Problem 106 - Chemistry and Chemical Reactivity

Question

Hydrolysis of an unknown ester of butanoic acid, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{R},\) produces an alcohol \(\mathrm{A}\) and butanoic acid. Oxidation of alcohol A forms an acid B that is a structural isomer of butanoic acid. Give the names and structures for alcohol \(A\) and acid B.

Step-by-Step Solution

Answer

Alcohol A: Ethanol \(\mathrm{C_2H_5OH}\); Acid B: Propanoic acid \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}\).

Step 1: Understanding Hydrolysis

Hydrolysis of the ester results in breaking the ester into an alcohol and butanoic acid. If the unknown ester is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{R}\), the R group represents the hydrocarbon chain that becomes the alcohol part upon hydrolysis. Since the ester hydrolyzes into butanoic acid and an alcohol, the structure of alcohol \(A\) will be \(\mathrm{R}-\mathrm{OH}\).

Step 2: Identifying Alcohol A

The next piece of information in the problem is that the alcohol \(\mathrm{A}\) upon oxidation forms an acid \(\mathrm{B}\). Acid \(\mathrm{B}\) is a structural isomer of butanoic acid. A common alcohol resulting from ester hydrolysis is ethanol or methanol. However, in this case, since we need \(\mathrm{B}\) to be a four-carbon isomer of butanoic acid, \(A\) is more likely to be \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{OH}\) (ethanol). Therefore, Alcohol \(A\) is ethanol \(\mathrm{C_2H_5OH}\).

Step 3: Oxidation to Acid B

Oxidation of ethanol \(\mathrm{C_2H_5OH}\) results in acetic acid \(\mathrm{CH}_3\mathrm{COOH}\). Therefore, Acid \(B\) must be propanoic acid \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}\), which has the same formula \(\mathrm{C_3H_6O_2}\) and is a structural isomer of butanoic acid \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2\mathrm{COOH}\).

Step 4: Verification of Structural Isomerism

To confirm, check the carbon chain arrangement: Butanoic acid has four carbons in a straight chain. In propanoic acid, the carbon framework is different but it contains three carbons consistent with the identified acetate pathway, showing that propanoic acid \(\mathrm{CH}_3\mathrm{CH}_2\mathrm{COOH}\) is indeed a structural isomer of butanoic acid. Therefore, our identification is correct.

Key Concepts

Alcohol Oxidation

Alcohol oxidation is a fundamental reaction in organic chemistry where an alcohol is converted into a ketone or an acid.
In this particular case, alcohol A, which is determined to be ethanol \(\text{CH}_3\text{CH}_2\text{OH}\), undergoes oxidation to form an acid.
When ethanol is oxidized, it typically forms acetic acid (ethanoic acid), represented as \(\text{CH}_3\text{COOH}\). This conversion happens because the primary alcohol is oxidized step by step: ethanol first becomes acetaldehyde, and then further oxidizes to become acetic acid.

Oxidation involves the gain of oxygen or the loss of hydrogen.

• In a laboratory setting, oxidizing agents such as potassium dichromate (\(\text{K}_2\text{Cr}_2\text{O}_7\)) or potassium permanganate (\(\text{KMnO}_4\)) are commonly used for this reaction.
• In biological systems, enzymes like alcohol dehydrogenase facilitate these conversions.

Isomer Identification

Isomers are compounds that have the same molecular formula but different structures. This results in different chemical and physical properties.
In the exercise, acid B, resulting from the oxidation of alcohol A, must be identified as a structural isomer of butanoic acid.

Butanoic acid has the formula \(\text{C}_4\text{H}_8\text{O}_2\). To be a structural isomer, acid B must share this formula but differ in structure.

• For example, butanoic acid has a continuous four-carbon chain.
• If acid B has a different arrangement of these carbon atoms, such as a branched chain or a different functional group, it will qualify as an isomer.

Identifying structural isomers requires knowledge of how functional groups and atomic arrangements alter molecular identity without changing the elemental composition.

Butanoic Acid

Butanoic acid (also known as butyric acid) is a carboxylic acid with the chemical formula \(\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}\).
It consists of a four-carbon straight chain with a carboxyl group \(\text{COOH}\) at the end, making it a saturated acid.

This organic compound occurs naturally in butter, milk, and cheeses in low concentrations. It has a rancid smell when concentrated.

• The presence of the carboxyl group gives butanoic acid its acidic properties.
• The four-carbon chain classifies it within the fatty acid group, often referred to by its alternative common name.

In contexts where structural changes are discussed, like in the original exercise, butanoic acid serves as the reference structure for identifying possible isomers.

Structural Isomerism

Structural isomerism is a fascinating concept that emerges when molecules with the same chemical formula have different connections among their atoms, leading to distinct structures.
This form of isomerism is crucial for understanding organic chemistry as it significantly affects a molecule's properties and behavior.

In the case study, identifying acid B as a structural isomer of butanoic acid involves inspecting possible changes to how carbon, hydrogen, and oxygen atoms are connected in the formula \(\text{C}_4\text{H}_8\text{O}_2\).

• For example, an isomer may shift a \(\text{COOH}\) functional group from one end of the carbon chain to a secondary position, or change the chain branched structure.
• Exploring these structural variations can sometimes result in forming entirely different acids, each with unique characteristics, though maintaining the same basic formula.

Understanding structural isomerism helps simplify topics like ester hydrolysis and subsequent reactions, such as those given in the textbook solution.