Polarity molecular

Experimentally, the polarity of molecules is measured indirectly by measuring the dielectric constant, which is the ratio of the capacitance of a cell filled with the substance to be measured to the capacitance of the same cell with a vacuum between the electrodes. Orientation of polar molecnles in the electric field partially cancels out the effect of the field and resnlts in a larger dielectric constant. Measurements at different temperatures allow calculation of the dipole moment for the molecule, defined as = Qr, where r is the distance between the centers of positive and negative charge and Q is the difference between these charges.  

Dipole moments of diatomic molecules can be calculated directly. In more complex molecules, vector addition of the individual bond dipole moments gives the net molecular dipole moment. However, it is usually not possible to calculate molecular dipoles directly from bond dipoles. Table 3.8 shows experimental and calculated dipole moments of chloro-methanes. The values calculated from vectors use C—H and C—Cl bond dipole moments of 1.3 X 10 ° and 4.9 X 10 C m, respectively, and tetrahedral bond angles. Clearly, calculating dipole moments is more complex than simply adding the vectors for individual bond moments. However, for many purposes, a qualitative approach is sufficient. 

Source Ejqierimental data, CRCHeaidbook. of Chemistry and Physics,92nd ed.,Taylor and Francis Group, LLC, 2011-2012, 

The SI unit for dipole moment is a Coulomb meter (C m), but a commonly used unit is the debye (D). One 

TABLE 7.2 H—X Bonds (X Varies along a Period), the Electronegativity (EN) of Their Constituents, and EN Differences 

SCHEME 7.12 Individual bond dipoles in (a) CO2 and (b) CCI4. Both molecules have zero molecular dipoles because the individual bond dipoles are oppositely directed and hence cancel one another. 

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Clausius-Mosottf Jaw The molecular polarization (P) of a substance of molecular weight M, density d and dielectric constant O is ... 

Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

Further subclassification of nonlinear optical materials can be explained by the foUowiag two equations of microscopic, ie, atomic or molecular, polarization,, and macroscopic polarization, P, as power series ia the appHed electric field, E (disregarding quadmpolar terms which are unimportant for device appHcations) ... 

Steps 3 and 4, however, can be described as chemical plasticization since the rate at which these processes occur depends on the chemical properties of molecular polarity, molecular volume, and molecular weight. An overall mechanism of plasticizer action must give adequate explanations for this as weU as the physical plasticization steps. 

Asphalts characteristically contain very high molecular weight molecular polar species, called asphaltenes, which are soluble in carbon disulfide, pyridine, aromatic hydrocarbons, chlorinated hydrocarbons, and tetrahydrofiiran. 

The physical and chemical characteristics of the adsorbate, that is, molecular size, molecular polarity, chemical composition, and so on the concentration of the adsorbate in the liquid phase (solution). 

As follows from Table I (see Section VII Index of Tables), the yields in 4-acetylenyl compounds depend both on the reaction time and on the structure of the aromatic and acyclic components (molecular polarity). If more than one equivalent of diazomethane is used, N-methylation of pyrazole occurs. 

Just as individual bonds are often polar, molecules as a whole are often polar also. Molecular polarity results from the vector summation of all individual bond polarities and lone-pair contributions in the molecule. As a practical matter, strongly polar substances are often soluble in polar solvents like water, whereas nonpolar substances are insoluble in water. 

Net molecular polarity is measured by a quantity called the dipole moment and can bethought of in the following way assume that there is a center of mass of all positive charges (nuclei) in a molecule and a center of mass of all negative charges (electrons). If these two centers don t coincide, then the molecule has a net polarity. 

Predict relative solubilities from molecular polarity (Section 8.9). 

What Do We Need to Know Already This chapter builds on Chapter 18 and requires the same preparation, as well as the concepts of acid-base and redox reactions (Sections J and K). It also makes use of the concepts of molecular polarity (Section 3.3) and solubility (Section 8.9). 

Ethers are more volatile than alcohols of the same molar mass because their molecules do not form hydrogen bonds to one another (Fig. 19.2). They are also less soluble in water because they have a lower ability to form hydrogen bonds to water molecules. Because ethers are not very reactive and have low molecular polarity, they are useful solvents for other organic compounds. However, ethers are flammable diethyl ether is easily ignited and must be used with great care. 

Before leaving the subject of distribution of electrons within molecules, and its attribution to the origin of molecular polarity, with consequent effect on intermolec-ular forces (with further consequent effects on solubilities and melting points), it is pertinent to remind ourselves of two significant challenges faced by chemistiy instractors (i) to graphically represent forces of attraction between molecules and (ii) to develop the imagery that in the liquid state, orientation of molecules toward each other because of polarities is transitory, even if more probable, as they move past each other. 

Ertl P, Rohde B, Selzer P. Fast calculation of molecular polar surface area as a sum of fragment-based contributions and its application to the prediction of drug transport properties. I Med Chem 2000 43 3714-7. 

Dipole moments depend on bond polarities. For example, the trend in dipole moments for the hydrogen halides follows the trend in electronegativity differences the more polar the bond (indicated by Ax), the larger the molecular polarity (indicated by the dipole moment, fi ... 

Predicting and sketching molecular shapes Predi cting and explaining bond angles Identifying molecular polarity... 

These boiling points can be explained in terms of dispersion forces and dipolar forces. First, assess the magnitudes of dispersion forces, which are present in all substances, and then look for molecular polarity. 

Masia M, Probst M, Rey R (2004) On the performance of molecular polarization methods. I. Water and carbon tetrachloride close to a point charge. J Chem Phys 121(15) 7362-7378... 

The general or universal effects in intermolecular interactions are determined by the electronic polarizability of solvent (refraction index n0) and the molecular polarity (which results from the reorientation of solvent dipoles in solution) described by dielectric constant z. These parameters describe collective effects in solvate s shell. In contrast, specific interactions are produced by one or few neighboring molecules, and are determined by the specific chemical properties of both the solute and the solvent. Specific effects can be due to hydrogen bonding, preferential solvation, acid-base chemistry, or charge transfer interactions. 

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