| MadSci Network: Chemistry |
I am explaining The differrence in much detail giving examples as well.Please let me know if you have further enquiry. van der Waals forces Relatively weak electric forces that attract neutral molecules to one another in gases, in liquefied and solidified gases, and in almost all organic liquids and solids. The forces are named for the Dutch physicist Johannes van der Waals, who in 1873 first postulated these intermolecular forces in developing a theory to account for the properties of real gases. Solids that are held together by van der Waals forces characteristically have lower melting points and are softer than those held togetherby the stronger ionic, covalent, and metallic bonds. Van der Waals forces may arise from three sources. First, the molecules of some materials,although electrically neutral, may be permanent electric dipoles. Because of fixed distortion in the distribution of electric charge in the very structure of some molecules, one side of a molecule is always somewhat positive and the opposite side somewhat negative. The tendency of such permanent dipoles to align with each other results in a net attractive force. Second, the presence of molecules that are permanent dipoles temporarily distorts the electron charge in other nearby polar or nonpolar molecules, thereby inducing further polarization. An additional attractive force results from the interaction of a permanent dipole with a neighbouring induced dipole. Third,even though no molecules of a material are permanent dipoles (e.g., in the noble gas argon or the organic liquid benzene), a force of attraction exists between the molecules, accounting for condensing to the liquid state at sufficiently low temperatures. The nature of the attractive force in molecules, which requires quantum mechanics for its correct description, was first recognized (1930) by the Polish-born physicist Fritz London, who traced it to electron motion within molecules. London pointed out that at any instant the centre of negative charge of the electrons and the centre of positive charge of the atomic nuclei would not be likely to coincide. Thus, the fluctuation of electrons makes molecules time-varying dipoles, even though the average of this instantaneous polarization over a brief time interval may be zero. Such time-varying dipoles, or instantaneous dipoles, cannot orient themselves into alignment to account for the actual force of attraction, but they do induce properly aligned polarization in adjacent molecules, resulting in attractive forces. These specific interactions, or forces, arising from electron fluctuations in molecules (known as London forces, or dispersion forces) are present even between permanently polar molecules and produce, generally, the largest of the three contributions to intermolecular forces. The hydrogen bond. The interactions described so far are not limited to molecules of any specific composition. However, there is one important intermolecular interaction specific to molecules containing an oxygen, nitrogen, or fluorine atom that is attached to a hydrogen atom. This interaction is the hydrogen bond, an interaction of the form A - H - B, where A and B are atoms of any of the three elements mentioned above and the hydrogen atom lies on a straight line between the nuclei of A and B. A hydrogen bond is about 10 times as strong as the other interactions described above,and when present it dominates all other types of intermolecular interaction. It is responsible, for example, for the existence of water as a liquid at normal temperatures; because of its low molar mass, water would be expected to be a gas. The hydrogen bond is also responsible for the existence as solids of many organic molecules containing hydroxyl groups (-OH); the sugars glucose and sucrose are examples. Many interpretations of the hydrogen bond have been proposed. One that fits into the general scheme of this article is to think of the A- H unit as being composed of an A atomic orbital and a hydrogen 1s orbital and to consider a lone pair of electrons on B as occupying a B orbital. When the three atoms are aligned, these three orbitals can form three molecular orbitals: one bonding, one largely non-bonding, and one anti-bonding There are four electrons to accommodate (two from the original A-H bond and two from the lone pair). They occupy the bonding and non-bonding orbitals, leaving the anti-bonding orbital vacant. Hence, the net effect is to lower the energy of the AHB grouping and thus to constitute an intermolecular bond. Once again, on encountering the hydrogen bond, one encounters a twist in the conventional attitude; the question raised by this interpretation is not why such a bond occurs but why it does not occur more generally. The explanation lies in the small size of the hydrogen atom, which enables the balance of energies in the molecular orbital scheme to be favourable to bonding. Hydrogen bonding occurs to atoms other than nitrogen, oxygen, and fluorine if they carry a negative charge and hence are rich in readily available electrons. Thus, hydrogen bonding is one of the principal mechanisms of hydration of anions in aqueous solution (the bonding of H O molecules to the solute species) and hence contributes to the ability of water to act as a good solvent for ionic compounds. It also contributes to the hydration of organic compounds containing oxygen or nitrogen atoms and thus accounts for the much greater aqueous solubility of alcohols than hydrocarbons. Hydrogen bonds are of great significance in determining the structure of biologically significant compounds, most notably proteins and deoxyribonucleic acid (DNA). An important feature of the structure of proteins (which are polypeptides, or polymers formed from amino acids) is the existence of the peptide link, the group -CO-NH-,which appears between each pair of adjacent amino acids. This link provides an NH group that can form a hydrogen bond to a suitable acceptor atom and an oxygen atom, which can act as a suitable receptor. Therefore, a peptide link provides the two essential ingredients of a hydrogen bond. Corey, who formulated a set of rules, the Pauling-Corey rules, for its implementation. The implication of these rules is the existence of two types of structure for a polypeptide, which is either a helical form (the helix) or a pleated sheet form (the pleated sheet). All polypeptides have one structure or the other and often have alternating regions of each. Since the properties and behaviour of an enzyme molecule (a particular class of polypeptides) are determined by its shape and, in particular, by the shape of the region where the molecule it acts on needs to attach, it follows that hydrogen bonds are centrally important to the functions of life. Hydrogen bonds are also responsible for the transmission of genetic information from one generation to another, for they are responsible for the specific keying together of cytosine with guanine and thymine with adenine moieties that characterizes the structure of the DNA double helix. Zia Khan
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