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Thread: Base strength

  1. #1
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    Default Base strength

    Chad, I'm having a little trouble understanding the trends of nucleophilicity and basicity.
    At the end of the HD Sn1/2 videos you state, that basicity increases up and to the left. Shouldn't basicity increase going down a column? A lewis base is generally a specie that can donate electrons, If this is true should'nt the basicity trend in a column go as follows: I>Br>Cl>F because one could argue that I has electrons that are less tightly bound by the nucleus, since they are located further away from the nucleus, making it MORE reactive thus more basic. I would appreciate if you could clarify this for me. Thanks!

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    Chad
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    The trend in basicity is technically for Bronsted/Lowry bases, not Lewis bases. When an ochem textbook gives a trend in basicity, this is what they're referring to. This trend not only takes into account the stability of a base but of the strength of the bond it forms with hydrogen (longer bonds are weaker bonds which is why larger anions are weaker bases). Overall this is a thermodynamic property.
    The other trend you'll need to know for ochem is for nucleophiles, not Lewis bases (similar but not quite the same thing). This trend deals with the kinetics (not thermodynamics) of how fast a nucleophile reacts with an electrophile (which is why simply looking at which anion has the electrons "less tightly bound by the nucleus" isn't quite enough to predict this trend). Ignoring solvent effects, a smaller anion will have a larger charge density and will typically react faster. But we do have to take into account solvent effects. Protic solvents stabilize (solvate) all ions, but smaller ions the most. In aprotic solvents, nucleophiles increase in strength going up the periodic table as stated above. But due to the stabilizing effect of protic solvents, the vertical trend is reversed in protic solvents and nucleophiles increase in strength going down the periodic table.
    Hope this helps. Knowing the trends is of primary importance and having a general understanding of why is ideal, but a detailed understanding is overkill as this can get even more technical than I've made it.

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    Quote Originally Posted by chad View Post
    The trend in basicity is technically for Bronsted/Lowry bases, not Lewis bases. When an ochem textbook gives a trend in basicity, this is what they're referring to. This trend not only takes into account the stability of a base but of the strength of the bond it forms with hydrogen (longer bonds are weaker bonds which is why larger anions are weaker bases). Overall this is a thermodynamic property.
    The other trend you'll need to know for ochem is for nucleophiles, not Lewis bases (similar but not quite the same thing). This trend deals with the kinetics (not thermodynamics) of how fast a nucleophile reacts with an electrophile (which is why simply looking at which anion has the electrons "less tightly bound by the nucleus" isn't quite enough to predict this trend). Ignoring solvent effects, a smaller anion will have a larger charge density and will typically react faster. But we do have to take into account solvent effects. Protic solvents stabilize (solvate) all ions, but smaller ions the most. In aprotic solvents, nucleophiles increase in strength going up the periodic table as stated above. But due to the stabilizing effect of protic solvents, the vertical trend is reversed in protic solvents and nucleophiles increase in strength going down the periodic table.
    Hope this helps. Knowing the trends is of primary importance and having a general understanding of why is ideal, but a detailed understanding is overkill as this can get even more technical than I've made it.
    Sorry to further pursuit this, but I am still a little confused. If this is more Bronsted/Lowry definition, then the nucelophile are proton acceptor. However, when you (Chad) stated the nucelophile is "stronger" does it simply mean "more reactive"? Can electronegativity be use to explain the reactivity? If so, why is the trend towards the right?

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    Chad
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    Quote Originally Posted by pchou View Post
    If this is more Bronsted/Lowry definition, then the nucelophile are proton acceptor.
    Sorry for the confusion. I was originally trying to state that when you think of the trend for basicity, think of it in terms of the Bronsted/Lowry definition, not the Lewis definition. The definition for a nucleophile is something else entirely.

    Quote Originally Posted by pchou View Post
    However, when you (Chad) stated the nucelophile is "stronger" does it simply mean "more reactive"?
    We have to be careful here in knowing what we mean when we say 'more reactive.' It means something slightly different for a base vs a nucleophile. When we talk about a base being 'more reactive,' we mean one that will form more products in an acid/base rxn. This is related to the concepts of equilibrium and thermodynamics. But when we talk about a nucleophile being 'more reactive,' we mean one that will react faster with an electrophile which is all about the kinetics of the rxn, not the thermodynamics. It makes sense that a base that forms more products (stronger base) is often also a nucleophile that reacts faster (stronger nucleophile), but not always. How much products are formed at equilibrium is independent of what solvent an acid/base rxn takes place in. But the solvent can totally affect the rate of rxn between a nucleophile and an electrophile which is why the verticle part of the nucleophile trend is different in protic vs aprotic solvents (while the base trend is always the same).

    Quote Originally Posted by pchou View Post
    Can electronegativity be use to explain the reactivity? If so, why is the trend towards the right?
    The answer is yes. A less electronegative base or nucleophile is typically less stable when negatively charged and therefore more reactive as a base and as a nucleophile and so both trends actually show an increase going to the left on the periodic table.
    Hope this helps and let me know if I need to clarify further.

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    Quote Originally Posted by chad View Post
    We have to be careful here in knowing what we mean when we say 'more reactive.' It means something slightly different for a base vs a nucleophile. When we talk about a base being 'more reactive,' we mean one that will form more products in an acid/base rxn. This is related to the concepts of equilibrium and thermodynamics. But when we talk about a nucleophile being 'more reactive,' we mean one that will react faster with an electrophile which is all about the kinetics of the rxn, not the thermodynamics. It makes sense that a base that forms more products (stronger base) is often also a nucleophile that reacts faster (stronger nucleophile), but not always. How much products are formed at equilibrium is independent of what solvent an acid/base rxn takes place in. But the solvent can totally affect the rate of rxn between a nucleophile and an electrophile which is why the verticle part of the nucleophile trend is different in protic vs aprotic solvents (while the base trend is always the same).
    Thanks, I understand the concept better...in Ochem lab, since all the reagent are specifically picked, I never really contemplated difference because the more reactive reagent given generally yielded highest product.

    Quote Originally Posted by chad View Post
    The answer is yes. A less electronegative base or nucleophile is typically less stable when negatively charged and therefore more reactive as a base and as a nucleophile and so both trends actually show an increase going to the left on the periodic table.
    Hope this helps and let me know if I need to clarify further.
    Sorry for the confusion, I meant the trend for electronegativity was to the right. (Towards Fluorine)

    Thank you for explaining, now I just need to make sense of this in my head.

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