Actually, Mr. Cool, you are correct about the pH being a necessity. The removal of the formed formic acid is trivial to the reaction, and proceeds merely by atomic opportunism, so to speak. The base is not optional, it is completely necessary. Ca(OH)2 is often used because it gives rise to the best results.
This reaction is known as the Tollen's Condensation, and this reaction is most classically known for producing our beloved product here: pentaerythitol. It involves two seperate mechanisms known as the Mixed Aldol Condensation, and the Crossed Cannizzaro Reaction as the last step. It really is a lovely sequence, and I can gladly illustrate the mechanics operating here if you like. I'll even draw some pretty pictures for everyone. :D
The reaction scheme looks like this on paper:
CH3CHO + 4HCHO --> C(CH2OH)4 + HCOOH
Due to the calcium hydroxide that facilitates this reaction, the actual products are:
2CH3CHO + 8HCHO + Ca(OH)2 --> 2C(CH2OH)4 + Ca(OOCH)2 + 2H2O
The first step that occurs is the Mixed Aldol Condensation, in a sort of runaway chain reaction that is often hard to stop, and so it happens three times. This is good, because that's exactly what we need to happen to make pentaerythitol.
First, in solution our Ca(OH)2 dissociates to Ca+2 and 2OH-. This basic environment is of 'medium strength' and is exactly what is needed. Acetaldehyde's alpha hydrogen extrudes a proton to neutralize the OH- in the environment, in the reaction OH- + H+ --> H2O. This reaction is reversible, and is based on an equilibrium factor that we are exploiting here.
You can see what's going on in this picture. The alpha proton neutralizes the hydroxide, leaving the alpha carbon with a negative charge. Due to electron resonance, we can see a proton shift back and forth called tautomerization. This is the enol isomer of acetaldehyde. The form on the right is more stable in very basic solutions, and its the one we DONT want here, and that is exactly why we don't use very strong bases, because then the equilibrium resides on the left, with the carbon bearing the negative charge and the carbonyl NOT being reduced to an alcohol.
<img src="http://www.methinfo.com/boards/general/binaries/18/18561.gif" alt=" - " />
So right now we got ourselves a (preferably) enolized acetaldehyde with an alpha carbon that keeps shuttling the negative charge back and forth between itself and the oxygen a zillion times a second. What happens next is this:
<img src="http://www.methinfo.com/boards/general/binaries/18/18563.gif" alt=" - " />
What the fuck is going on??? you say, right? Well, on the top left we got our formaldehyde. That oxygen is pretty electronegative, a lot more than the carbon at least, and polarizes the molecule to that the oxygen is slightly negative and the carbon is slightly positive. The positive carbon (carbocation) is attracted to the negative enol carbon (carbanion) of the acetaldehyde + base, and they couple. The oxygen on the formaldehyde retracts an electron from the carbon, giving the oxygen a negative charge, and the two carbons that are charged meet and are neutralized. The C-C bond is very tough. Once you slap it together, it ain't gonna come off so easily, so once it's made, there's no going back.
Now remember the last step of the previous picture, the hydroxide was neutralized to water, and I said this was reversible. This is where it gets reversed. The negative oxygen on the compound just made reacts with water, neutralizing it's charge to become an alcohol, and regenerating the hydroxide. You see now that the hydroxide is a catalyst, without it the reaction cannot occur. Stronger bases such as -OC2H5 (ethoxide) are often too strong, because they force the equilibrium far to the right with the original enol having the oxygen carry the charge, and this does not produce the right product, at least in this case. There are exceptions.
Now we have set the stage for the same reaction to occur again, two more times in fact. The same process occurs with the hydroxide abstracting a proton from the alpha carbon, same as before, and then the carbanion reacts with another molecule of formaldehyde. This alpha carbon has three alpha protons, so occordingly the reaction occurs three times, replacing all three to get our final compound in that picture, with the three methylol functions added to the structure and that blasted hydroxy that wont fucking go away, seemingly. Good for him, he's working hard for us.
Its really a good thing we WANT this to happen three times, because its actually hard to STOP the reaction. The only reason it stops at all is because the carbon only has three protons to lose. With each methylol function added, the next formed carbanion is that much more stable, meaning it is more likely to be present, and therefore more reactions occur and it is harder to stop at each stage.
Now the estute will realize that this nifty little compound is very very close to the pentaerythitol of our heart's desire, but is still not quite there. We still need to reduce that aldehyde to the final alcohol group. Now that our Mixed Aldol reaction is kaput, what happens?
The Crossed Cannizzaro Reaction. This one is really weird, but hey if it works dont bitch, right? Ok, stay with me, here.
Notice we've used up three of the four formaldehyde molecules in our chemical formula. We'll use one more here to reduce our aldehyde to the last alcohol, and oxidize this last formaldehyde to a formic acid. The mechanism of the Crossed Cannizzaro is a hydride ion shift, whereas in an Aldol Condensation, the mechanism involves a proton shift. A proton shift is a movement of H+, and a hydride shift is a movement of H-, a much more rare entity.
<img src="http://www.methinfo.com/boards/general/binaries/18/18567.gif" alt=" - " />
Alright, the first molecule on the top left is the last formaldehyde. That workhorse hydroxide ion is attracted to that carbon atom because the oxygen of the formaldehyde is pulling on the carbon's electrons, making the oxygen negatively polarized and the carbon positively polarized. The hydroxide attaches to the carbon, and the carbon yields one electron to the oxygen, making a negative oxygen ion. Pay attention to the hydrogen in the second picture of the formaldehyde molecule, the one I circled. Notice I circled the H and also the bond with it. The bonds are the electrons. This hydrogen doesnt leave the formaldehyde as a proton, it leaves as a hydride ion H- so that the negative oxygen atom can neutralize the charge to become a carbonyl group like it was before that irritating hydroxy butted in. So H- leaves the formaldehyde, and the double bond is restored. The product is a formic acid molecule.
In our "almost pentaerythitol" we still have that last carbonyl group (the aldehyde part, C=O) to make into an alcohol. Well, just like in formaldehyde, that oxygen is hoarding those damn carbon's electrons for itself, causing minor polarization. This hydride ion is VERY strongly attracted to this carbocation (positive carbon ion) and adds to it, and the oxygen of the aldehyde gets its electron, forming a familiar negative charge. At last, we can see the light at the end of the tunnel. This is an alkoxide ion. Remember I said earlier that alkoxides like ethoxide ions are VERY VERY strong bases. And bases react with acids; in the last step, the alkoxide steals a proton from the formic acid, becoming a neutral alcohol: pentaerythitol, at last! :)
Our newly made formate ion has replaced the original hydroxide ion from the calcium hydroxide, and pairs up with it to form calcium formate.
These are the mechanics behind our equation:
2CH3CHO(acetaldehyde) + 8HCHO(formaldehyde) + Ca(OH)2(calcium hydroxide) --> C(CH2OH)4(pentaerythitol) + Ca(OOCH)2(calcium formate)
So you see, base is not trivial at all, but the "neutralization of formic acid" is. It is simply a coincidence.
I certainly hope everyone enjoyed this post, and learned something. I tried to make it interesting and explain why it happens this way. I put nearly two freaking hours into this post, so I hope it was worth that to you guys.
I'm new here, and I look forward to working out lots of chemistry stuff for all kinds of spiffy explosives. :D
Feedback is appreciated. Cya, Im off to bed to dream about other chemical reactions and other nifty products I can tweak out of shit under my sink. Toodles! <img border="0" title="" alt="[Wink]" src="wink.gif" />
PrimoPyro
<small>[ August 06, 2002, 04:04 AM: Message edited by: PrimoPyro ]</small>