Give the structure of (a) a three-carbon alcohol with the -OH group on the center carbon. (b) a four-carbon branched-chain carboxylic acid. (c) the ester formed when these two compounds react.

Understanding the structure of alcohols is crucial in organic chemistry, as they are a common functional group in many organic compounds. An alcohol is characterized by the presence of one or more hydroxyl (-OH) groups attached to a saturated carbon atom. The simplest alcohols are methanol and ethanol, with one and two carbon atoms, respectively.

When discussing a three-carbon alcohol with the -OH group on the center carbon, as in the exercise, we're referring to a compound with a specific placement of the hydroxyl group. This particular structure is known as isopropanol or 2-propanol, where the number 2 indicates the position of the -OH group on the second carbon atom. The carbon chain can be visualized as a straight line of three carbon atoms (C-C-C), with the central carbon bonded to the hydroxyl group. Hydrogen atoms are added to ensure that each carbon forms a total of four bonds, adhering to the octet rule.

The correct drawing of isopropanol would look like this: CH₃CHOHCH₃. It's pivotal that the hydroxyl group is correctly placed to ensure proper identification and understanding of the molecule's properties and reactivity.

Carboxylic acids are another fundamental functional group in organic chemistry, recognized by the carboxyl group (-COOH) within their structure. This group consists of a carbon double-bonded to an oxygen atom and also bonded to a hydroxyl (OH) group. In a four-carbon branched-chain carboxylic acid, we have more complexity due to the branching.

The branching creates an additional layer of structural variety, making it imperative to understand the nomenclature and drawing of such molecules. The carboxyl group is often placed at the end of the longest carbon chain, named according to the number of carbon atoms. For instance, a four-carbon branched-chain carboxylic acid might have a three-carbon main chain and a methyl group as a branch on the second carbon. Such a compound is named 2-methylpropanoic acid, where '2-methyl' denotes the position and nature of the branch, and 'propanoic acid' reflects the carboxylic acid with three carbon atoms in the main chain.

The structure of 2-methylpropanoic acid would be drawn as CH₃C(CH₃)COOH, highlighting the importance of including the carboxyl group and branch in accurate locations to depict the correct molecule.

Ester formation, known as esterification, is a key reaction in organic chemistry that involves the combination of an alcohol and a carboxylic acid to form an ester. This reaction typically occurs in the presence of an acid catalyst and is generally reversible. In the formation of an ester, the hydroxyl group (-OH) from the carboxylic acid and a hydrogen atom from the alcohol's hydroxyl group combine to release a water molecule, leaving the remaining parts of the two reactants to join together.

The general structure of an ester is denoted as RCOOR', where 'RCOO' represents the acyloxy part coming from the carboxylic acid minus the OH, and 'R'' denotes the remaining alkyl group from the alcohol. The specific ester derived from the given three-carbon alcohol, isopropanol, and the four-carbon branched-chain carboxylic acid, 2-methylpropanoic acid, would be termed isopropyl 2-methylpropanoate, with the structure CH₃C(CH₃)COOCH₂CH₃. It's crucial to accurately remove the correct atoms from the reactants to form the ester structure and the by-product water. Understanding esterification enhances one's ability to predict and identify the outcomes of such organic synthesis reactions.