All the acids and bases we have been discussing so far have been protic, or Brønsted, acids and bases. In fact, the defi nition of an acid and a base we gave you on p. 165 is a definition of a Brønsted acid and a Brønsted base. When a carboxylic acid gives a proton to an amine, it is acting as a Brønsted acid while the amine is a Brønsted base. The ammonium ion produced is a Brønsted acid while the carboxylate anion is a Brønsted base.

• Brønsted acids donate protons.

 • Brønsted bases accept protons.

But there is another important type of acid: the Lewis acid. These acids don’t donate pro tons—indeed they usually have no protons to donate. Instead, they accept electrons. It is indeed a more general defi nition of acids to say that they accept electrons and of bases that they donate electrons. Lewis acids are usually halides of the higher oxidation states of metals, such as BF3, AlCl3, ZnCl2, SbF5, and TiCl4. By removing electrons from organic compounds, Lewis acids act as important catalysts in important reactions such as the Friedel–Crafts alkyl-ation and acylation of benzene (Chapter 21), the SN1 substitution reaction (Chapter 15), and the Diels–Alder reaction (Chapter 34).

• Lewis acids accept electrons.

 • Lewis bases donate electrons.

A simple Lewis acid is BF3. As you saw in Chapter 5, monomeric boron compounds have three bonds to other atoms and an empty p orbital, making six electrons only in the outer shell. They are therefore not stable and BF₃ is normally used as its ‘etherate’: a complex with Et₂O. Ether donates a pair of electrons into the empty p orbital of BF3 and this complex has tetrahedral boron with eight electrons. In this reaction the ether donates electrons (it can be described as a Lewis base) and BF₃ accepts electrons: it is a Lewis acid. No protons are exchanged. The complex is a stable liquid and is the form usually available from suppliers.

Lewis acids often form strong interactions with electronegative atoms such as halides or oxygen. In the Friedel–Crafts acylation, which you will meet in Chapter 21, for example, AlCl₃ removes the chloride ion from an acyl chloride to give a species, the acylium ion, which is reactive enough to combine with benzene.

Lewis acid–base interactions are very common in chemistry and are often rather subtle. You are about to meet, in the next chapter, an important way of making C–C bonds by adding organometallics to carbonyl compounds, and in many of these reactions there is an interaction at some point between a Lewis acidic metal cation and a Lewis basic carbonyl group.

مواضيع ذات صلة

Organometallics as bases

How to make organolithium reagents

Making organometallics, How to make Grignard reagents

Organometallic compounds contain a carbon–metal bond

pKa in action—the development of the drug cimetidine

Why do we need to compare acid strengths of O and N acids?

Carbon acids

Substituents affect the pKa

The ‘pKas’ of bases

Nitrogen compounds as acids and bases

Delocalization of the negative charge stabilizes the conjugate base

Constructing a pKa scale