Ochem Strong base/Strong Nucleophile

[sujithp]

When reviewing Ochem, I came across a constant problem that I faced. I was wondering what "strong bases" and "strong nucleophiles" does the AAMC expect us to know are strong? I know the basic strong bases that we are taught in general chemistry (like NaOH) but how am I supposed to know for instance that sodium ethoxide is a strong base? Or that sodium azide is a strong nucleophile for Sn2 reactions? Is there a definitive list anywhere or are these common bases/nucleophiles I just don't know? Thanks ahead of time!!!

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[yoyohomieg5432]

When reviewing Ochem, I came across a constant problem that I faced. I was wondering what "strong bases" and "strong nucleophiles" does the AAMC expect us to know are strong? I know the basic strong bases that we are taught in general chemistry (like NaOH) but how am I supposed to know for instance that sodium ethoxide is a strong base? Or that sodium azide is a strong nucleophile for Sn2 reactions? Is there a definitive list anywhere or are these common bases/nucleophiles I just don't know? Thanks ahead of time!!!

i never memorized any strong bases/nucleophiles. negatively charged species you should know will be a very good base/nucleophile so it can lose some of the negative charge.

the line is hard to draw sometimes when something is a strong base or a strong nucleophile. as far as i know many compounds can act as both. there are some factors that can favor something acting as a nucleophile or as a base.

for example, things like LDA (http://en.wikipedia.org/wiki/Lithium_diisopropylamide) or tert butoxide are very bulky. it will be hard for them to act as a nucleophile because the large carbon groups will create steric issues as it tries to approach to act as a nucleophile. as a result, it can act as a base and pick off a hydrogen that is sticking out but won't be able to approach into the bulk of the molecule where the carbons are to act in an sn2 manner

 

[Resilience]

It really isn't a question about which bases and nucleophiles do I have to memorize are strong. It's really just a matter of knowing what a base or nucleophile is.Well, what is a base? A base is a molecule that can donate electrons. So, if a molecule has a negative charge on it, it is probably a base. It doesn't have to have a charge in order to be a base, as it could have a lone pair of electrons that do not give it a charge, however in almost every single case this will be a weak base (e.g. ammonia). To determine whether or not it is a strong base, look at the structure of the molecule. Does it look to be moderately stable with the negative charge? Does it have electron withdrawing groups close to the negative charge? Is it resonance stabilized? If so, it might not be the strongest base. If not, then there is a high chance that it is a strong base. Electron donating groups can also indicate a strong base. Sodium ethoxide, the example you gave, is a strong base because there is a negative charge on the oxygen, and the only thing the oxygen is attached to is an ethyl group that is slightly electron donating. So, the oxygen readily donates it's electrons.

A strong nucleophile is basically a strong base that isn't sterically hindered. This is a simplified definition, but it will almost always work.

I hope this helps, and I hope it was clear. I'm in a rush so I had to be quick. Ask if you have any questions.

 

[sujithp]

Thanks for the information guys! That really does clear it up pretty well for me. I was just getting confused when deciphering between the Sn1/Sn2/E1/E2 stuff when it came down to what type of nucleophile/base was used. If either of you have time, could you explain how you would know Sodium Azide (NaN3) would be a strong nucleophile and favor a Sn2 pathway over an E2 pathway? Thanks again!!!

 

[Resilience]

Thanks for the information guys! That really does clear it up pretty well for me. I was just getting confused when deciphering between the Sn1/Sn2/E1/E2 stuff when it came down to what type of nucleophile/base was used. If either of you have time, could you explain how you would know Sodium Azide (NaN3) would be a strong nucleophile and favor a Sn2 pathway over an E2 pathway? Thanks again!!!

you're welcome, glad it helped. So, sodium azide is a tricky one because it is one of the few strong nucleophiles that is a weak base (the halides also follow this rule). It is a strong nucleophile because it has that negative charge. I'm not entirely sure why it is a weak base, but I believe it is because the molecule is stabilized due to resonance. So, if it is a strong nucleophile, it will favor SN2, which needs a good nucleophile. It does not favor E2 because E2 needs a strong base that will donate to the hydrogen. Think of it as you would with a halide. You know that halides are strong nucleophiles and they occur frequently in substitution reactions, but you would never use a halide for an elimination reaction because it is a weak base.

Sorry I do not have a better reason as to why exactly azide is a strong nucleophile but a weak base. It's a tricky one.

 

[sujithp]

Thanks again for replying Resilience! What you said made sense to me. I guess for NaN3 I'll just memorize it as being a strong nucleophile haha. Hope you have a great day!