Ammonia lone pair’ electrons

The simplest type of Lewis acid-base reaction is the combination of a Lewis acid and a Lewis base to form a compound called an adduct. The reaction of ammonia and trimethyl boron is an example. A new bond forms between boron and nitrogen, with both electrons supplied by the lone pair of ammonia (see Figure 21-21. Forming an adduct with ammonia allows boron to use all of its valence orbitals to form covalent bonds. As this occurs, the geometry about the boron atom changes from trigonal planar to tetrahedral, and the hybrid description of the boron valence orbitals changes from s p lo s p ... 

The simplest compounds to consider here are ammonia and water. It is apparent from the above electronic configurations that nitrogen will be able to bond to three hydrogen atoms, whereas oxygen can only bond to two. Both compounds share part of the tetrahedral shape we saw with 5/ -hybridized carbon. Those orbitals not involved in bonding already have their full complement of electrons, and these occupy the remaining part of the tetrahedral array (Figure 2.21). These electrons are not inert, but play a major role in chemical reactions we refer to them as lone pair electrons. 

These orbital pictures tend to get a little confusing, in that we really need to put in the elemental symbol to distinguish it from carbon, and we usually wish to show the lone pair electrons. We accordingly use a compromise representation that employs the cleaner line drawings for part of the structure and shows the all-important orbital with its lone pair of electrons. These are duly shown for ammonia and water. 

One final point about covalent bonds involves the origin of the bonding electron pair. Although most covalent bonds form when two atoms each contribute one electron, bonds can also form when one atom donates both electrons (a lone pair) to another atom that has a vacant valence orbital. The ammonium ion (NH4+), for example, forms when the two lone-pair electrons from the nitrogen atom of ammonia, NH3, bond to H +. Such bonds are called coordinate covalent bonds. 

In 1947 Walsh (68) proposed that because the ionization potential of the rr electrons in ethylene and of the lone pair in ammonia are both around 10.5 eV, the it electrons in the olefins should be equally capable of donation to acceptor centers. This implied that olefin complexes should be much more widespread than they were. 

In the presence of an adequate amount of water, aliphatic amines are generally dealkylated by anodic oxidation. llius, a tertiary amine is successively dealkylated to a secondary amine, a primary amine and finally to ammonia, llie mechanism involves initial removal of one electron ftom the lone pair electrons of nitrogen leading to a cation radical, though a variety of mechanisms have been proposed depending on the structures of the amines and the reaction conditions. 

The lone pair electrons of water (O atom), ammonia (N atom), and amino groups (N atom) influences the behavior and concentrations of hydrogen ions (H+) in water. Hydrogen ions, produced either by dissociation of water or by dissociation of acids, do not occur as free entities in aqueous solutions. They associate with the lone pair electrons of other water molecules to form hydronium ions, H3O+. This association involves the formation of an electron donor-acceptor bond. 

Valence electrons that are not used for bonding are called uonbond-ing electrons, or lone-pair electrons. The nitrogen atom in ammonia, fbr instance, shares six valence electrons in three covalent bonds and has its remaining two valence electrons in a nonbonding lone pair. 

Here ammonia, the Lewis base, donates lone-pair electrons to BF3, the Lewis acid or electron acceptor. The bond that forms is called a coordinate covalent bond, in which both electrons are supplied by a lone pair on the Lewis base. 

Ammonia has strong proton affinity, thus has strong H-bond interactions with the water monomer compared with the interaction in the water dimer. In NH3 H2O interaction, polarizable lone-pair electrons in NH3 make NH3 better H-accepting than H2O. H2S is a weaker proton-acceptor than H2O and plays a role of H-donor in H2O-H2S interaction. The third elements P and S atoms have relatively weak H-bond interactions due to large atomic radius. 

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