Читать книгу Organic Mechanisms - Xiaoping Sun - Страница 36

Many anions that contain a lone pair of electrons (e.g., OH⁻, OR⁻, RCO₂⁻, HS⁻, Br⁻, and CN⁻) are good (strong) nucleophiles. On the other hand, many electrically neutral molecules that contain a lone pair of electrons (e.g., H₂O, ROH, and RCO₂H) act as poor (weak) nucleophiles.

When we determine the relative strength of nucleophiles, consideration is focused on the reactivity for the nucleophiles toward electrophilic carbon atoms of organic substrates. The general guideline is The greater the electron density does the species have on its reactive center and the larger is the size of the reactive center, the more nucleophilic is the species. On the basis of this guideline, we have the following general rules:

1 An anion is stronger in nucleophilicity than a neutral molecule given that the reactive (nucleophilic) centers for both the anion and neutral molecule are the atoms of the same element. This is because in general an anion, due to its negative charge, has greater electron density than does a neutral molecule. It can be readily seen by comparison of hydroxide (OH−, a strong nucleophile) and water (H2O, a weak nucleophile) and by comparison of an alkoxide (RO−, a strong nucleophile) and an alcohol (ROH, a weak nucleophile).For both pairs of species, the reactive centers are in oxygen atoms. The above Lewis structures show that each of the anions has three lone pairs of electrons in the oxygen atom, while the corresponding neutral molecule has two lone pairs of electrons in the oxygen atom. Clearly, for each pair of the species, the anion has greater electron density in the reactive center than the corresponding neutral molecule, giving rise to stronger nucleophilicity.

2 For the same group of elements, the atomic radii increase from the top to bottom. As a result, for species containing the atoms of the same group of elements as reactive centers and having the same number of electron pairs in the reactive centers, their nucleophilicity increases as one moves from the top to the bottom along the group of elements. For example, the nucleophilicity of halides increases in the order of fluoride (F−), chloride (Cl−), bromide (Br−), and iodide (I−). All of them have four electron pairs. Another example is that the nucleophilicity of hydrogen sulfide (SH−) is stronger than hydroxide (OH−). For both of them, the central atoms contain three lone pairs of electrons.

Some molecules do not contain any lone pairs of electrons. However, a bonding electron pair in these molecules may be donated to an electrophile making them nucleophilic. In this category, compounds containing a C–M [M = Li or MgX (X = Cl, Br, or I)] bond are strong nucleophiles because the C─M bonding electron pair is active and has strong tendency to be donated to an electrophilic center. For example, a Grignard reagent undergoes nucleophilic addition to a carbonyl group (a carbon electrophile) as shown below:

The C=C π bond in alkenes is nucleophilic. They are characterized by electrophilic addition reactions (e.g., Reaction 1.64). The aromatic rings are nucleophilic due to the activity of the conjugate π electrons. Therefore, arenes undergo extensive electrophilic substitution reactions as illustrated below: