I came across this definition.

When discussing nucleophilicity we’re specifically talking about donating a pair of electrons to an atom other than hydrogen (usually carbon). When a species is donating a pair of electrons to a hydrogen (more specifically, a proton, H+) we call it a base.



Does this make this drawing of mine wrong? I labeled the base here a nucleophile even though the nucleophile is lending n electron pair to the hydrogen. Are all bases nucleophiles? Or should I stick with the definition above? (Forgive the forgotten arrow from the bond between oxygen and hydrogen which should be pointing at oxygen's valence environment.

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2 Answers 2


All nucleophiles are Lewis bases. They are just electron pair donor.

A molecule or ion is called a "base" (or, “Brønsted base”) when it interact with proton, it is called a "nucleophile" when it interact with other atom centers, which can be carbon, nitrogen, sulfur and so on. So the row "base" and "nucleophile" may overlap. For example, $\ce{OH-}$ is a base when it take proton off an acid, and it can also be a nucleophile when it hydrolyzes ester (nucleophilic attack to carbonyl group).

The broad nucleophilicity definition is how good the nucleophile interact the positive or partial positive atom center. When the atom center is proton, nucleophilicity is then basicity. Because proton center is hard and usually less steric hindred, a bulky and hard (for hardness $O$ > $N$ > $S$ center) electron donor will be likely to prefer proton over the softer and more hindred non-proton center. These electron donors are called Non-nucleophilic bases. Potassium tert-Butoxide is a good example.


It does not exclude a base of being a nucleophile. It is just that a specific label is provided for a pair of electrons being donated to the hydrogen ion.

Are all bases nucleophiles? There are 2 definitions for a base, a lone pair doner and/or a hydrogen ion accepter. For many compounds, a base is also a nucleophile. I hesitate to say all bases are nucleophiles, but it may be true.

  • 1
    $\begingroup$ Non-nucleophilic strong bases are an important part of organic chemistry. While thermodynamically all bases are nucleophiles, kinetically this can not be true. Usually, non-nucleophilic bases are compounds designed to have a large amount of steric hindrance, which precludes the possibility of them attacking an electrophilic non-hydrogen atom in the middle of an structure. There are also a few examples of non-basic strong nucleophiles, such as the halide anions, cyanide and derivatives, and azide. $\endgroup$ May 11, 2014 at 12:30
  • $\begingroup$ But I thought at least some of the halide ions - i.e. fluoride ion - were bases - in the Bronsted sense and in the Lewis sense. $\endgroup$
    – Dissenter
    May 12, 2014 at 0:10

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