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CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds Weaker Than Covalent- Addition Compounds

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CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds Weaker Than Covalent- Addition Compounds Subject Chemistry Paper No and Title Paper 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module No and Module 10: Bonds weaker than covalent- addition Title compounds Module Tag CHE_P1_M10 CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Non-Covalent Interactions and Associated Compounds 4. Summary CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds 1. Learning Outcomes After studying this module, you shall be able to Learn about different aspects of bonding involved in supramolecular chemistry Know about non covalent interactions Understand the concept and energies associated with different classes of non-covalent interactions. Analyse various types of compounds which are associated with non-covalent interactions. 2. Introduction Supramolecular chemistry is a highly interdisciplinary field of science covering the chemical, physical, and biological features. They are the assemblies of organized chemical species and have greater complexity than individual molecules themselves. These assemblies are held together and organized by means of intermolecular (non-covalent) binding interactions. In other words, supramolecular chemistry can also be defined as ‘chemistry of molecular assemblies and intermolecular non-covalent interactions’. Professor Jean-Marie Lehn, won the Nobel Prize in 1987 for his work in the area of supramolecular chemistry. The non-covalent interactions such as electrostatic interactions, hydrogen bonding and van der Waals forces define the inter component bond between the molecular individuals and populations. These non-covalent interactions are as important in supramolecular chemistry as covalent interactions in classical organic chemistry. The energy of these non-covalent interactions is much smaller than 200-400 kJ mol-1 which is typical for covalent chemical bonds. In addition to relatively strong ion-ion electrostatic interactions of ca. 100-350 kJ mol- 1 and hydrogen bonding of ca. 10-120 kJ mol-1, they include much smaller London dispersion forces, ion-induced dipole and dipole-dipole interactions that are in the range of 5-50 kJmol-1 strong. Supramolecular chemistry covers the crystals, solutions and also the polymers in which non-covalent interactions play an important role. In general, supramolecular chemistry involves the self-assembly and host-guest systems using a variety of interactions, some of which are clearly non-covalent (e.g. hydrogen bonds) and some of which possess a significant covalent component (e.g. metal–ligand interactions) CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds It also has diversified enormously and includes charge-transfer complexes, inclusion complexes (e.g. Cram's hemicarcerands and cyclodextrins), mono- and polylayers, micelles, vesicles, liquid crystals and cocrystals consisting of at least two different kinds of molecules which form highly specific domains. The specificity and separateness of the charge-transfer complexes and those of liquid crystals seem generally recognized. On the other hand, inclusion complexes or other molecular aggregates consisting of only few molecules, higher molecular aggregates, and cocrystals formed by at least two types of molecules the situation is not that clear. 3. Non-Covalent Interactions and Associated Compounds The supramolecular chemistry generally concerns non-covalent bonding interactions such as ion-ion interactions, ion-dipole interactions, dipole-dipole interactions, hydrogen bonding, cation-π interactions, anion-π interactions, π-π interactions, closed shell interactions, van der Waals forces, crystal close packing and closed shell interactions. The term ‘non-covalent’ encompasses an enormous range of attractive and repulsive effects. Following figure compares molecular chemistry with supramolecular chemistry. As shown, in molecular chemistry we study properties such as chemical nature, shape, redox properties, polarity, magnetism etc and in supramolecular chemistry, we study properties like recognition capability, catalysis, transportation, degree of order etc.(the properties specific to the molecular assembly) CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds Fig. 1: Comparison between the scope of molecular and supramolecular chemistry according to Lehn Let us study some of the most common types of non-covalent interactions. 3.1 Ion-Ion Interactions: The ion-ion interaction as in ionic cubic lattice of solid sodium chloride in which each Na+ cation is surrounded by six Cl- anions. The Na+ cation is able to organise six complementary donor atoms about itself in order to maximize non-covalent ion–ion interactions. The ionic cubic lattice of solid sodium chloride breaks down in solution because of solvation effects to + give species such as the labile, octahedral Na(H2O)6 . The bond strength of ionic bonding is comparable in strength to covalent bonding and bond energy in the range of 100–350 kJ mol- 1. Fig. 2: (a) The ionic cubic lattice of solid sodium chloride (b) showing supramolecular ion-ion interactions by anion and an organic cation and (c) showing the supramolecular ion-dipole interactions by metal cation and crown ether 3.2 Ion-Dipole Interactions: The bonding of an ion, such as Na+, with a polar molecule, such as water, is an example of an ion–dipole interaction, which range in strength from ca. 50 – 200 kJ mol-1. This kind of bonding is seen both in the solid state and in solution. The ion-dipole interactions in supramolecular structures of the complexes of alkali metal cations with macrocyclic (large CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds ring) ethers termed crown ethers is shown in figure 3. The ether oxygen atoms play the same role as that of polar water molecules, although the complex is stabilized by the chelate effect and the effects of macrocyclic pre-organization. Fig. 3: Supramolecular ion-dipole interactions by metal cation and crown ether 3.3 Dipole-Dipole Interactions: Alignment of one dipole with another can result in significant attractive interactions from matching of either a single pair of poles on adjacent molecules (type I) or opposing alignment of one dipole with the other (type II) (Figure 4). The energies for such interactions lie in the range 5–50 kJ mol-1. Organic carbonyl compounds show this behaviour well in the solid state and calculations have suggested that type II interactions have an energy ~20 kJ mol-1 which is comparable to a moderately strong hydrogen bond. The boiling point of ketones such as acetone (56 ºC), however, demonstrates that dipole–dipole interactions of this type are relatively weak in solution. Fig. 4: Dipole-dipole interactions in carbonyls CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds 3.4 Hydrogen Bonding A hydrogen bond (H-bond) may be regarded as a particular kind of dipole–dipole interaction in which a hydrogen atom attached to an electronegative atom (or electron withdrawing group) is attracted to a neighbouring dipole on an adjacent molecule or functional group. H- bonds are commonly written D–H··A and usually involve a hydrogen atom attached to an electronegative atom such as O or N as the donor (D) and a similarly electronegative atom, often bearing a lone pair, as the acceptor (A). The typical range for H-bond strength is from ca. 10–120 kJ mol-1 and the typically H-bonded O···O distances are 2.50–2.80 Å in length, though interactions in excess of 3.0 Å may also be significant. An excellent example of H- bonding in supramolecular chemistry is the formation of carboxylic acid dimers. Figure 5(a), which results in the shift of the ν(OH) infrared stretching frequency from about 3400 cm-1 to about 2500 cm-1, accompanied by a significant broadening and intensifying of the absorption. H-bonds are ubiquitous in supramolecular chemistry. In particular, H-bonds are responsible for the overall shape of many proteins, recognition of substrates by numerous enzymes (along with π-π stacking interactions) and also for the double helix structure of DNA. Fig. 5: Supramolecular Hydrogen bonding (H-bonding) interactions in (a) carboxylic acid dimers (b) organic molecule and (c) & (d) guanine and cytosine base pairs of DNA 3.5 π-π interactions The π-π interactions occur between aromatic rings so sometimes it is also called aromatic π-π stacking interactions, often in situations where one is relatively electron rich and one is electron poor. The typical range for π-π interactions strength is from ca. 50–500 kJ mol-1. There are two types of π-interactions: face-to-face and edge-to-face (Figure 6). Face-to-face π-stacking interactions are responsible for the slippery feel of graphite and its useful lubricant properties. Similar π-stacking interactions between the aryl rings of nucleobase pairs also CHEMISTRY Paper No. 1: ORGANIC CHEMISTRY- I (Nature of Bonding and Stereochemistry) Module 10: Bonds weaker than covalent- addition compounds help to stabilize the DNA double helix. Edge-to-face interactions may be regarded as weak forms of
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