In this case of moving up and down a column, nucleophilicity does not always follow basicity. It depends on the type of solvent you are using. In the section Nucleophilic Substitution, we assigned a relationship to leaving groups containing C, N, O, and F, showing that the strength of the leaving group follows electronegativity.
This is based on the fact that the best leaving groups are those that are weak bases that do not want to share their electrons. The best nucleophiles however, are good bases that want to share their electrons with the electrophilic carbon.
The relationship shown below, therefore, is the exact opposite of that shown for the strength of a leaving group. In general chemistry, we classified solvents as being either polar or nonpolar. 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.
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.
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. Good nucleophiles. Fair nucleophiles. Weak nucleophiles. H 2 O, ROH.
VERY weak nucleophiles. RCO 2 H. As shown above, as a general rule, the anion of a reactant will be a better nucleophile than the neutral form. Think about it for a second…. Bases accept protons, with a negative charge or lone pair.
This is a major consideration when looking at SN vs E reactions. Not to humble brag, but it is pretty good. What are some examples of nucleophiles and strong bases? Can a strong nucleophile be a weak base? How do you rank nucleophiles? How do you determine a nucleophile or a base? Are alkenes nucleophiles? Do nucleophiles add to carbonyl carbons? Why is ammonia a nucleophile?
Is water a nucleophile?
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