INTER MOLECULAR BONDING
Intermolecular bonding takes place between different molecules. This can take the form of ionic bonding, hydrogen bonding, dipole–dipole inter-actions and van der Waals interactions. The type of bonding involved depends on the functional groups present.
Ionic bonds are possible between ionized functional groups such as car-boxylic acids and amines.
Intermolecular hydrogen bonding is possible for alcohols, carboxylic acids, amides, amines, and phenols. These functional groups contain a hydrogen atom bonded to nitrogen or oxygen. Hydrogen bonding involves the inter-action of the partially positive hydrogen on one molecule and the partially negative heteroatom on another molecule. Hydrogen bonding is also possi-ble with elements other than nitrogen or oxygen.
Dipole–dipole interactions are possible between molecules having polariz-able bonds, in particular the carbonyl group (C=O). Such bonds have a dipole moment and molecules can align themselves such that their dipole moments are parallel and in opposite directions. Ketones and aldehydes are capable of interacting through dipole–dipole interactions.
van der Waals interactions are weak intermolecular bonds between regions of different molecules bearing transient positive and negative charges. These transient charges are caused by the random fluctuation of electrons. Alkanes, alkenes, alkynes and aromatic rings interact through van der Waals interactions.
Intermolecular bonding is the bonding interaction which takes place
between different molecules. This
can take the
form of ionic bonding,
hydrogenbonding, dipole–dipole interactions or van der Waals interactions. These bonding forces are weaker than
the covalent bonds,but they have an important influence on the physical and
biological properties of a compound.
Ionic bonding takes place between molecules having opposite charges and involves an electrostatic interaction between the two opposite charges. The functional groups which most easily ionize are amines and carboxylic acids (Fig. 1).
Ionic bonding is possible between a molecule containing an ammonium ion and a molecule containing a carboxylate ion. Some important naturally occurring mol-ecules contain both groups – the amino acids. Both these functional groups are ionized to form a structure known as a zwitterion (a neutral molecule bearing both a positive and a negative charge) and intermolecular ionic bonding can take place (Fig. 2).
Hydrogen bonding can take place when molecules have a hydrogen atom attached to a heteroatom such as nitrogen or oxygen. The common functional groups which can participate in hydrogen bonding are alcohols, phenols, carboxylic acids, amides, and amines. Hydrogen bonding is possible due to the polar nature of the N–H or O–H bond. Nitrogen and oxygen are more electronegative than hydrogen. As a result, the heteroatom gains a slightly negative charge and the hydrogen gains a slightly positive charge. Hydrogen bonding involves the partially charged hydrogen of one molecule (the H bonddonor) interacting with the partially charged heteroatom of another molecule (the H bond acceptor) (Fig. 3).
Dipole–dipole interactions are possible between polarized bonds other than N–H or O–H bonds. The most likely functional groups which can interact in this way are those containing a carbonyl group (C=O). The electrons in the carbonyl bond are polarized towards the more electronegative oxygen such that the oxygen gains a slight negative charge and the carbon gains a slight positive charge. This results in a dipole moment which can be represented by the arrow shown in Fig. 4. The arrow points to the negative end of the dipole moment. Molecules containing dipole moments can align themselves with each other such that the dipole moments are pointing in opposite directions (Fig. 4b).
van der Waals interactions are the weakest of the intermolecular bonding forces and involve the transient existence of partial charges in a molecule. Electrons are continually moving in an unpredictable fashion around any molecule. At any moment of time, there is a slight excess of electrons in one part of the molecule and a slight deficit in another part. Although the charges are very weak and fluctuate around the molecule, they are sufficiently strong to allow a weak interaction between molecules, where regions of opposite charge in different molecules attract each other.
Alkane molecules can interact in this way and the strength of the interaction increases with the size of the alkane molecule. van der Waals interactions are also important for alkenes, alkynes and aromatic rings. The types of molecules involved in this form of intermolecular bonding are ‘fatty’ molecules which do not dissolve easily in water and such molecules are termed hydrophobic (water-hating). Hydrophobic molecules can dissolve in nonpolar, hydrophobic solvents due to van der Waals interactions and so this form of intermolecular bonding is sometimes referred to as a hydrophobic interaction.