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Nucleophilicity Trend

Discussion in 'MCAT Study Question Q&A' started by StarryNights, Nov 25, 2009.

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  1. StarryNights

    StarryNights

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    Hey you guys, so this is a High Yield Kaplan OChem question on nucleophilicity trend. It says that a methoxide anion (CH3O-) is more basic than an acetate anion (CH3CO2-) which is more basic than methanol (CH3OH). These three are in a polar, protic solvent. So the answer says that more basic molecules are better H+ acceptors, so they will be surrounded by solvent molecules and therefore less available to attack the substrate --> less basic = better nucleophile. Yet it arranges CH3O- > CH3CO2- > CH3OH in order of nucleophilicity for both polar, protic solvent and polar, aprotic solvent (follow up example at the end). I'm soooo confused!
  2. McEMCDB

    McEMCDB

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    Yup, fairly confusing that they would make that point and follow it up with an example where the point doesn't seem to apply.

    What you'll need to do is keep in mind that nucleophilicity and basicity do not go hand in hand. Nucleophilicity is a reflection of the rate of reaction, while basicity is determined by equilibrium values. Greater basicity values do not automatically make a molecule a better nucleophile. While basicity does decrease nucleophilicity in a protic solvent, it doesn't automatically mean the molecule with the greater (basic) pH will become a weaker nucleophile than one with a lesser pH. There are other factors to consider, like substitution and whatnot.

    So, CH3O- obviously has a greater nucleophilicity in a polar, aprotic solvent than a protic one due to it being highly basic. I believe this is all you can determine from the details given.

    Despite the reduced nucleophilicity, it is still possible for a basic compound to act as a nucleophile in a protic solvent.. and it seems methoxide is still a better nucleophile than an acetate ion, which is still better than the methanol.

    This is my interpretation of the details given with a little research of my own. I'm not an expert on the topic quite yet, so you'll definitely want another opinion. I could be completely wrong here :laugh:
  3. wanderer

    wanderer

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    There is certainly a correlation between basicity and nucleophilicity (remember a nucleophile is always a Lewis base), so that all other things equal stronger bases are stronger nucleophiles. However the solvent makes huge difference. If the solvent is polar protic it stabilizes the negative charges of anionic nucleophiles. So for example comparing fluorine and bromine, fluorine is a stronger base but is also a smaller ion, so it is more readily solvated in a protic solvent. Since it is solvated it is not free to attack the electrophile as readily as Bromine is. In an aprotic solvent (like ether for example), neither anion is surrounded by a protective shell of hydrogens so that the stronger base - Fluorine - is more nucleophilic. When comparing acetate to methoxide there are several things to keep in mind when considering nucleophilicity. One is stability of the basic form. The other is size of the overall molecule. The acetate ion is pretty bulky compared to the methoxide ion and is less nucleophilic even in a protic medium. The fact that each oxygen of the acetate ion only has half a negative charge likely plays a role in determing its nucleophilicity as well.

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