1. Organic Molecules and Functional Groups A functional group is an atom or a group of atoms with characteristic chemical and physical properties. It is the reactive part of the molecule. Most organic compounds have C—C and C—H bonds A functional group determines all of the following properties of a molecule:  Bonding and shape  Type and strength of intermolecular forces  Physical properties  Nomenclature  Chemical reactivity

2. 2 Ethane: This molecule has only C — C and C — H bonds, so it has no functional group. Ethane has no polar bonds, no lone pairs, and no  bonds, so it has no reactive sites. Consequently, ethane and molecules like it are very unreactive. Ethanol: This molecule has an OH group attached to its backbone. This functional group is called a hydroxy group. Ethanol has lone pairs and polar bonds that make it reactive with a variety of reagents. The hydroxy group makes the properties of ethanol very different from the properties of ethane.

3. 3 Aromatic hydrocarbons are so named because many of the earliest known aromatic compounds had strong characteristic odors. The simplest aromatic hydrocarbon is benzene. The six-membered ring and three  bonds of benzene comprise a single functional group. When a benzene ring is bonded to another group, it is called a phenyl group.

4. 4 An organic compound is soluble in water only if it contains one polar functional group capable of hydrogen bonding with the solvent for every five C atoms it contains. For example, compare the solubility of butane and acetone in H 2 O and CCl 4 : Physical Properties—Solubility