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Discussion in 'DAT Discussions' started by melmu001, May 15, 2014.
why is NaCN considered a weak base since its on the top of the periodic table?
That is one of the ones you just need to memorize. The other strong nucleophile/weak bases that are worth memorizing: NaCl, NaBr, NaI, and NaN3. There is also a poor nucleophile/strong base you should memorize: t-butyl ethoxide.
These don't follow the CARDIO rules, so just memorizing is easier than the discussing "why". I believe it has to do with pKb.
I just remembered the main strong bases: OH-, OR- (& NR3?)
If it wasn't a strong base, it was weak. (CN-, N3-, SH-, SR-)
Other possible strong bases include grignards, Na/NaI, but these are usually used for certain reactions.
it honestly depends on what type of reaction you're trying to run. Cyanide ions are great nucleophiles for Sn2 reactions. They're small, linear, and have no problem coming in and displacing an electrophile by backside attack.
A cyanide ion is also a good Bronsted base, b/c they have a pka of ~25. As long as a Bronsted acid has a lower pka than this, cyanide will quickly deprotonate it.
So in summary: A Cyanide ion - great nucleophile, and a decent base. There's a lot more to it (and it still doesn't answer your question) but this is as much as you need to know.
What am I supposed to do about regarding the knowledge of NaCN as a nucleophile/base? Just remember it off of my heart instead of understanding the periodic trend of it?
Actually, wow, I'm really sorry. I was talking about the acetylene ion. I confused acetylene for cyanide WOOPS!! My bad. Disregard everything I said haha.
The two people that answered before me - what they said is correct.
The cyanide anion has a pka ~ 9. But it is still a good nucleophile!! Just a bad Bronsted base. The periodic trend doesn't apply here. There are a bunch of nucleophiles/bases in ochem in which the periodic trend doesn't apply. Like I said, for cyanide, there's a lot more to it.
If you really want to know just message me. Just be aware that I'll have to mention MO theory. Or maybe you can just ask chad cause he'll probably do a way better job of explaining.