Nucleophiles vs Bases: Understanding Strong/Weak Reactions

[CB4]

I am going through Sn1, E1, Sn2, E2 reactions and I'm trying to figure out if the reaction involves a nucleophile (strong/weak) or a base (strong/weak). I understand that you can differentiate nucleophiles and bases by their pKa value but that doesn't quite help me out, since I'm not given molarity. I am strictly speaking from the standpoint of seeing a question on a test and being able to tell if it's a nucleophile or base and whether it's strong or not. All my teacher did was give me a list of "Strong Bases" and "Good Nucleophiles" to memorize which is not my thing at all. I want to be able to look at the molecule and KNOW. I hope someone can help me!

I am looking at my worksheets and a question is:

1-Chloro-1-ethylpentane + HO-CH₃ ------->

I am not told whether HO-CH₃ is a nucleophile or base, but in the solutions it says it's a nucleophile. What I don't know is how that is concluded.

 

[CrimpJiggler]

Yeah I know what you mean, I'm also incapable of mindlessly memorising something so I can parrot it off later. I have to truly understand the concepts. This is a good question, I'd like to know the difference between bases and nucleophiles too. I just skimmed through this:
http://www.crystalclearchemistry.com/organic/concepts/nucbase.php
they provide a good example:
[PLAIN]http://www.crystalclearchemistry.com/organic/concepts/images/nucbase_example.gif
the alkyl group of alkyl halide is the electrophile and the other reactant has 2 nucleophilic groups, the amino group and the thiol group. Amino groups are basic but thiols aren't but the thiol group is more nucleophilic. Nucleophilicity is all about reaction rate while basicity is all about the stability of the product formed. In other words the thiol group will react with the electrophile a lot faster than the amino group will because thiols are more nucleophilic. The product that forms though won't be as strong as the product that would be formed when the amino group reacts with the electrophile. So let's pretend the reaction above is irreversible. In that case the product will be the methyl group (from the alkyl halide) bonded to the thiol portion of the group. If the reaction is reversible on the other hand, the product will be the methyl group bonded to the amino group because although the thiol product is forming faster, the amino product is more stable so every time the amino product forms, its a lot more likely to stay like that. That was a terrible explanation, sorry about that.

 

[CB4]

Yeah I'm still confused on how to differentiate the two. I know both bases and nucleophiles donate an electron pair to the electrophile, but that's as far as I am right now.

 

[Highway]

Honestly, you KNOW from experience, and doing a **** TON of practice problems...

My ochem book had a solutions manual, and the answer to every problem in it, so I literally worked through every problem at the back of each chapter, and checked my answer.

The short method is to memorize those lists your were given, and then implement their use in all of those practice problems. By the end of a chapter or two, you will just KNOW.

Practice problems are literally the make it or break it of ochem. . .

 

[mishrashubham]

Highway is right, with enough practice you sort of begin to intuitively understand which ones are nucleophiles and which ones are the bases. However the basic principles always apply. Nuceophiles are characterised by their willingness to form bonds while bases try to simply give away the electrons (i.e. extract hydrogen).
So what encourages or discourages a molecule or ion to form bonds?
1.) Stability of the molecule or ion as a free species. Greater the stability lesser is its nucleophilicity. For example, Cl^- is quite stable in aqeous environment as HCl is highly acidic. Therefore it is not a good nuceophile.
2.) Steric Hinderance - This is also important and depends on both substrate and attacking group. The classic example of bad nucleophile is C(CH₃)₃. The bulkiness prevents it from forming a bond.
3.) Solvent - You need to consider whether the solvent is polar protic or polar aprotic to decide between SN1 and SN2 or between E1 and E2 - same goes for concentration.
4.) Substrate - Presence or absence of good leaving group.

 

[Highway]

Highway is right, with enough practice you sort of begin to intuitively understand which ones are nucleophiles and which ones are the bases. However the basic principles always apply. Nuceophiles are characterised by their willingness to form bonds while bases try to simply give away the electrons (i.e. extract hydrogen).
So what encourages or discourages a molecule or ion to form bonds?
1.) Stability of the molecule or ion as a free species. Greater the stability lesser is its nucleophilicity. For example, Cl^- is quite stable in aqeous environment as HCl is highly acidic. Therefore it is not a good nuceophile.
2.) Steric Hinderance - This is also important and depends on both substrate and attacking group. The classic example of bad nucleophile is C(CH₃)₃. The bulkiness prevents it from forming a bond.
3.) Solvent - You need to consider whether the solvent is polar protic or polar aprotic to decide between SN1 and SN2 or between E1 and E2 - same goes for concentration.
4.) Substrate - Presence or absence of good leaving group.

A+ answer. My ochem is too rusty to give an answer like this :P

 

[mishrashubham]

Another important factor I forgot to mention is polarizability. Larger and less electronegative anions i.e. highly polarizable anions are better nucleophiles (like to share electrons).

 

[CrimpJiggler]

Yeah I'm still confused on how to differentiate the two. I know both bases and nucleophiles donate an electron pair to the electrophile, but that's as far as I am right now.

When you get answers like the one highway gave you, you know the issue is that contemporary scientific knowledge hasn't got far enough to answer your question yet. I have the same problem with solubilities. I hate having to refer to the CRC handbook or whatever every time I want to know the solubility of a compound in a particular solvent. I intuitively know we will find algorithms in the future to predict all this but until then, I'll have to make do with what we've got: solubility databases.