Guide Specific Intermolecular Interactions of Organic Compounds

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Therefore, it is part of the evaluation margins of the values of enthalpy characteristics of vaporization. The implementation of structural-energetic analysis depends on the information of the structural parameters of the crystal and liquid conditions as the enthalpy and entropy of vaporization are interrelated with the energy contribution of each func- tional group or, more precisely, with the bond vacancy forming the specific interac- tion with pseudo-atomic essentially unshared 2s2-electron pair of carbon atoms, lying in a pentacoordinated condition; with hydrogen atoms, forming a hydrogen bond; and with atoms of oxygen and nitrogen, forming hydrogen or specific interactions dependending on the second participant of this interaction.

Considering the structure of the liquid, being a unique system of where the elementary volume acquires a definite sense containing extremely few particles on condition of its subordination to the law of statistics [73], knowledge of the numbers and types of bond vacancies and the interacting particles lead to the possibility of determining the energies of specific interactions. The correct choice of the specific interactions in combination with other parameters for example, with the results of the spectral method of research predetermines the ground for the network of corresponding interactions.

In the choice of a probabilistic structure of liquid it is necessary to ignore the model of sp3- hybridization of the carbon atom and take into account the presence of the essentially unshared 2s2 electron pair, forming a specific interaction and providing the carbon atom with the pentacoordinated condition.

In Fig. Note that at the melting temperature of the crystalline and liquid condition the compounds have insignificant differences. Cocoa butter is a mixture of triglycerides in which stearoyl, oleoyl and palmitoyl groups predominate. It is the polymorphs of this matrix that influence the quality of chocolate. Low melting polymorphs feel too sticky or thick in the mouth.

Unfortunately, the higher melting form VI is more stable and is produced over time. Water Solubility.

Polar & Non-Polar Molecules: Crash Course Chemistry #23

Water has been referred to as the "universal solvent", and its widespread distribution on this planet and essential role in life make it the benchmark for discussions of solubility. Water dissolves many ionic salts thanks to its high dielectric constant and ability to solvate ions. The former reduces the attraction between oppositely charged ions and the latter stabilizes the ions by binding to them and delocalizing charge density.

Many organic compounds, especially alkanes and other hydrocarbons, are nearly insoluble in water. Organic compounds that are water soluble, such as most of those listed in the above table, generally have hydrogen bond acceptor and donor groups. Even so, diethyl ether is about two hundred times more soluble in water than is pentane.

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The chief characteristic of water that influences these solubilities is the extensive hydrogen bonded association of its molecules with each other. This hydrogen bonded network is stabilized by the sum of all the hydrogen bond energies, and if nonpolar molecules such as hexane were inserted into the network they would destroy local structure without contributing any hydrogen bonds of their own.

Of course, hexane molecules experience significant van der Waals attraction to neighboring molecules, but these attractive forces are much weaker than the hydrogen bond. Consequently, when hexane or other nonpolar compounds are mixed with water, the strong association forces of the water network exclude the nonpolar molecules, which must then exist in a separate phase.

This is shown in the following illustration, and since hexane is less dense than water, the hexane phase floats on the water phase. It is important to remember this tendency of water to exclude nonpolar molecules and groups, since it is a factor in the structure and behavior of many complex molecular systems. A common nomenclature used to describe molecules and regions within molecules is hydrophilic for polar, hydrogen bonding moieties and hydrophobic for nonpolar species.

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Associated Data

Return to Table of Contents. More about Intermolecular Forces. The attractive forces that exist between molecules are responsible for many of the bulk physical properties exhibited by substances. Some compounds are gases, some are liquids, and others are solids. The melting and boiling points of pure substances reflect these intermolecular forces, and are commonly used for identification.

Of these two, the boiling point is considered the most representative measure of general intermolecular attractions. Thus, a melting point reflects the thermal energy needed to convert the highly ordered array of molecules in a crystal lattice to the randomness of a liquid. Spherically shaped molecules generally have relatively high melting points, which in some cases approach the boiling point, reflecting the fact that spheres can pack together more closely than other shapes.

Boiling points, on the other hand, essentially reflect the kinetic energy needed to release a molecule from the cooperative attractions of the liquid state so that it becomes an unencumbered and relative independent gaseous state species. This attractive force has its origin in the electrostatic attraction of the electrons of one molecule or atom for the nuclei of another, and has been called London dispersion force. The following animation illustrates how close approach of two neon atoms may perturb their electron distributions in a manner that induces dipole attraction.

The induced dipoles are transient, but are sufficient to permit liquefaction of neon at low temperature and high pressure. In general, larger molecules have higher boiling points than smaller molecules of the same kind, indicating that dispersion forces increase with mass, number of electrons, number of atoms or some combination thereof. The following table lists the boiling points of an assortment of elements and covalent compounds composed of molecules lacking a permanent dipole.

The number of electrons in each species is noted in the first column, and the mass of each is given as a superscript number preceding the formula. Two ten electron molecules are shown in the first row. Methane is composed of five atoms, and the additional nuclei may provide greater opportunity for induced dipole formation as other molecules approach. The ease with which the electrons of a molecule, atom or ion are displaced by a neighboring charge is called polarizability , so we may conclude that methane is more polarizable than neon.

In the second row, four eighteen electron molecules are listed. The remaining examples in the table conform to the correlation of boiling point with total electrons and number of nuclei, but fluorine containing molecules remain an exception. The anomalous behavior of fluorine may be attributed to its very high electronegativity. The fluorine nucleus exerts such a strong attraction for its electrons that they are much less polarizable than the electrons of most other atoms.

Of course, boiling point relationships may be dominated by even stronger attractive forces, such as those involving electrostatic attraction between oppositely charged ionic species, and between the partial charge separations of molecular dipoles. Molecules having a permanent dipole moment should therefore have higher boiling points than equivalent nonpolar compounds, as illustrated by the data in the following table.

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In the first row of compounds, ethane, ethene and ethyne have no molecular dipole, and serve as useful references for single, double and triple bonded derivatives that do. Formaldehyde and hydrogen cyanide clearly show the enhanced intermolecular attraction resulting from a permanent dipole. Methyl fluoride is anomalous, as are most organofluorine compounds. In the second and third rows, all the compounds have permanent dipoles, but those associated with the hydrocarbons first two compounds in each case are very small.

Large molecular dipoles come chiefly from bonds to high-electronegative atoms relative to carbon and hydrogen , especially if they are double or triple bonds. Thus, aldehydes, ketones and nitriles tend to be higher boiling than equivalently sized hydrocarbons and alkyl halides. The atypical behavior of fluorine compounds is unexpected in view of the large electronegativity difference between carbon and fluorine.

When a hydrogen atom is part of a polar covalent bond to a more electronegative atom such as oxygen, its small size allows the positive end of the bond dipole the hydrogen to approach neighboring nucleophilic or basic sites more closely than can components of other polar bonds.

5. Specific Intermolecular Interactions of Sulfur, Oxygen, and Nitrogenated Organic Compounds

The table of data on the right provides convincing evidence for hydrogen bonding. In each row the first compound listed has the fewest total electrons and lowest mass, yet its boiling point is the highest due to hydrogen bonding. Other compounds in each row have molecular dipoles, the interactions of which might be called hydrogen bonding, but the attractions are clearly much weaker. The first two hydrides of group IV elements, methane and silane, are listed in the first table above, and do not display any significant hydrogen bonding.

Organic compounds incorporating O-H and N-H bonds will also exhibit enhanced intermolecular attraction due to hydrogen bonding.

Bibliographic Information

Some examples are given below. Water is the single most abundant and important liquid on this planet. The miscibility of other liquids in water, and the solubility of solids in water, must be considered when isolating and purifying compounds. To this end, the following table lists the water miscibility or solubility of an assortment of low molecular weight organic compounds.

Specific Intermolecular Interactions of Element-Organic Compounds -

The influence of the important hydrogen bonding atoms, oxygen and nitrogen is immediately apparent. The fundamentals of an unconventional approach to the theory of H-bonding and specific interactions are formulated based on a concept of pentacoordinate carbon atoms. New types of hydrogen bonds and specific interactions are substantiated and on the basis of the developed methodology their energies are determined.

The system of interconnected quantitative characteristics of the stability of specific intermolecular interactions is presented. The laws of their transformations are discussed and summarized. The new concept of the extra stabilizing effect of isomeric methyl groups on the structure and stability of organic molecules is introduced and the destabilization action on specific interactions is outlined.

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