary.
2, Place either a damp red litmus paper or some paper soaked in copper sulfate solution near the opening of the reaction vessel. Both of these show the presence of ammonia. The red litmus paper would turn blue and the light blue colour of copper sulfate would turn a deep blue/purple.

Rich_Insane - 3-2-2013 at 14:37

I have tons of Cu(SO4).5H2O! Thanks very much. I will soak a small filter paper in concentrated copper sulfate to prepare this! Will report back with results.

One question though: Will phenolphthalein turn pink in the presence of ammonia gas?

I know that aq. ammonia will turn the paper pink, but will the vapor turn the paper pink, perhaps when the paper is dampened slightly?

[Edited on 3-2-2013 by Rich_Insane]

Rich_Insane - 3-2-2013 at 16:45

Apologies for the double post -- I wanted to post my results separately.

Procedure: Copper sulfate pentahydrate was dissolved in cold water until no more dissolved. Filter paper was cut length-wise and immersed 90% into the solution. Note that it will most likely not dye blue (but there is copper sulfate on there). Meanwhile, a 2.5 M solution of sodium hydroxide was prepared and inserted into a small vial. The sample (about 50-100 mg) was placed in the vial and right away, a test strip was immersed approximately 1/4 the way up. A light blue-green layer of copper hydroxide should form promptly on the paper. Then a lighter was held under the vial until boiling occurred. After 10-30 seconds of heating, the flame was removed and the test strip was removed. On the opposite end of the test strip, a drop of 2.5 M NaOH was placed on the other end to replicate the color of copper hydroxide (as a negative control).

Results: There was a black colored precipitate on the surface of the paper, which made results difficult to interpret, but with this sample, there was most certainly a color change to a dark purple-blue color. The black precipitate is assumed to be copper oxide and mixed organic gunk. Interestingly enough, ammonia vapor was not sufficient enough to induce a color change and did not do so with phenolphthalein paper either. This is most likely due to the small sample size.

Questions for the pros: One side-product of this amidation reaction is the nitrile -- Is it possible for the nitrile to cause a false positive on this test (via decomposition to ammonia at ~100oC)?

Pictures:



The sample; Green coloration due to copper salts





Test strips; The dark colored side was the one immersed in the sample while the pale colored side was the negative control.

weiming1998 - 4-2-2013 at 06:44

Quote: Originally posted by Rich_Insane  
I have tons of Cu(SO4).5H2O! Thanks very much. I will soak a small filter paper in concentrated copper sulfate to prepare this! Will report back with results.

One question though: Will phenolphthalein turn pink in the presence of ammonia gas?

I know that aq. ammonia will turn the paper pink, but will the vapor turn the paper pink, perhaps when the paper is dampened slightly?

[Edited on 3-2-2013 by Rich_Insane]


Yes. Like red litmus, phenolphthalein will turn change colour in the presence of ammonia gas, as long as your test strip is damp, as the ammonia dissolves in the moisture and form hydroxide ions, which turns phenolphthalein pink.

The black colour on the test strip is most likely copper (II) oxide, which is formed by the decomposition of the dark blue copper ammonia complex. Its presence shows that ammonia (or at least an alkaline gas) is present.

Nitriles will give a false positive, unfortunately, as this reference states: http://www.chemguide.co.uk/organicprops/nitriles/hydrolysis....




[Edited on 4-2-2013 by weiming1998]

Rich_Insane - 4-2-2013 at 08:59

Is there any good way for me to differentiate between the nitrile and the amide? I don't have access to H-NMR, which the references used to differentiate between the two.

I know that this nitrile (if nitrile is present) is relatively hydrophobic and rather large (MW > 200 g/mol). Recrystallization from water yields white flakes constituted by many small needles and is difficult because the material is only soluble in very hot water and will readily precipitate out upon cooling to room temperature or even ~30oC below the BP of water.

mnick12 - 4-2-2013 at 10:22

The hydrolysis of nitriles in acidic or alkaline conditions produces amides before they are further hydrolyzed. Why not just do a mp?

Rich_Insane - 5-2-2013 at 10:57

Is there any data on the rate at which nitriles hydrolyze to amides and then to carboxylic acids occurs?

I'm curious as to whether this reaction occurs quickly (both for nitriles and amides.

From this site, I take that the conversion to the COOH occurs slowly (Nitrile Hydrolysis), but I'm not sure if the amide forms quickly or slowly.

I recrystallized the "amide," by the way. I plan to take a melting point shortly. I do not have an MP apparatus, so I plan to use an oil bath, a thermometer and a test tube (sadly, no capillary tubes are available to me).

Anyone tips on measuring the MP using an oil bath?

mnick12 - 5-2-2013 at 11:48

I am not sure about the rate of hydrolysis for the compounds in question, I am assuming you are talking about 3-(1,3-benzodioxol-5-yl)-2-methylpropanenitrile and 3-(1,3-benzodioxol-5-yl)-2-methylpropanamide.

As for melting point apparatuses, usually an oil bath a small test tube and a thermometer is good enough for imprecise measurements. I used a broken piece of pyrex tubing from a gas adapter and melted the bottom with a torch. This tubes ID was slightly larger that the OD of my thermometer so is made for a good little apparatus.

You may also want to try some TLC on the compound to get a rough idea of how pure your stuff is.

Rich_Insane - 5-2-2013 at 16:46

Do you recommend that I put the thermometer directly in the pile of densely packed crystals?

I do not have silica gel at the moment -- otherwise I'd certainly be making some of those nifty little TLC plates. So MP is probably the only good measurement of purity I can get.

Rich_Insane - 6-2-2013 at 21:48

Hey everyone: I've actually found a capillary tube! I'm not sure how to use the oil bath to heat it though. I know that I secure the tube to a thermometer and heat slowly within 10-20oC of the predicted MP, but should I place everything directly in the bath? I know that the predicted MP will be around 120oC so I must use an oil bath and not a water bath.

mnick12 - 6-2-2013 at 22:23

Crush up some of your sample, seal one end of your capillary with a torch. Then pack in about 3mm of your sample by "tapping" the crushed sample with the open end of the capillary. Invert the capillary and drop it down a glass tube or something similar. When the sample is dropped on the sealed end this causes the sample at the top of the tube to fall down. Once this is done seal off the other end of the capillary with a torch. Next attach your sample to a thermometer using a rubber band or piece of thin wire, and immerse into your oil bath. This is not going to give a super accurate reading, but it should give you an idea.

If that was hard to follow look on youtube, there are a lot quick videos pertaining to lab technique.

Rich_Insane - 7-2-2013 at 21:31

So I did the imprecise MP test, and I came up with a range of 110oC to 125oC. The material was mostly (50%) molten at 110-112oC and the last crystals melted at around 115-125oC.

Literature reports an MP of 122oC when recrystallized from benzene. Am I way off, or can freezing point depression from impurities indeed cause such a decrease in MP (10% difference from that of literature)?