Structure and Physical Properties

Return to the Organic Chemistry Page.
<= Back Next =>
To predict the properties from molecular structure is one of the goals of organic chemistry. Functional groups, heteroatoms, and molecular size influence physical properties in ways that permit some broad generalizations.

Effects of Functional Groups on Solubility in Water

Piperidine and tetrahydropyran, shown in the previous section, are both soluble in water, but cyclohexane and other alkanes are insoluble. The reason why is because, the alkanes have only C-C and C-H bonds, which are relatively nonpolar and do not tend to form hydrogen bonds. Hydrogen bonds usually form when the H is attached to O or N atoms, two of the three most electronegative atoms. The delta- charge (relatively negative charge) of the O and the N atoms attract the delta+ charge of the H, since their is a sizable difference in their electronegativities.
Since piperidine and tetrahydropyran have heteroatoms of N or O, the water molecules (and other polar fluids) tend to cluster around each molecule. When they cluster around them, the delta+ charge of the H in water tends to form hydrogen bonds (shown as · ·) with the delta- charge in the N or O. Therefore, the molecules of piperidine and tetrahydropyran become dissolved in the water.
Piperidine hydrogen bonded to two water molecules
Tetrahydropyran hydrogen bonded to a water molecule
Because alkanes are relatively nonpolar, they do not dissolve in water, a polar molecule, because hydrogen bonds do not form and water does not cluster around each alkane molecule.

Effects of Molecular Size on Physical Properties

A molecule of cholesterol, which has an OH group, is expected to dissolve in water, because it is polar due to its OH group. But it is practically insoluble, because since the large size of the alkane-like part of the molecule, the polarity of the OH group is overwhelmed by the huge hydrocarbon group. It is more soluble in nonpolar solvents.
Next =>