Intermolecular forces Relatively weak hydrogen bonding

B. The best solvents for a solute have intermolecular bonds of similar strength to the solute ( like dissolves like ). Bromine is a non-polar molecule with intermolecular attractions due to weak London dispersion forces. The relatively strong hydrogen bonding in H20 and NH3 and the very strong electrostatic attractions in molten NaCI would make each of them a poor solvent for Br2 because these molecules would prefer to remain attracted to one another. CS2 is a fairly small non-polar molecule. 

Molecular solids consist of atoms or molecules held together by dipole—dipole forces, dispersion forces, and/or hydrogen bonds. Because the.se intermolecular forces are weak, molecular solids are soft and have relatively low melting points (usually below 200 °C). Most substances that are gases or liquids at room temperature form molecular solids at low temperature. Examples include Ar, H2O, and CO2. 

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

Covalent compounds, by contrast, are low-melting solids, liquids, or even gases. A sample of a covalent compound, such as hydrogen chloride, consists of discrete HCI molecules. The covalent bond within an individual molecule may be very strong, but the attractive forces between the different molecules are fairly weak. As a result, relatively little energy is required to overcome these forces and cause a covalent compound to melt or boil. We ll look at the nature of intermolecular forces and the boiling process in Chapter 10. 

These principles are best recognized when studying relatively simple molecular systems that have an ability to exploit weak interactions to create structure. Among many, peptides are the perfect choice for such studies considering their versatility in make up given the 20+ natural and synthetic amino acid residues, and their functional diversity. In addition, the amino acid sequence of the primary structure combined with the ability of forming secondary (3-sheet or a-helix structures provide substantial room for the creation of hierarchical structures based on weak intermolecular forces, mainly hydrogen bonds. A limited sequence of residues also prevents additional complication from tertiary and quaternary structures as seen with proteins. 

A nonpolar liquid like heptane (C7H16) has intermolecular bonds with relatively weak London dispersion forces. Heptane is immiscible in water because the attraction that water molecules have for each other via hydrogen bonding is too strong. Unlike Na+ and CP ions, heptane molecules cannot break these bonds. Because bonds of similar strength must be broken and formed for solvation to occur, nonpolar substances tend to be soluble in nonpolar solvents, and ionic and polar substances are soluble in polar solvents like water. Polar molecules are often called hydrophilic and non-polar molecules are called hydrophobic. This observation is often stated as like dissolves like. Network solids (e.g., diamond) are soluble in neither polar nor nonpolar solvents because the covalent bonds within the solid are too strong for these solvents to break. 

The spontaneous formation of structures by self-assembly relies on a delicate balance of relatively weak intermolecular forces such as the hydrophobic force, electrostatic and van der Waals interactions, and hydrogen bonding. Building blocks for self-assembly can range from small atoms to much larger macroscale objects. A wide variety of two- and three-dimensional... 

---