Uneven distribution of electrons

Molecular Polarity Results from an Uneven Distribution of Electrons... 

MOLECULAR POLARITY RESULTS FROM AN UNEVEN DISTRIBUTION OF ELECTRONS... 

Oxygen molecules are attracted to water molecules when the oxygen molecules are brought close to the water molecule. The negative side of a water molecule pushes oxygen s electrons farthest from water and results in a temporary uneven distribution of electrons in the oxygen molecule known as an induced dipole. 

At any one instant, a nonpolar molecule like CH3CH3 can have an uneven distribution of electrons—an instantaneous dipole.This dipole can induce a dipole in a nearby molecule.This instantaneous dipole-induced dipole interaction is the weakest of all. 

Reactions of nucleophiles with protons and with carbonyl groups are heavily influenced by electrostatic attraction (as well as by HOMO-LUMO interactions). The proton is, of course, positively charged. The carbonyl group too has a substantial positive charge on the carbon atom, which comes from the uneven distribution of electrons in the C=0 k bond (Chapter 4). 

Whichever name it is given, the origin of this attraction is the mushy electron cloud that surrounds the nitrogen molecule. Because the electrons can be considered mobile in the electron cloud, they can be pictured as congregating momentarily at one end of the molecule or the other. This momentary uneven distribution of electrons is termed a temporary dipole, but it acts in the same manner as a permanent dipole. It is attracted to other dipoles, temporary or otherwise. The redistribution of electrons may be spontaneous, or if there is an ion or a molecule with a permanent dipole in the vicinity, this species might induce a momentary dipole, too. This situation is shown in figure 1.8.2. 

Unlike charges attract, and like charges repel each other. The uneven distribution of electrons in a molecule can result in a pair of partial charges, represented by 6+ and 6- or by H— , a dipole moment arrow with the plus end indicated. Polar covalent bonds create... 

A molecule that has an uneven distribution of electrons is a polar molecule. 

One of the deficiencies of the MO methods, especially the simple ones, is that they tend to exaggerate uneven distribution of electrons in a molecule and thus make it more polar (with a higher dipole moment) than it actually is. The result is that dipole moments which are based on charge densities obtained from eigenfunctions of the MO approximations are usually considerably higher than the actual experimental dipole moments. Two old, well-known examples of theoretical dipole moments obtained by the HMO method are fiilvene (11) whose calculated dipole moment is 4.7 D [72-74] and the experimental value is 1.2 D [68], and azulene (15), with a calculated dipole moment of 6.9 D [72] and the experimental value of 1.0 D [68]. 

Water. Water is distributed between intracellular and extracellular compartments, the latter comprising interstitial fluids, blood, and lymph. Because water is a dipolar molecule with an uneven distribution of electrons between the hydrogen and oxygen atoms, it forms hydrogen bonds with other polar molecules and acts as a solvent. 

Relatively weak intermolecular forces resulting from a temporarily uneven distribution of electrons that induces a dipole in a neighbor... 

All atoms and molecules attract each other, even in the absence of charged groups, as a result of mutual interactions related to induced polarization effects. Such polarization is sometimes indicated by symbols 5 and 5 . Attraction results from the uneven distribution of electrons when the positive end of one dipole (polarized bond) is attracted to the negative end of another dipole. This arises from three types of interactions ... 

Figure 122 The uneven distribution of electron density in the water molecule.

The property of molecules of having an uneven distribution of electrons, so that one part has a positive charge and the other a negative charge. 

FIGURE 2.8 Temporary dipoles and induced dipoles in nonpolar molecules resulting from an uneven distribution of electrons at a given instant. 

This dynamic distribution of electrons is not the only way to achieve an uneven distribution of electron density. Because electrons have very little mass and are charged, they can also be influenced by electric fields external to the molecule, such as those from a dipole in a neighboring molecule. When a molecule encounters an external electric field, the negatively charged electrons will tend to migrate away from the negative pole of the applied field. This response establishes a new dipole within the molecule itself. Thus the external field has forced a dipole to exist where it had not existed previously it has induced the dipole. This process is shown schematically in Figure 8.15. 

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