Consider the following organic substances: ethanol, propane, hexane, and propanol. (a) Which of these molecules contains an OH group? (b) Which of these molecules contains three carbon atoms?

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The presence of a functional group gives an organic compound its reactivity and properties.

For instance, the OH group, also known as the hydroxyl group, is a common functional group in organic chemistry. It consists of an oxygen atom bonded to a hydrogen atom (–OH). In the exercise, ethanol and propanol were identified as the molecules containing this functional group. The hydroxyl group is characteristic of alcohols, like the given examples, and dictates how these compounds will behave in chemical reactions.

Identifying functional groups within organic molecules is crucial for understanding their behavior and predicting the outcomes of chemical reactions. By knowing that ethanol and propanol contain an OH group, students can infer that these compounds will have similar properties, such as being polar and having the ability to form hydrogen bonds with water, which affects their solubility and boiling points.

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. The molecular structure determines the physical and chemical properties of the compound and how it interacts with other molecules. The arrangement of atoms dictates the molecular geometry, which can be linear, trigonal planar, tetrahedral, and so on.

In the organic substances discussed in the exercise—ethanol, propane, hexane, and propanol—the structure of these molecules can be visualized by drawing their chemical structures. These visual representations help us understand aspects such as the molecule's size, shape, functional groups, and the connectivity of atoms.

Understanding molecular structure is key for students as it allows them to predict the molecule's properties and the types of chemical reactions it can undergo. For example, hexane's long-chain structure makes it non-polar and hydrophobic, meaning it doesn't mix well with water, as opposed to ethanol and propanol, which have hydroxyl groups making them more polar and hydrophilic.

Carbon atoms are the backbone of organic compounds. They can form four covalent bonds, allowing for a diversity of complex organic molecules. The number of carbon atoms within a molecule is a defining characteristic and can indicate the substance's class and name.

In the exercise, propane and propanol were identified as the substances containing three carbon atoms. This means that both propane and propanol are classified as a part of the 'prop-' family (from the Greek word for 'three'), which signifies organic molecules with a three-carbon atom chain. The structural formulae provided in the solution (\(C_{3}H_{8}\) for propane and \(C_{3}H_{7}OH \) for propanol) show the carbon atoms and their connections.

Structural isomerism is an important concept here, where compounds like propane and propanol have the same number of carbon atoms but differ in arrangement and functional groups. This isomerism results in varied physical and chemical properties that students need to understand when studying organic chemistry.