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A New Perspective on Borane Chemistry: The Nucleophilicity of the B−H Bonding Pair Electrons

Polarizability is directly related to the size of the atom and the number of electrons that are distant from the nucleus. So, if we look at halogens in the Periodic Table, the iodide will be the best nucleophile, and fluoride will be the worst. The ability of nucleophiles to participate in hydrogen bonding decreases as we go down the periodic table.

Hence fluoride is the strongest hydrogen bond acceptor, and iodide is the weakest. In a polar protic solvent iodide is the best nucleophile, followed by bromide, followed by chloride, and then last of all is the fluoride.

BUT the opposite is true in aprotic solvents. Under these conditions, nucleophilicity correlates well with basicity. Thus, fluoride ion , being the most unstable of the halide ions, reacts fastes t with electrophiles. In polar aprotic solvents, this is not an issue, so basicity is the most important variable.

  • Meaning of "nucleophilicity" in the English dictionary.
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When discussing reactions in organic chemistry, we must take into account that orbitals at the carbon that participate in reactions are generally less accessible than protons are. Synonyms and antonyms of nucleophilicity in the English dictionary of synonyms. Examples of use in the English literature, quotes and news about nucleophilicity.

Nucleophilicity and Solvent Effects - Chemistry LibreTexts

This trend reflects the relative basicities, as well. Eric V. Anslyn, Dennis A. Dougherty, The relationship between the concentration of amines and substrate enol tosylate has been examined. Sitaram Bhavaraju, Francis A. Carey, Richard J. Sundberg, Relationship between Nucleophilicity and Reactivity Most of the protein modification reactions are nucleophilic reactions, involving a direct displacement of a leaving group by the attacking nucleophile, which in this case is the amino acid side Shan S. Wong, Nucleophilicity and Solvent Effects The term nucleophilicity refers to the effect of a Lewis base on the rate of a nucleophilic substitution reaction and may be contrasted with basicity, which is defined in terms of the position of an equilibrium Polar solvents can be further subdivided into protic and and aprotic solvents.

A protic solvent is a solvent that has a hydrogen atom bound to an oxygen or nitrogen. The diagram below shows a few examples of protic solvents we will see.

Nucleophilicity vs. Basicity

Since oxygen and nitrogen are highly electronegative atoms, the O-H and N-H bonds that are present in protic solvents result in a hydrogen that is positively polarized. When protic solvents are used in nucleophilic substitution reactions, the positively polarized hydrogen of the solvent molecule can interact with the negatively charged nucleophile. In solution, molecules or ions that are surrounded by these solvent molecules are said to be solvated. Solvation is the process of attraction and association of solvent molecules with ions of a solute. The solute, in this case, is a negatively charged nucleophile.

The following diagram depicts the interaction that can occur between a protic solvent and a negatively charged nucleophile. The interactions are called hydrogen bonds. A hydrogen bond results from a from a dipole-dipole force between between an electronegative atom, such as a halogen, and a hydrogen atom bonded to nitrogen, oxygen or fluorine. In the case below, we are using an alcohol ROH as an example of a protic solvent, but be aware that this interaction can occur with other solvents containing a positively polarized hydrogen atom, such as a molecule of water, or amides of the form RNH 2 and R 2 NH.

What Makes A Good Nucleophile?

Why is this important? Solvation weakens the nucleophile; that is, solvation decreases nucleophilicity. This is because the solvent forms a "shell" around the nucleophile, impeding the nucleophile's ability to attack an electrophilic carbon. Furthermore, because the charge on smaller anions is more concentrated, small anions are more tightly solvated than large anions. The picture below illustrates this concept.

Notice how the smaller fluoride anion is represented as being more heavily solvated than the larger iodide anion. This means that the fluoride anion will be a weaker nucleophile than the iodide anion.

In fact, it is important to note that fluoride will not function as a nucleophile at all in protic solvents. It is so small that solvation creates a situation whereby fluoride's lone pair of electrons are no longer accessible, meaning it is unable to participate in a nucleophilic substitution reaction.

Hydrogen bonding lowers intrinsic nucleophilicity of solvated nucleophiles.

Previously we learned how nucleophilicity follows basicity when moving across a row. In our discussion on the effect of protic solvents on nucleophilicity, we learned that solvation weakens the nucleophile, having the greatest effect on smaller anions. In effect, when using protic solvents, nucleophilicity does not follow basicity when moving up and down a column.

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