Question

Write the structural formula for the product of (a) the reaction of glycerol \((1,2,3\)-trihydroxypropane) with stearic acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH}\), to produce a saturated fat; (b) the oxidation of 4-hydroxybenzyl alcohol by sodium dichromate in an acidic organic solvent.

Short answer

The structural formula of the product for (a) is a triglyceride formed by three ester bonds between the glycerol and three stearic acid molecules. The structural formula for (b) is 4-hydroxybenzoic acid, resulting from the oxidation of the benzyl alcohol to a carboxylic acid.

Step-by-step solution

- Understanding the Reaction Between Glycerol and Stearic Acid

Recognize that the reaction between glycerol and stearic acid is an esterification reaction. For every hydroxyl (-OH) group on glycerol, a molecule of stearic acid can react to form an ester bond, releasing a water molecule. Glycerol has three hydroxyl groups, so it can react with three stearic acid molecules.

- Writing the Structural Formula for the Saturated Fat

Draw the glycerol backbone with its three hydroxyl groups. Then, draw three stearic acid molecules, but replace the hydroxyl group of each with an oxygen linking to the hydroxyl groups on glycerol to form ester bonds. The resulting compound is a triglyceride, a type of saturated fat.

- Understanding the Oxidation of 4-Hydroxybenzyl Alcohol

Recognize that the oxidation of 4-hydroxybenzyl alcohol by sodium dichromate in an acidic environment typically leads to the formation of a carbonyl compound, specifically an aldehyde or carboxylic acid. Given the reagent's strength and the reaction conditions, a carboxylic acid is the likely product.

- Writing the Structural Formula for the Oxidation Product

The benzene ring from 4-hydroxybenzyl alcohol remains intact. The hydroxyl group remains unchanged, as it is on the benzene ring, and is not the primary target of the oxidation. The benzyl alcohol functional group is oxidized to a carboxylic acid. Place a carboxyl group (-COOH) where the -CH2OH group was initially located on the benzene ring.

Key concepts

Esterification Reaction

An esterification reaction is a key organic process where an alcohol and a carboxylic acid combine to form an ester and water. This type of reaction is integral to the synthesis of various fats and oils and plays a crucial role in organic synthesis. In an esterification process, the hydroxyl (-OH) group from the acid and a hydrogen atom from the alcohol's hydroxyl group are removed, forming a molecule of water ( \[\mathrm{H}_2\mathrm{O}\]) and leaving behind an ester linkage.

For example, when glycerol reacts with stearic acid, each of the three hydroxyl groups on the glycerol can link with a stearic acid molecule, yielding triglycerides - the primary constituents of many saturated fats. As water is released, the immediate product, known as a monoglyceride, further reacts until all three hydroxyl groups are esterified, resulting in a triglyceride.

Saturation of Fats

The concept of saturation in fats refers to the absence of double bonds between the carbon atoms in the fatty acid chains. In saturated fats, all carbon atoms are fully 'saturated' with hydrogen atoms, creating a straight chain with no double bonds. This structural makeup allows the fat molecules to pack closely together, making them solid at room temperature.

In contrast, unsaturated fats contain one or more double bonds, which introduce kinks in the fatty acid chains, preventing tight packing and typically resulting in a liquid state at room temperature. Saturated fats, such as the product of glycerol and stearic acid esterification, are common in animal fats and some vegetable fats, and have different health and nutritional implications compared to unsaturated fats.

Oxidation Reaction

Oxidation reactions in organic chemistry involve the loss of electrons by a molecule, atom, or ion. In the context of organic compounds, this often translates to an increase in the oxygen content or a decrease in hydrogen content, changing the organic molecule's functional groups.

For example, in the oxidation of alcohols, primary alcohols can be oxidized to aldehydes and further to carboxylic acids, depending on the reaction conditions and the strength of the oxidizing agent. Sodium dichromate, a strong oxidizing agent used in an acidic environment, is potent enough to convert 4-hydroxybenzyl alcohol all the way to a carboxylic acid by replacing the alcohol group with a carbonyl functional group ( \[-\mathrm{COOH}\]).

Structural Formula

The structural formula of an organic compound is a graphical representation that describes the exact number and location of atoms, the types of bonds between them, and the overall geometry of the molecule. It serves as a blueprint that allows chemists to visualize how atoms are organized and how they are connected within a molecule.

Unlike a simple molecular formula, which only indicates the number of each type of atom present, the structural formula conveys the molecule's structure, which is critical for understanding the substance's properties and reactivity. One can represent structures using various methods, including line-angle formulas and condensed structures to show the complete spatial arrangement of atoms.

Triglyceride Synthesis

Triglyceride synthesis is a vital biological process and a fundamental concept in nutrition and biochemistry. It results in the formation of triglycerides, which are the main form of stored energy in animals and an essential part of human nutrition.

As seen in the esterification of glycerol and fatty acids like stearic acid, triglyceride synthesis is a sequential process. Starting with glycerol, each of its three hydroxyl groups forms an ester linkage with the carboxylic acid portion of a fatty acid. The resulting triglycerides can differ widely in their properties, such as melting point and solubility, depending on the nature and saturation of the constituent fatty acids. Importantly, triglycerides are a form of long-term energy storage, and during metabolic needs, the body breaks them down into fatty acids and glycerol to be used as an energy source.