6.10   Summary

New Concepts

A measure of a compound’s acidity—its pK_a value—is introduced. The lower the pK_a value of a compound, the more acidic it is.

The solvent is an important factor in stabilizing ions. For example, both the conjugate bases of alcohols (alkoxides) and the conjugate acids of amines (ammonium ions) depend greatly on solvation for stabilization. In assessing basicity or acidity, you must take account of the solvent.

A general discussion of solvents and solvation also appears. Protic solvents that can participate in hydrogen bonding dissolve other protic, polar molecules well. By contrast, polar solvents do a poor job of dissolving “greasy” nonpolar organic molecules such as hydrocarbons, which are dissolved by nonpolar, similarly “greasy” solvents. Like dissolves like—polar solvents dissolve polar molecules, and nonpolar solvents dissolve nonpolar molecules.

Organic halogenated molecules can be used to form organometallic reagents. Many of the important synthetic reactions of organic chemistry use organometallic reagents. In this chapter, we see them only as very strong bases and as sources of hydrocarbons through their reaction with water.

Amines, like alkanes with sp³-hybridized carbons, are tetrahedral. Unlike alkanes, amines invert their “umbrellas” through an sp²-hybridized transition state.

We also introduced the hydrogen bond. This kind of bond is nothing more than a partially completed acid–base reaction in which a partial bond is formed between a pair of electrons on one atom and a hydrogen on another (Fig. 6.68).

 

Key Terms

Common Errors

People sometimes get the connection between pK_a and acid strength wrong. A strong acid has a low pK_a; a weak acid has a high pK_a.

It is easy to get confused when using the pK_a of an ammonium ion as a measure of the basicity of the related amine.

Review the discussion (p. 250) or use the more direct (but less common) pK_b values.

Syntheses

1. Grignard Reagents

2. Hydrocarbons

3. Organolithium Reagents