Electron-rich sigma bonds

We can consider a partially ionic sigma bond as an already partially broken bond. An organometallic bond, for example, has electrons that are very available. Conversely, a carbon-carbon covalent bond is completely covalent its electrons reside in a very stable sigma bonding orbital and are not available. The main exception to this is the C-C bonds of cyclopropane, which are destabilized by ring strain, and thus are more reactive. 

To determine the reactivity of an organometallic on the basis of R when the metal is the same, compare the carbanion stability (Section 3.4.1). As the number of electron-donating alkyl groups on the carbanion increases, the carbanion gets less stable and more reactive. The alkyllithium general reactivity trend is tertiary secondary primary methyl. The less stable the carbanion, the more reactive it is as an electron source. For carbanions that are resonance-stabilized by electron-withdrawing groups, the identity of the metal is of less importance than the stabilization of the carbanion. 

Simple alkyl carbanions, like CH3Li, are sp hybridized at the carbanionic center and commonly exist as the tetramer, (CH3Li)4. This tetramer exists as a tetrahedron of lithium ions with a carbanion snuggled in between the three lithium atoms of each face. 

Which of the following, CH3Li, CH3MgBr, CH3CdCl, is the most reactive organometallic Which is the least  

Answer The most ionic is the most reactive. The least electronegative metal in this group is lithium, so CH3Li is the most reactive. The next most reactive is CH3MgBr the least reactive and the most covalent organometallic is CH3CdCl. 

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The pi bond as a nucleophile. A strong electrophile attracts the electrons out of the pi bond to form a new sigma bond, generating a carbocation. The (red) curved arrow shows the movement of electrons, from the electron-rich pi bond to the electron-poor electrophile. 

The best electron sources are usually nonbonding electron pairs. They are electron rich, and no bonds need be broken to use them as electron sources. Other excellent electron sources are highly ionic sigma bonds and also pi bonds highly polarized by excellent electron-releasing groups. 

Bonding in the carbonyl group (a) carbonyl carbon is sp -hybridized, (b) C=0 group consists of sigma and pi bonds, (c) electrostatic potential map illustrates the electron-poor nature (blue) of the carbonyl carbon and the electron-rich nature (red) ot the oxygen. 

When an electron-rich functional group comes into close proximity to an electron-deficient species (an acid), donation of two electrons to the electron-deficient center leads to a new sigma bond. This process is illustrated in Figure 5.8, where it is compared to a typical Lewis base-Lewis acid reaction between a generic base, B , and BF3 to give the complex shown (see Chapter 2, Section 2.5 and Chapter 6, Section 6.5). The concept of electron flow involves the transfer of two electrons from the electron rich base to the electron poor boron to give the Lewis-acid-Lewis base complex formed as a new bond. 

The nitrene nitrogen atom of imidogen is sp hybridized. Both the NH bond and the sigma lone pair of electrons use nitrogen orbitals that are rich in 2s character. 

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