The polarity of a covalent bond between two different elements is determined by electronegativity. The more electronegative an element is, the more it attracts the electron density in the bond. So, the greater the difference between the electronegativities, the greater the difference between the attraction for the bonding electrons, and the more polarized the bond becomes. In the extreme case of complete polarization, the covalent bond ceases to exist and is replaced by electrostatic attraction between ions of opposite charge. We discussed this in Chapter 4 (p. 96), where we considered the extreme cases of bonding in NaCl.

How important are organometallics for making C–C bonds?

As an example, let’s take a molecule known as ‘juvenile hormone’. It is a compound that prevents several species of insects from maturing and can be used as a means of controlling insect pests. Only very small amounts of the naturally occurring compound can be isolated from the insects, but it can instead be made in the laboratory from simple starting materials. At this stage you need not worry about how, but we can tell you that, in one synthesis, of the 16 C–C bonds in the final product, seven were made by reactions of organometallic reagents, many of them the sort of reactions we will describe in this chapter. This is not an isolated example. As further proof, take an important enzyme inhibitor, closely related to arachidonic acid which you met in Chapter 7. It has been made by a succession of C–C bond-forming reactions using organometallic reagents: eight of the 20 C–C bonds in the product were formed using organometallic reactions.

When we discussed (in Chapter 6) the electrophilic nature of carbonyl groups we saw that their reactivity is a direct consequence of the polarization of the carbon–oxygen bond towards the more electronegative oxygen, making the carbon a site for nucleophilic attack. In Chapter 6 you also met the two most important organometallic compounds—organolithiums and organ magnesium halides (known as Grignard reagents). In these organometallic rea gents the key bond is polarized in the opposite direction—towards carbon—making carbon a nucleophilic centre. This is true for most organometallics because, as you can see from this edited version of the periodic table, metals (such as Li, Mg, Na, and Al) all have lower electro negativity than carbon.

Pauling electronegativities of selected elements

The molecular orbital energy level diagram—the kind you met in Chapter 4—represents the C–Li bond in methyllithium in terms of the sum of the atomic orbitals of carbon and lithium. The more electronegative an atom is, the lower in energy are its atomic orbitals (p. 96). The f i lled C–Li σ orbital is closer in energy to the carbon’s sp3 orbital than to the lithium’s 2s orbital, so we can say that the carbon’s sp³ orbital makes a greater contribution to the C–Li σ bond and that the C–Li bond has a larger coefficient on carbon. Reactions involving the filled

σ orbital will therefore take place at C rather than Li. The same arguments hold for the C–Mg bond of organo-magnesium or Grignard reagents, named after their inventor Victor Grignard.

We can also say that, because the carbon’s sp³ orbital makes a greater contribution to the C–Li σ bond, the σ bond is close in structure to a filled C sp³ orbital—a lone pair on carbon. This useful idea can be carried too far: methyl lithium is not an ionic compound Me⁻Li⁺ although you may sometimes see MeLi or MeMgCl represented in mechanisms as Me⁻.

مواضيع ذات صلة

Organometallics as bases

How to make organolithium reagents

Making organometallics, How to make Grignard reagents

Lewis acids and bases

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