I see this at a Wikipedia article:
Probably means nucleophile.
In the insert drawing, the Nu- is water acting as a nucleophile and beginning the polymerization of the electrophilic double bond of the cyanoacrylate. The lone pair on H2O's oxygen acts as the partially negative charged nucleophile. The new partial negative charge is on the carbon alpha to the stabilizing effect of the CN, and it acts as a new nucleophile to attack another molecule of cyanoacrylate, and so on, and so on... One catalytic Nu- can become amplified by the mass of thousands of the cyanoacrylate monomer units polymerized.
The article speaks of two instances where the reaction becomes strongly exothermic- One is with NaHCO3 a mild base that will increase the nucleophilicity of adsorbed moisture- the second is cotton which can also act as a nuclephile along with its tendency to have adsorbed moisture.
Other possible nucleophiles are ammonia, or amine based epoxy catalysts. the amino acids in skin, and in sweat and skin oils from fingerprints.
I guess that begs the question of what a nucleophile is. Wikipedia's description isn't too good.
Wiki spells it out in the beginning and the end of the article- the different nucleophilicity measures and equations aren't necessary for the concept. It does require a simple understanding of bond formation, and is narrowed to the covalent bonds formed with carbon in organic chemistry.
Nucleophiles are reactive species for organic chemistry where the reaction forms a stable covalent bond with the other reactant (the cyanoacrylate). If the Nucleophile brings in a negative charge in forming the new bond, the new intermediate also has a negative charge. In the diagram the arrows are used to show the direction of electron flow. Lines between the Nu and end C of the cyanoacrylate represent a covalent bond. This is the method that allows the cyanoacrylate to polymerize or "grow" by addition with new carbon carbon bonds linking the monomers (individual cyanoacrylate molecules that make up the" poly"-mer).
The concept of Lewis Acids and Bases may help you with the concept- The Nu- is a Lewis base and donates electrons to bond formation when reacted with a Lewis acid (an electron pair acceptor). The oxygen nucleophiles in the Wiki page represent the starter or reactant that starts the polymerization of cyanoacrylate. The cyanoacrylate double bond is polarized to accept electron pairs from Nu- due to presence of the CN as an electron withdrawing substituent on the double bond. The nitrogen nucleophiles are also represented in the Wiki and are what makes up your skin in contact with crazy glue!
OK, I think I'm beginning to understand. Looking at the Wikipedia article for Lewis acids & bases, it seems that a Lewis acid reacts with a regular covalently bonded molecule to produce a cation (+), while a Lewis base reacts with that to produce an anion (-) - so basically a Nu- ends up creating an anion, a Nu+ an cation.
You are getting it;
I don't want to throw in a wrinkle but for polymerizations and for simple reactions that form only one covalent bond, the intermediate finds something else to act as a electrophile than another monomer unit. This reaction is known as a termination step in polymerization and involves any Lewis acid like H2O for this reaction... The covalent reaction abstracts the H+ from H2O and leaves a new hydroxide (-OH). The intermediate is now satisfied to be a stable covalently bonded molecule and has been terminated. (note that you have a new Lewis base to start the whole thing over).... The point is that trace moisture does a good job of activating but not terminating the reaction.... Too much moisture terminates the reaction before too much polymer chain enlargement reactions have taken place, and there are no polymeric molecules to act as a "glue".
If you feel ready to understand organic chemical reactions a good book I'd recommend is "Pushing Electrons" by Daniel Weeks- I didn't really learn this until grad school! It is the 2nd year chem i.e. yet, it is "the org chem" key tool, IMHO.
OK, I think I have the understanding I am looking for. I'll take a look at that book.
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