Triangle Reactions
Triangle reactions
There are two types of reactions that I call “triangle reactions”. It is important to have a general understanding of them before we encounter them in the remainder of this chapter. An analogy helps explain how triangle reactions work. This analogy is the story of a victim and two criminals. This analogy is a little bizarre, but I think it helps us remember how these reactions happen, so bear with me. The victim of the crime is somebody who is totally honest. He is so honest; he cannot help but announce to the world the amount of money he has by carrying a sign above his head. The victim has two options available to him. One is to hold a sign that reads “O” indicating that he has zero money on him. The other option is a sign that reads “+” indicating that he is carrying a positive amount of money on him. The victim sometimes has bags slung over his shoulders. In order to protect any money he holds, the victim puts his money in the pocket that is hidden under the most bags.
The victim of a Triangle Reaction
One of the two criminals is a weak, scared, but greedy pickpocket. This weak pickpocket will not attack a victim unless he knows the victim is definitely carrying money. If he doesn’t know this, he will leave the victim alone. So, if the victim is carrying the “O” sign, the weak criminal leaves him alone. If the victim is carrying a “+” sign, the pickpocket will reach into his pocket under the bags, where the money is kept.
The second criminal is a strong, aggressive, violent criminal. The strong criminal doesn’t really care about money. He is so violent; he wants to punch the victim in the gut immediately. This aggressive criminal will attack any type of victim. If the strong criminal encounters the victim carrying the “O” sign, he punches the victim immediately in the gut. The strong criminal wants to do as much damage as quickly as he can to the victim, so he punches him in the side that has the fewest bags on it.
How criminals attack in a triangle reaction
If we have a “triangle reaction” the top atom is usually either neutral or it has a positive charge. If the top atom is neutral, it is like a victim that is holding an “O” sign. If the top atom has a positive charge, it is like a victim that is holding a “+” sign. The criminals in the analogy are either strong nucleophiles (the strong criminal) or weak nucleophiles (the weak criminal). Strong nucleophiles tend to attack the least crowded side of the triangle because it is easiest to get to. Weak nucleophiles tend to attack the side of the triangle with the most alkyl groups on it. The bags the victim is carrying are “R” groups or alkyl groups. If the top atom has a positive charge, the positive charge is shared somewhat on the other two atoms of the triangle. The atom of the triangle with the most R groups attached holds more of the partially positive charge. This is because more substituted carbon atoms are more stable carbocations. That more substituted carbon atom of the triangle has enough positive charge (+ money) that even a weak nucleophile (weak criminal) cannot resist and will attack it. Below is an example of a neutral epoxide on the left and a positive, protonated epoxide on the right.
How strong or weak nucleophiles attack epoxides
Example of a strong nucleophile attacking a neutral epoxide
Example of a weak nucleophile attacking a protonated epoxide