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I am confused on the topic of electronegative nucleophiles, and polar aprotic solvents.

From my understanding, the more electronegative a nucleophile is, the weaker of a nucleophile it will be, because atoms will be less likely to share its electron density.

However, in a polar aprotic solvent, because H-bonding cannot occur, it seems as if the trend of nucleophilicity is such that $\ce{F- > Cl- > Br- > I-}$.

Can someone please clarify how this is possible? I would have expected $\ce{F-}$ to be the LEAST nucleophilic nucleophile in a polar aprotic solvent, because it is the most electronegative atom. Therefore according to the first point I learned, I thought it should be the weakest nucleophile.

All this is what I've learned through reading Kaplan's Organic Chemistry Review.

orthocresol
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lost-in-nucleophiles
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  • https://chemistry.stackexchange.com/questions/47072/nucleophilicity-with-respect-to-the-solvent https://chemistry.stackexchange.com/questions/95710/order-of-halide-nucleophilicity-in-dmf – Mithoron Dec 29 '18 at 16:49

1 Answers1

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When talking about elements of a group, the size of the atom is more effective than the electronegativity factor. As $\ce{F-}$ size is too short so its electron density will be much higher than other hallogens.

In aprotic solvent the order of nucleophillicity: $\ce{F- > Cl- > Br- >I-}$(No hydration occurs) In protic solvant(like $\ce{H2O}$) its reversed because of hydration of ions having much electron density.

A.K.
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Rahul Gill
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  • Welcome to Chemistry.SE! Please note that formulas can be better expressed with $\ce{ }$ for chemical formulas/equations, $ $ for math term/equations, and $\pu{ }$ for units. More information is available in this meta post Also, take a minute to look over the help center and tour page to better understand our guidelines and question policies if you haven't already. – A.K. Jan 26 '19 at 02:13