Problem 86 - Chemistry: The Central Science

Question

Write a condensed structural formula for each of the following: (a) an acid with the formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\), (b) a cyclic ketone with the formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}\), (c) a dihydroxy compound with formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{2}\), (d) a cyclic ester with formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2}\)

Step-by-Step Solution

Answer

(a) CH\(_3\)CH\(_2\)CH\(_2\)COOH; (b) cyclopentanone, C_5H_8O; (c) CH\(_2\)OH-CH\(_2\)-CH\(_2\)OH; (d) \(\delta\)-valerolactone, C_5H_8O_2.

Step 1: (a) Acid with the formula \(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{2}\) (Acid group: Carboxylic Acid)

Since the compound is an acid, it must contain a carboxylic acid functional group (-COOH). The condensed structural formula is: CH\(_3\)CH\(_2\)CH\(_2\)COOH The structural formula indicates a carboxylic acid with 4 carbon atoms.

Step 2: (b) Cyclic ketone with the formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}\) (Ketone group: C=O)

Since the compound is a cyclic ketone, it must contain a ketone functional group (C=O) within a cyclic structure. The condensed structural formula is: cyclopentanone, which is represented as C_5H_8O. The structural formula indicates a cyclopentanone with 5 carbon atoms within the ring.

Step 3: (c) Dihydroxy compound with the formula \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}_{2}\) (Two hydroxyl groups: -OH)

Since it's a dihydroxy compound, it must contain two hydroxyl groups (-OH). The condensed structural formula is: CH\(_2\)OH-CH\(_2\)-CH\(_2\)OH The structural formula indicates a dihydroxy compound with 3 carbon atoms and two hydroxyl groups.

Step 4: (d) Cyclic ester with the formula \(\mathrm{C}_{5} \mathrm{H}_{8} \mathrm{O}_{2}\) (Ester Group: -COO-)

Since the compound is a cyclic ester, it must contain an ester functional group (-COO-) within a cyclic structure. The condensed structural formula is: \(\delta\)-valerolactone, which is represented as C_5H_8O_2. The structural formula indicates a cyclic ester with 5 carbon atoms and an ester group within the ring.

Key Concepts

Carboxylic Acid

Carboxylic acids are a class of organic compounds characterized by the presence of at least one carboxyl group (-COOH). This functional group consists of a carbon (C) atom double-bonded to an oxygen (O) atom and also bonded to a hydroxyl group (OH). Their general formula is R-COOH, where 'R' can be a variety of alkyl or aryl groups.

Carboxylic acids are known for their sour taste and are commonly found in vinegar (acetic acid) and citrus fruits (citric acid). They are weak acids, meaning they only partially dissociate in water to give off protons (H+ ions). This acidity is due to the polar nature of the carboxyl group, which facilitates the release of a hydrogen ion.

The compound given in the exercise (CH₃CH₂CH₂COOH) exemplifies a simple straight-chain carboxylic acid, where the 'R' group is a propyl chain (CH₃CH₂CH₂-). It illustrates how structural formulas convey not just the number of each atom but also the specific arrangement, leading to a clearer understanding of the molecule's properties and reactivity.

Cyclic Ketone

Cyclic ketones are ketones that contain a carbonyl group (C=O) within a ring structure. The carbonyl group is highly reactive and is characterized by a carbon atom double-bonded to an oxygen atom. Ketones, in general, are represented by the formula R₁-(C=O)-R₂, where R₁ and R₂ are alkyl or aryl groups. In the case of cyclic ketones, the R groups form part of a ring.

These compounds are found in various natural and synthetic substances. For example, the hormone progesterone includes a cyclic ketone in its structure. Cyclic ketones are important in the fragrance industry and as intermediates in organic synthesis.

As per the exercise solution, cyclopentanone (C₅H₈O) is a five-membered ring with the ketone group. It demonstrates how the simplicity of a compound's molecular formula can belie the complexity of its geometric structure, which affects its chemical behavior.

Dihydroxy Compound

Dihydroxy compounds contain two hydroxyl (-OH) groups attached to their carbon skeleton. The presence of multiple hydroxyl groups makes these compounds polyols or polyhydric alcohols. Their general formula can be represented as (CHOH)_n where 'n' is the number of hydroxyl groups.

These compounds are of interest due to their solubility in water and their ability to form hydrogen bonds, both of which lead to applications in various industries, including food, pharmaceutical, and chemical manufacturing.

An example of a dihydroxy compound provided in the exercise is ethane-1,2-diol (CH₂OH-CH₂-CH₂OH), which features a three-carbon chain with hydroxyl groups on the first and last carbon atoms. Such compounds are also key intermediates in organic chemistry reactions and can act as precursors to more complex molecules.

Cyclic Ester

Cyclic esters, often referred to as lactones, are formed when a carboxylic acid reacts with an alcohol within the same molecule, resulting in the formation of a ring structure containing an ester linkage (-COO-). The general structure of a cyclic ester involves a carbonyl group (C=O) adjacent to an ether linkage (-O-).

These compounds are notable for their distinct aromas and flavors and are found in a variety of natural products, such as some fruit flavors and pheromones. They also play a crucial role in polymer chemistry as monomers for the production of biodegradable plastics.

The molecule (delta)-valerolactone (C₅H₈O₂), described in the exercise, is a compound with a five-membered ester ring. This structure highlights the importance of ring size in determining the chemical properties of the lactone, with smaller rings typically being more strained and reactive.