Condensation: It's Not Just For Soda Anymore, Part 1
Condensation reactions incidentially have jack all to do with actual condensation (that happens because the cold temp of your drink causes the gaseous water in the air to cool down when it hits the surface and become liquid. So the next time someone tells you to put a coaster under a room temp drink, laugh at them.). Condensation reactions are called condensation reactions because they managed to squeeze out a water molecule during the reaction. You may think that it's the same thing as dehydration, and I suppose technically it is, but the reactions are much more complicated. These reactions are actually a combination of Nu Add'n and alpha substitution. I'd review those, but you can go back and check the previous entries for them. I'm getting better at tagging, BTW. :) At the moment though there's only like 11 entries, so it shouldn't be too much of a challenge to go look backwards.
Note: The order of carbons goes like this: carbonyl --> Alpha --> Beta. It's really important to keep this in mind for this chapter.
In short, to quote my teacher directly: "There is Nu Add'n of the enolate ion of the donor molecule to the carbonyl C of the acceptor molecule".
Man, that's fancy sounding. My teacher is a brilliant and kind women who cares about us and our learning, but one of her few weak spots is that she doesn't often relate these complex concepts in simple language. She calls a thing what it is, which makes sense, but doesn't always help you really understand.
Maybe if she learned Tumblr speak and used more gifs.
Kidding (ok not really.). ;)
Alright, so in my last massive post we covered enolate ions with their jumpy bonds, right? Yeah. That part of the molecule ("the donor") attaches to the C that's next to the O of the carbonyl group (the "carbonyl carbon" of the "acceptor" molecule.). So let's get to it, shall we?
1. Aldol reaction to produce beta-hydroxy alds and kets - If you'll remember, hydroxy is alcohol (ROH's substituate name is "hydroxyl".) So essentially what this means is that we'll be sticking an alcohol to the carbon that's in a beta position relative to the aldehyde or ketone (completely unrelated: I just googled ketones and found raspberry ketones. What a load of horse shit. Shame on you Dr. Oz, you should KNOW BETTER. Proof he's only after the money. I hate TV doctors.). The mechanism here is completely and utterly different from the grignard, and I do want to stress that because you won't see that in this reaction, but the result of it is very similar. If you, like me, do not have to memorize mechanisms for your test then it is probably safe to at least categorize them together in your head. The reagents are Small, catalytic amounts of NaOH, EtOH. If you over saturate with base, the reaction doesn't work. You need just enough to get the hydrogens off of the carbons. If you have more, it starts reacting with the enolate ions and you don't want that to happen.
Now, I am not gonna lie. I had to stare at this for awhile to understand what it's saying. Then I went to the bathroom and for some reason the change of scenery or something made it click in my head: for some reason I thought we were reacting beta carbons, and we're not, we're *creating* a hydroxy group ON the beta carbon. Honestly, the base way to explain it is with an example:
MS PAINT SKILLZ ACTIVATE LOOOOOOOOOLZ
Here we have two ethaldehyde (otherwise known as acetaldehyde, but I don't like this name because this is NOT ACETONE.) molecules combining like Voltron (<--showing my age.) to form a new molecule, and an alcohol appearing at the site where the old ketone of the second molecules was. Important things to know about these reactions:
It's an equilibrium reaction (as you might have noticed in my drawing), and the equilibrium depends on how substituted the alpha carbon is. Mono favors the product, and di substitution of the alpha and carbonyl C favor reactants. That means this reaction works better with aldehydes.
The single difference between ending up with the aldol product you want instead of an alpha alkylated product is the amount of base. The reaction conditions are critical, and you need to be exact. You use a weak base at room temp, instead of a strong base at lower temps.
Alright, that's it for those reactions. On to the next! Afterall, there are only so many hours in the day and this is only reaction #1.
2. Dehydration of Aldols: synthesis of enones - What does that mean? Well, we'll be removing water from the aldol that we just made to produce an enone. A hint here: use your words. Aldol can be broken down to it's parts to understand what it is: ald - aldehyde. -ol - alcohol. An alcohol on an aldehyde. enone is the same way: en is short for -ene, which if you dredge that up from first semester is a C=C. -one is the name for a KetONE. A ketone with an alkene. Dehydration - DE, removal, hydra - water. Removal of water. So, just by the name you can guess that we're removing a water molecule from the aldol to create an enone. Since the only place to get the OH for the water is the alcohol, that's where the double bond ends up. The reagents are: H or OH/heat.. Meaning simply dump it in an acid or a base. So your teacher may decide to write NaOH over the arrow...it's the same thing. See my big list of reagents for a list of bases, and for acids. The double bond ends up on the left side of where the OH was not because of markovnikov, but because of the mechanism and the way that an enolate works.
Some notes about this reaction:
Usually this reaction happens in conjunction with the previous ones because of the difficulty involved in controlling the reaction conditions.
This is an equilibrium reaction, and the product can actually react with the water that is created from the reaction, so if you remove the water you will get more product
We did this in lab (practical, whatever you call it.), so you might as well. We have a microscale (small) lab, so let me show you the ridiculousness of this method:
Ok, so the green stuff is the product and the blue is water. So we removed the water by having the condensation generated by the heat slide down into the little teeny-tiny arm of that piece of glass piping (not what it's generally used for, FYI.), along with some product, We were supposed to decant what we could off the top back into the flask, but to get the rest of it we were supposed to manage to somehow magically remove that cork and ONLY LET THE WATER OUT. Cause that happens. Even tho we have those little valve things in our kit...there's got to be a better way. Luckily, I'm the boss at decanting so I got most of my product back. Good thing % yield isn't THAT important. x.x Ha...I should post more lab photos for you guys. :) Synthesis in second semester is way better than all the testing you do 1st semester. Prepare to get really friendly with IR testing though.
Ok, so there are still a TON of reactions in this chapter, so I'm going to post this now in an effort to control the length. Part 2 will follow in probably like an hour or two, depending on how long it takes me to write it. I'm at work and people (also the internet. My eyes are literally burning from my lazer-like focus.) keep interrupting me so we'll see.