Nucleophile vs Base

[deleted483737]

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I know this has probably been asked 5 million times, but I've looked through the archives and haven't found a lot of useful information.

How do I distinguish between a nucleophile and a base to determine whether a reaction will proceed in an Sn2 vs E2 reaction?

Is there a list that I can memorize with strong nucleophiles and strong bases that are commonly used? It just seems to me like a lot of them are so similar. I was told that a negative charge can be used to indicate a nucleophile, but then ethoxide ion has a negative charge and is more likely to participate in E2 reactions because it's a strong base.

 

[MedWonk]

I know this has probably been asked 5 million times, but I've looked through the archives and haven't found a lot of useful information.

How do I distinguish between a nucleophile and a base to determine whether a reaction will proceed in an Sn2 vs E2 reaction?

Is there a list that I can memorize with strong nucleophiles and strong bases that are commonly used? It just seems to me like a lot of them are so similar. I was told that a negative charge can be used to indicate a nucleophile, but then ethoxide ion has a negative charge and is more likely to participate in E2 reactions because it's a strong base.

EtO can be used for Sn2, but you'll also have competition with E2 and you end up with multiple products. It's often not an either/or proposition. KOtBu is commonly used for E2 since its bulky and will not participate in Sn2. MeO, EtO, and I are very good nucleophiles. Acetylide and nitrile anions are also very good. Non-bulky carbanions are generally good nucleophiles. Amines are good. SH/OH are so-so. Br is also OK. Cl and F are crap as nucleophiles.

It's not a matter of distinguishing between a nucleophile and base, since nucleophiles usually are bases, and reactions will often compete. What you need to look for are steric hindrance (favors E2), antiperiplanar protons (required for E2), alpha/beta branching (favors E2), and the strength of the base (stronger bases, E2 dominates). Ideal nucleophiles that favor Sn2 are weak bases, whereas for E2, you want a strong base.

Someone fresher on the material can probably flesh this out better than I can, but that's the gist of it.

 

[LucasJason]

Everyone just quotes "if this reacts with this" and ya da ya da ya because this is what was told to them. A nucleophile in an equation attaches itself to a carbons nucleolus. It is the definition of lewis base; meaning it donates one of its electrons🙂) to form the bond to the carbon . A base in an equation attacks and de-protonates a hydrogen. is the definition of a Bronston base; meaning it takes the actual H away from the molecule but leaves its share of the electron behind; forming a double bond. DONE. as far as which is which in a specific equation, you really have to look at everything in the equation. if you don't have the total concentration of all of the substrate, catalyst, solution it is in, as well as the attacking molecule there is NO way to figure it out. A lot of professors give you this general bogus problem that there is no real right answer just to see how you will react to it. is it wrong? in a sense yes. Figuring out how this works takes a great deal of thought and time. some people it comes to quicker but then there are things that will come to you quicker in which that individual wont get. Only jerks laugh at others struggles. They are called bullies. My suggestion to mastering organic chemistry is the same as my suggestion in learning a new language. NEVER GIVE UP!! and surround yourself with it. read these threads. even though some are wrong it helps, watch videos on youtube, I suggest leah4sci via youtube. She is wonderful. DO NOT JUST RELY ON TEXT BOOKS. get as many books as you can. lectures, videos.. it is the exact same thing as learning a new language. HOPE this helps. keep up the fight!

 

[Teleologist]

It is the definition of lewis base; meaning it donates one of its electrons🙂)

You talkin' about radicals or Lewis bases?