Functional groups: Properties and reactions

PROPERTIES AND REACTIONS

Key Notes

Properties

The presence of functional groups affect such properties as melting points, boiling points, polarity, dipole moments, and solubility. Molecules with strongly polar functional groups tend to have higher melting points and boiling points than molecules with nonpolar functional groups, and prefer to dissolve in polar solvents rather than nonpolar solvents.

Reactions

The sorts of reactions which compounds undergo are determined by the sorts of functional groups which are present. Functional groups undergo characteristic reactions, but the rates of these reactions are affected by stereoelectronic factors and conjugation.

Properties

The chemical and physical properties of an organic compound are determined by the sort of intermolecular bonding forces present, which in turn depends on the functional group present. A molecule such as ethane has a low boiling point and is a gas at room temperature because its molecules are bound together by weak van der Waals forces (Fig. 1a). In contrast, methanol is a liquid at room temperature since hydrogen bonding is possible between the alcoholic functional groups (Fig. 1b).

The polarity of molecules depends on which functional groups are present. A molecule will be polar and have a dipole moment if it contains polar functional groups such as an alcohol, amine, or ketone. Polarity also determines solubility in different solvents. Polar molecules prefer to dissolve in polar solvents such as water or alcohols, whereas nonpolar molecules prefer to dissolve in nonpolar sol-vents such as ether and chloroform. Polar molecules which can dissolve in water are termed hydrophilic (water-loving) while nonpolar molecules are termed hydrophobic (water-hating).

In most cases, the presence of a polar functional group will determine the phys-ical properties of the molecule. However, this is not always true. If a molecule has a polar group such as a carboxylic acid, but has a long hydrophobic alkane chain, then the molecule will tend to be hydrophobic.

Reactions

The vast majority of organic reactions take place at functional groups and are characteristic of that functional group. However, the reactivity of the functional group is affected by stereoelectronic effects. For example, a functional group may be surrounded by bulky groups which hinder the approach of a reagent and slow down the rate of reaction. This is referred to as steric shielding. Electronic effects can also influence the rate of a reaction. Neighboring groups can influence the reactivity of a functional group if they are electron-withdrawing or electron-donating and influence the electronic density within the functional group. Conjugation and aromaticity also has an important effect on the reactivity of functional groups. For example, an aromatic ketone reacts at a different rate from an aliphatic ketone. The aromatic ring is in conjugation with the carbonyl group and this increases the stability of the overall system, making it less reactive