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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.