Problem 54 - Chemistry and Chemical Reactivity

Question

Aldehydes and carboxylic acids are formed by oxidation of primary alcohols, and ketones are formed when secondary alcohols are oxidized. Give the name and formula for the alcohol that, when oxidized, gives the following products: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\) (b) 2-hexanone

Step-by-Step Solution

Answer

(a) 1-propanol (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)) (b) 2-hexanol (\(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHOH} \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3\))

Step 1: Understanding Primary Alcohol Oxidation

Primary alcohols are oxidized to aldehydes and further to carboxylic acids. To identify the alcohol precursor of an aldehyde, look for a primary alcohol structure: the CHOH group at the end of the hydrocarbon chain indicates a primary alcohol. The aldehyde given is \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\), meaning we need a 3-carbon chain for the alcohol.

Step 2: Identify the Precursor of Aldehyde

To synthesize \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\) from an alcohol, consider the alcohol must be 1-propanol (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)). Oxidation of the terminal OH group in 1-propanol yields the aldehyde \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CHO}\).

Step 3: Exploring Secondary Alcohol Oxidation

Secondary alcohols oxidize to form ketones. When addressing 2-hexanone, look for the structure of a secondary alcohol where the OH group is on the second carbon of a six-carbon chain.

Step 4: Identify the Precursor of Ketone

The ketone 2-hexanone (\(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COCH}_2 \mathrm{CH}_2 \mathrm{CH}_3\)) arises from the oxidation of the alcohol 2-hexanol (\(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CHOH} \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3\)). The OH group at the second carbon ensures oxidation results in the ketone.

Key Concepts

aldehydes

Aldehydes are organic compounds that contain a carbonyl group bonded to at least one hydrogen atom. The general formula for aldehydes is \(\text{RCHO}\), where \( ext{R}\) represents a hydrocarbon group or hydrogen. Aldehydes are typically formed through the oxidation of primary alcohols. During the oxidation process, the hydroxyl group \(\text{-OH}\) on the primary alcohol is converted into a carbonyl group \(\text{C=O}\).

This transformation often involves using an oxidizing agent like potassium dichromate \((\text{K}_2\text{Cr}_2\text{O}_7)\) with sulfuric acid. This method oxidizes the primary alcohol to the desired aldehyde without further oxidation to a carboxylic acid.

For example, when 1-propanol \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) is oxidized, it produces propanal \(\text{CH}_3\text{CH}_2\text{CHO}\). This example highlights the importance of understanding chemical structures and the placement of functional groups when predicting the outcome of oxidation reactions.

ketones

Ketones are characterized by having a carbonyl group \(\text{C=O}\) bonded to two carbon atoms. The general formula for ketones is \( ext{RCOR}\), where each \(\text{R}\) can be the same or different hydrocarbon chains or rings. Unlike aldehydes, ketones form from the oxidation of secondary alcohols.

In secondary alcohols, the hydroxyl group is attached to a carbon atom that is itself bonded to two other carbon atoms. This structure is key in predicting that the product of oxidation will result in a ketone rather than an aldehyde. Typically, an oxidizing agent like chromium trioxide \((\text{CrO}_3)\) is employed for this transformation.

When oxidizing 2-hexanol \(\text{CH}_3\text{CH}_2\text{CHOH}\text{CH}_2\text{CH}_2\text{CH}_3\), the product is 2-hexanone \(\text{CH}_3\text{CH}_2\text{CO}\text{CH}_2\text{CH}_2\text{CH}_3\). By understanding the placement of the hydroxyl group in the original molecule, one can predict that oxidation will lead to the creation of a ketone at the central carbon atom of the chain.

primary alcohols

Primary alcohols are a class of alcohols where the hydroxyl group \(\text{-OH}\) is bonded to a primary carbon atom. A primary carbon is a carbon attached to only one other carbon. This simple structure makes primary alcohols particularly susceptible to oxidation.

In chemical reactions, primary alcohols can be oxidized first to aldehydes and then, if conditions permit further oxidation, to carboxylic acids. The choice of reaction conditions and oxidizing agents influences the extent of oxidation.

Consider 1-propanol \(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\) as a representative of primary alcohols. Using mild oxidants, 1-propanol oxidizes to produce propanal \(\text{CH}_3\text{CH}_2\text{CHO}\). By choosing stronger oxidants, further oxidation can convert propanal into propanoic acid. Such reactions are foundational knowledge in organic chemistry, highlighting how the transformation of primary alcohols depends on controlled oxidizing conditions.

secondary alcohols

Secondary alcohols feature the hydroxyl group \(\text{-OH}\) attached to a secondary carbon, which is a carbon bonded to two other carbon atoms. This configuration distinguishes them from primary and tertiary alcohols and is crucial to understanding their reactivity.

When secondary alcohols undergo oxidation, they produce ketones. This is because the carbon atom with the hydroxyl group is between two other carbons, making the formation of a carboxylic acid unfavorable. The oxidation process typically uses strong oxidizing agents like \(\text{CrO}_3\) or acidified potassium dichromate.

An example of secondary alcohol oxidation is the conversion of 2-hexanol \(\text{CH}_3\text{CH}_2\text{CHOH}\text{CH}_2\text{CH}_2\text{CH}_3\) to 2-hexanone \(\text{CH}_3\text{CH}_2\text{CO}\text{CH}_2\text{CH}_2\text{CH}_3\). The position of the hydroxyl group ensures the resulting product is a ketone. This transformation is central to the study of secondary alcohols and their application in the synthesis of diverse organic compounds.