In Chapter 11, you are introduced to the concept of intermolecular forces. Previ

ID: 998057 • Letter: I

Question

In Chapter 11, you are introduced to the concept of intermolecular forces. Previously, you learned about ionic bonds and covalent bonds. These are bonds that exist between atoms within a single compound. Intermolecular forces are forces that exist between the compounds. They are weaker than ionic bonds and covalent bonds. However, they do play an extremely important role in determining the physical characteristics of compounds. Looking at Section 11.3 and Table 11.4 of your book, please explain the various intermolecular forces. Please give examples of compounds that exhibit each of these forces.

Explanation / Answer

We must remember that the particular method of mentally building molecules

that we are learning to use is artificial: it is a purely intellectual process involving

imaginary overlap of imaginary orbitals. There are other, equally artificial

ways that use different mental or physical models. Our method is the one. that so

far has seemed to work out best for the organic chemist. Our kit of mental atomic

models will contain just three "kinds" of carbon: tetrahedral sp3 -hybridized),

trigonal (sp2-hybridized), and digonal (sp -hybridized). By use of this kit, we shall

find, one can do an amazingly good job of building hundreds of thousands of

organic molecules.

But, however we arrive at it, we see the actual structure of a molecule to be

the net result of a combination of repulsive and attractive forces,, which are related

to charge and electron spin.

(a) Repulsive forces. Electrons tend to stay as far apart as possible because

they have the same charge and also, if they are unpaired, because they have the

same spin (Pauli exclusion principle). The like-charged atomic nuclei, too, repel

each other.

(b) Attractive forces. eg. intramolecular hydrogen bonding

Electrons are attracted by atomic nuclei as are the

nuclei by the electrons because of their opposite charge, and hence tend to occupy

the region between two nuclei. Opposite spin permits (although, in itself, probably

does not actually encourage) two electrons to occupy the same region.

In methane, for example, the four hydrogen nuclei are as widely separated

as they can be. The distribution of the eight bonding electrons is such that each

one occupies the desirable region near two nuclei the bond orbital and yet,

except for its partner, is as far as possible from the other electrons. We can picture

each electron accepting perhaps reluctantly because of their similar charges

one orbital-mate of opposite spin, but staying as far as possible from all other electrons

and even, as it wanders within the loose confines of its orbital, doing its

best to avoid the vicinity of its restless partner. We must remember that the particular method of mentally building molecules