Organic Structures - Organic Chemistry Concepts: An EFL Approach - page 8

The most interesting bonding centers are usually carbon. As Figure 1.7 shows,
it is usual to indicate an inductive effect relative to the almost non-polar C-H
bond. The effect of other atoms or groups is then expressed as ±I.

FIGURE 1.7
Negative and positive inductive effects of carbon.

Note that an inductive effect refers to σ-bonded electrons only. The σ-bonded
electrons are localized . This means that they are found mostly between the
bonded nuclei. Because of this, an inductive effect is only felt over very short
distances, and is almost gone after one bond. Later chapters use the inductive
effect in discussions of molecular properties and reactivity.

1.6 FORCES BETWEEN MOLECULES
In ionic compounds, electrostatic attraction causes the ions to form large three-
dimensional arrangements called crystals. For organic compounds, in which the
bonding is mostly covalent, the unit is usually an uncharged single molecule.
The relatively weak attractive intermolecular interactions, the van der Waals
forces, between these molecules are of three types:

dipole/dipole (includes hydrogen bonding)

n

dipole/induced-dipole

n

induced-dipole/induced-dipole. n

These intermolecular forces break down at lower temperatures (lower energy)
than for ionic compounds. As a result, organic compounds generally have lower
boiling and melting points than inorganic compounds.

The strength of the intermolecular interactions depends on the polarization of
various parts of the organic molecule. One cause of polarization is the inductive
effects that come from the presence of electronegative heteroatoms. This polar-
ization leads to dipole/dipole interactions. Also, a dipole can affect the electron
field in a part of any nearby molecule. This can cause an induced-dipole to form
and lead to dipole/induced-dipole interactions.

Even non-polar molecules can have temporary distortions in their electron fields.
These short-lived induced-dipoles can cause distortions in a part of other nearby
molecules. As shown in Figure 1.8 , this can lead to induced-dipole/induced-
dipole interactions. Extended induced-dipole/induced-dipole interactions over
many molecules can add up to give significant intermolecular attraction.