Post Tagged with: "Organic Lab Demonstrations"

Chemistry Lab Demonstrations: Candy Chromatography

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Last lab of the semester today. Next week is the lab final and checkout. This week the students practiced column chromatography. They purified their crude product mixture from last week’s nitration lab. I’ve talked about the theory behind column chromatography before, so I won’t rehash it here in any detail. Suffice it to say that different organic compounds have differing affinities for a stationary phase versus a mobile phase. These differing affinities allow for one compound of interest to be separated from a mixture through the use of column chromatography. Students were aided this week in that their product was bright yellow. They could physically watch it run down the column, then only collect the yellow fractions.

Last lab of the semester means last demo of the semester.  This one’s a do-it-yourself demo, if you’d like.  You can separate the colors contained in M&M shells (or Skittles, or Reese’s Pieces, or Sharpies, etc) through chromatography.  I got my M&M proceedure here.  If you’re interested, other proceedures are available here, and here.   Basic rundown: put drops of water on wax paper, and put a piece of candy on each drop.  Allow for the water to strip the color off the colorful candy shell.  Cut a coffee filter into a rectangle.  Use a toothpick to spot each color onto the coffee filter.  Put the coffee filter into a 1% solution of table salt and allow the water to rise through the coffee filter.  Watch the colors separate like magic!

Couple’a observations I noticed.  Quite interestingly… the stationary phase matters.  A lot.  I started by spotting the colors on my silica gel TLC plates .  I was quite disappointed because the red and yellow both travelled with the solvent front and there was little separation.  I tried several different solvents… no luck.  I also noticed that according to the websites I was looking at, red should have travel the shortest distance.  Then I switched over to filter paper, and all of a sudden I got the results I was expecting.  Who knew?  Also, you should put a crease in the coffee filter before placing it in the solvent.  The paper will start to buckle and it will droop and fall over if it is not creased first.  The more distance you give the colors to separate, the better the results.  I used the largest filter paper we had, and ran the chromatograph twice to get the results shown.

Pop quiz, hot shot: Do you know what the difference between Red 40 and Red 40 Lake are?  I didn’t either.  Turns out… nothing.  At least, not as far as the compound responsible for the hue is concerned.  It’s all in the formulation:

Color additives are available for use in food as either “dyes” or “lakes”.

Dyes dissolve in water, but are not soluble in oil. Dyes are manufactured as powders, granules, liquids or other special purpose forms. They can be used in beverages, dry mixes, baked goods, confections, dairy products, pet foods and a variety of other products. Dyes also have side effects which lakes do not, including the fact that large amounts of dyes ingested can color stools.

Lakes are the combination of dyes and insoluble material. Lakes tint by dispersion. Lakes are not oil soluble, but are oil dispersible. Lakes are more stable than dyes and are ideal for coloring products containing fats and oils or items lacking sufficient moisture to dissolve dyes. Typical uses include coated tablets, cake and donut mixes, hard candies and chewing gums, lipsticks, soaps, shampoos, talc, etc.

There are 5 food coloring agents in M&Ms: Red 40, Yellow 5, Yellow 6, Blue 1, and Blue 2.  As you might expect, green separates into blue and yellow, but surprising the red and yellow of the orange M&M do not separate.  Rather, there is one orange spot with a larger Rf than red.  Brown separates to blue, red and orange.   But it looks like the blue in the blue M&M is a different blue than the blue in the green and brown M&M.

I’ve got lots of pictures from my experience (click for larger).  Note how poorly silica works and how different the Rf’s are between silica and filter paper.  the video is of separating components of felt tip pens, but it’s also neat.

There are no more demos planned, since the lab course is over.  Hope you enjoyed my miniseries.

silicaspotssilicaplatebeforefilterspotssilicarunningsilicaeluentcomparefilterrunning1

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filtersilicacompare

By April 10, 2009 17 comments fun

Chemistry Lab Demonstrations: Silver Nitrate/Copper Wire

nitration

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This week in lab, students performed electrophilic aromatic substitution (and here).  Dissolution of 4-methylacetanilide in 70% nitric acid gives mono nitration.  There are two possible products.  The acetamide is a better ortho/para director than the methyl group, so 2-nitro-4-methylacetanilide is the major product of the reaction.  The reaction was a bit touchy.  In my lab, for the most part the reactions were carried out at room temperature.  This resulted in the reaction not occuring!  Nearly every student got back unreacted starting material, instead of product.  The product is supposed to be a bright, crayon yellow solid.  Other labs ran the reaction on low heat and got excellent results…  But some students left the reaction on the heat too long or at too high of a temperature and the reaction decomposed into this ugly brown oil.  So there is a very small window for success in this reaction.  For the first time this semester (!) students analyzed the reaction mixture by Thin Layer Chromatography.

A brief safety warning.  Nitric acid is a very strong oxidizer.  It reacts explosively with readiliy-oxidizable small organic molecules like alcohols and acetone… acetone being of course what all good lab students rinse their glassware with before they start lab.  There was an explosion in our department last year as a result of improperly mixing nitric acid waste and acetone waste.  Nitric acid MSDS here, incident report on nitric acid explosion (not from our department) here (it’s probably worth glancing through the other incidents on that page as well), and video showing gas evolution from nitric acid oxidation (this time of copper) here – note how much gas is produced in a short amount of time.  Imagine this in a closed container.  Keep watching the video till the end, as it actually makes for a neat demo in itself.

The demo for the week was the silver nitrate/copper wire demo.  Silver nitrate (nitrate being the conjugate base of nitric acid… which is how I’m relating this demo to lab this week…) dissolves readily in water to give a solution of silver nitrate.  Just about everything silver nitrate touches gets stained black.  Not immediately… only after exposure to light.  For this reason, silver nitrate used to be used in early photography.  No stains for me, though it is always a concern.

Dropping copper wire into the silver nitrate solution initiates a redox reaction between the silver ion and copper metal.  The silver is reduced to elemental silver and the copper is oxidized to copper(II):

Cu(0) + 2AgNO3 = Cu(NO3)2 + 2Ag(0)

The silver crystallizes at the surface of the copper and the copper wire quickly becomes coated with a bunch of elemental silver.  At the same time, the copper ions go into solution and the colorless solution turns a characteristic blue as the concentration of copper ions builds.  It’s a pretty dramatic demo of a neat redox reaction.  I was testing the speed of the reaction before I went to lab (silver started to become visible within 30 seconds to a minute), and I ended up leaving the copper wire in the silver nitrate while I went to lab.  I came back several hours later and the crystals had plenty of time to grow and were very nice looking.  I let it go overnight to see how big the crystals would get.  I took a picture when I got to lab the next morning (click for larger).  And then I collected the silver in a scintillation vial to take home with me.  The pictures are from my experience, Noel posted a picture a while back as well, and there is a nice video below.

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By April 3, 2009 8 comments fun

Chemisry Lab Demonstrations: Nylon Rope Trick

amide

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This week’s lab involved taking the unknown carboxylic acid from experiments 1 (acid/base extraction) and 2 (purification by recrystallization) and converting it to a primary amide. The acid was refluxed in neat thionyl chloride and added dropwise to 30% ammonium hydroxide. Extract with dichloromethane and evaporate.  The fumes from this lab are pretty nasty.  The reagents are bad enough – the lab smelled like ammonia all day – but sulfur dioxide and HCl gas are liberated during the reaction.  The “fume hoods” in the ugrad labs aren’t so hot, so the students attach a long-stem funnel (where do I sign up to be a glassware photographer?!) to the water aspirator and invert the funnel over the reaction mixture as it refluxes.  This collects the fumes as an extra “mini fume hood” during the reaction.

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By March 27, 2009 10 comments fun

Chemistry Lab Demonstrations: Homemade Breathylizers…Sorta

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Undergrads were on spring break last week, so no lab last week.  This week, we performed the Jones oxidation of isoborneol to camphor.  The crude product was put in the bottom of a Hirsch funnel with a cold finger, and the product was purified by sublimation.  One of my favorite chemistry trivia facts is that the opposite of sublimation is deposition.  Now you know.

jones

I must say, I am really, really disappointed with the way this week’s demo turned out.  Plan A was following a patent prep to immobilize Jones reagent on silica gel.  That actually worked really well.  No problems there.  I had a nice orange granular solid.  The next step was to pack it into 5″ pipets between glass wool and plain silica gel to create a tall, narrow column of Jones reagent.  That also worked really well.  I meant to take a picture of the setup before I dumped everything, but I was pretty down after it didn’t work and just poured it all into my chromium waste bottle.  Sorry.

Anyway, the plan was to ask if anyone came to lab drunk that day (no one admitted they did), then have someone volunteer to swish around some Listerine for a while.  Then attach a clean drinking straw to the pipet and blow.  See, Listerine is 21% alcohol, so there’d still be some ethanol vapor in the breath which should flow past the Jones reagent.  The Jones reagent will oxidize the ethanol to acetic acid and will itself be reduced.  The reduced chromium reagent is greenish brown.  With the pipet system, the orange color would slowly change over to green from the start to the finish.  The amount of ethanol in a person’s breath (and therefore the BAC) can be determined by seeing how far up the column the green color extends.  The more solid that is reduced to green, the more wasted the person.

I know there are several potential safety issues with this setup.  Chromium is toxic and shouldn’t be ingested or released into the environment.  The goal was to have only me handle the glass and only the student with clean hands touch the drinking straw – which was not ot be unwrapped until just before use.  When I was testing this, I don’t know what was going on, but I could get no air pressure through the pipet.  With breath not flowing through the pipet, there’s no chance of the Jones reagent being reduced, so no demo.

See, they really did used to use Jones reagent in breathalyzers.  They don’t anymore because chromium is toxic, and well, lots of things can reduce chromium… not just ethanol.  This leads to false positives.   Now they use a fuel cell for better results.  The best part of the Jones reagent story is that drunk people would blow through a solution of Jones reagent, which would reduce some of the chromium.  A UV/Vis detector could measure the absorption of the solution before and after the test.  The absorption is related to the amount of ethanol because the measured absorption (A) is equal to the product of the concentration of chromium (c), the path length of the cell (l), and a constant unique to chromium (ε).  A =εcl.  This relationship, ironically enough, is known as Beer’s law.  That’s right.  Beer’s law can be used to tell how drunk a person is.

Plan B was to have a row of test tubes with a solution of Jones reagent to which was added increasing amounts of ethanol.  This would gradually change the color from yellow to greenish.  A separate test tube with a solution of Jones reagent in lab would have an arbitrary amount of ethanol added to it.  It could be titrated against the standards to show the color change and determine the level of drunkenness.  Problem is the color change is very subtle in dilute solutions.  Too subtle to really see.  No demo again.

I won’t bore you with plan C, but suffice it to say – fail.

bromocresolgreen

So finally, plan D.  Same as plan B, but with a surrogate for Jones reagent… and a surrogate for ethanol.  Basically none of the actual reagents, but would still give a color change.  I went with acid/base chemistry.  A row of test tubes was filled with an acidic solution of bromocresol green.  The acidic solution is yellow.  To each tube was added an increasing amount of sodium hydroxide so the color would gradually change to a deep blue.  The first test tube was yellow, then gradually orange, then muddy brown, then blue.  In fact, the series hit just about every color EXCEPT green (the image looks better.  Mine was not nearly so nice.  I was in a hurry).   Anyway, then an arbitrary amount of NaOH was added to a separate tube with an acidic solution of bromocresol green and titrated against the standard.

It wasn’t a great demo, but it looked like it was supposed to look.

By March 19, 2009 2 comments fun, synthetic chemistry