Double bond electron pairs

In Section 7.2, we saw that insofar as geometry is concerned, a multiple bond acts as if it were a single bond. In other words, the extra electron pairs in a double or triple bond have no effect on the geometry of the molecule. This behavior is related to hybridization. The extra electron pairs in a multiple bond (one pair in a double bond, two pairs in a triple bond) are not located in hybrid orbitals. 

The bonds may be single (one electron pair), double (two electron pairs), or triple (three electron pairs). 

Every group of electrons shared between two atoms constitutes a covalent bond. When one pair of electrons is involved, the bond is called a single bond. When two pairs of electrons unite two atoms, the bond is called a double bond. Three pairs of electrons shared between two atoms constitute a triple bond. Examples of these types of bonds arc given below ... 

Note In VSEPR theory the term bond pair is used for a single bond, a double bond, or a triple bond, even though a single bond consists of one pair of electrons, a double bond two pairs of electrons, and a triple bond three pairs of electrons. To avoid any confusion between the number of electron pairs actually involved in the bonding to a central atom, and the number of atoms bonded to that central atom, we shall occasionally use the term ligand" to indicate an atom or a group of atoms attached to the central atom. 

In addition, carbon atoms can form strong single, double, or triple bonds with other carbon atoms. In a single carbon-carbon bond, one pair of electrons is shared between two carbon atoms. In a double bond, two pairs of electrons are shared between two atoms. In a triple bond, three pairs of electrons are shared between two atoms. 

A double bond involves two pairs of electrons. In a double bond, one pair of electrons forms a single bond and the other pair forms an additional, weaker bond. The electrons in the additional, weaker bond react faster than the electrons in the single bond. Thus, carbon-carbon double bonds are more reactive than carbon-carbon single bonds. When an alkene reacts, the reaction almost always occurs at the site of the double bond. 

Besides a heteroatom substituent, which renders a double bond electron rich by resonance interaction of the lone pairs with the n system, other substituents can... 

The equilibrium constant for this reaction is sensitive to the substituents on the C=0 double bond. Electron-withdrawing substituents, such as the C13C group in chloral, drive the reaction toward the dialcohol, or diol (K 1). Electron-donating substituents, such as the pair of CH3 groups in acetone, pull the equilibrium back toward the aldehyde (Ka = 2 x 10 3). 

As might be expected, the first step in this reaction involves attack by a nucleophile at the positively charged end of the C=0 double bond. A pair of nonbonding electrons on the oxygen atom of the alcohol is donated to the carbon atom of the carbonyl to form a CO bond. As this bond forms, the electrons in the bond of the carbonyl are displaced onto the oxygen atom. A proton is then transferred back to the solvent to give a tetrahedral addition intermediate. 

The lone pair electrons from nitrogen can overlap with the double bond n electrons in a manner similar to the overlap of the n electrons of two conjugated double bonds. This electron contribution from nitrogen makes an enamine double bond electron-rich. 

This theory implies outer orbital involvement in the bonding. It assumes that each bond between the ligand and the central atom involves an electron pair. A double bond involves four electrons. Further, it assumes that all nonbonding valence electrons have a steric effect. These lone pairs repel other electron pairs more than do bonding electron pairs and have about the same steric requirements as a double bond. Gillespie predicted the shape of unknown species using this theory all later isolated species conformed to his predictions. 

Pyrrole is cyclic and planar, with a total of four ji electrons from the two 7i bonds. Is the non-bonded electron pair localized on N or part of a delocalized ji electron system The lone pair on N is adjacent to a double bond. Recall the following general rule from Section 16.5 ... 

The cyclopentadienyl anion is a cyclic and planar anion with two double bonds and a non-bonded electron pair. In this way it resembles pyrrole. The two n bonds contribute four electrons and the lone pair contributes two more, for a total of six. By HiickeTs rule, having six n electrons confers aromaticity. Like the N atom in pyrrole, the negatively charged carbon atom must be sp hybridized, and the nonbonded electron pair must occupy a p orbital for the ring to be completely conjugated. 

Double and triple bond domains which are composed of two and three electron pairs, respectively, are larger than single-bond electron pair domains. 

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