Question

Oxidation of an aldehyde yields a carboxylic acid: Draw the structures for the products of the following oxidation reactions. a. propanal \(\stackrel{\text { [ox] }}{\rightarrow}\) b. 2,3 -dimethylpentanal \(\stackrel{\text { [ox] }}{\longrightarrow}\) c. 3 -ethylbenzaldehyde \(\stackrel{\text { [ox] }}{\longrightarrow}\)

Short answer

a. Propanal oxidizes to propanoic acid: CH3-CH2-COOH. b. 2,3-dimethylpentanal oxidizes to 2,3-dimethylpentanoic acid: (CH3)2-CH-CH2-CH(CH3)-COOH. c. 3-ethylbenzaldehyde oxidizes to 3-ethylbenzoic acid: a benzene ring with an ethyl group (C2H5) attached to the third carbon, and a carboxylic acid group attached to the first carbon.

Step-by-step solution

Identify the functional group

First, we need to identify the functional group present in each of the given compounds. In all three cases, we are given aldehydes. The general structure of an aldehyde is R-CHO, where R is the alkyl or aryl group.

Oxidation of aldehydes

To convert the aldehyde to a carboxylic acid, we need to oxidize the carbonyl group of the aldehyde. Oxidation involves the addition of an oxygen atom to the carbonyl carbon, resulting in the formation of a carboxylic acid functional group (R-COOH).

a. Oxidation of propanal

Propanal has the structure CH3-CH2-CHO. When oxidized, the carbonyl group gets converted to a carboxylic acid group. The product will be CH3-CH2-COOH. This compound is called propanoic acid.

b. Oxidation of 2,3-dimethylpentanal

2,3-dimethylpentanal has the structure (CH3)2-CH-CH2-CH(CH3)-CHO. Upon oxidation, the carbonyl group gets converted to a carboxylic acid group. The product will be (CH3)2-CH-CH2-CH(CH3)-COOH. This compound is called 2,3-dimethylpentanoic acid.

c. Oxidation of 3-ethylbenzaldehyde

3-ethylbenzaldehyde has the structure of a benzene ring with an ethyl group (C2H5) attached to the third carbon, and an aldehyde group attached to the first carbon. Upon oxidation, the carbonyl group will be converted to a carboxylic acid group. The product will be a benzene ring with an ethyl group (C2H5) attached to the third carbon, and a carboxylic acid group attached to the first carbon. This compound is called 3-ethylbenzoic acid.

Key concepts

Carboxylic Acid Formation

In organic chemistry, the transformation of aldehydes to carboxylic acids is a fundamental reaction often seen in biochemical pathways and industrial processes. This transformation is an oxidation reaction, which means it involves the gain of oxygen or loss of hydrogen.
When an aldehyde is oxidized, the carbonyl carbon (the carbon double-bonded to oxygen) gains an additional oxygen atom. This additional oxygen comes from an oxidizing agent often represented by [ox] in chemical equations. As a result, the carbon goes from having a double bond with one oxygen in the aldehyde to having both a double bond and a single bond with hydroxyl group (-OH) in the carboxylic acid. The general reaction can be simplified as:
\[ R-CHO + [O] \rightarrow R-COOH \]

• Propanal Oxidation: When propanal (CH₃-CH₂-CHO) is oxidized, propanoic acid (CH₃-CH₂-COOH) is formed.
• 2,3-Dimethylpentanal Oxidation: Upon oxidation, 2,3-dimethylpentanal ((CH₃)₂-CH-CH₂-CH(CH₃)-CHO) yields 2,3-dimethylpentanoic acid ((CH₃)₂-CH-CH₂-CH(CH₃)-COOH).
• 3-Ethylbenzaldehyde Oxidation: Oxidizing 3-ethylbenzaldehyde produces 3-ethylbenzoic acid, where an ethyl group attaches to a benzene ring, along with a carboxylic acid group replacing the aldehyde.

Choosing an appropriate oxidizing agent is crucial for successful carboxylic acid formation. For instance, reagents such as potassium permanganate (KMnO₄), chromic acid (H₂CrO₄), and nitric acid (HNO₃) are commonly used in organic chemistry labs to achieve the desired oxidation.

Organic Chemistry Reactions

The oxidation of aldehydes to carboxylic acids is one among many organic chemistry reactions that are vital to understand for a working knowledge in the field. In general, organic reactions are characterized by the type of change that occurs in the organic molecule. These reactions can include addition, substitution, elimination, and rearrangement processes.
For students mastering organic chemistry, it's important to be familiar with various classifications and mechanisms. For example, the oxidation reaction discussed here proceeds through an electrophilic addition mechanism where the aldehyde acts as the electrophile (electron-poor center) and the oxidizing agent provides the nucleophilic (electron-rich) oxygen. This specificity in mechanism guides the prediction of products and understanding reaction conditions.
Other common organic reactions include:

• Hydrogenation, which involves the addition of hydrogen to unsaturated compounds like alkenes.
• Halogenation, where halogens are introduced into organic molecules typically at alkene or alkane sites.
• Esterification, a process by which carboxylic acids react with alcohols to form esters and water.

The variety and complexity of organic reactions make them fascinating, but also challenging. Breaking down reactions into smaller, understandable parts, like recognizing the role of functional groups, can make learning organic chemistry more manageable.

Aldehyde Structure and Reactivity

Aldehydes are a group of organic compounds characterized by the presence of a carbonyl group bonded to at least one hydrogen atom. The general formula for an aldehyde is R-CHO, where 'R' represents an alkyl or aryl group, and '-CHO' is the aldehyde functional group itself.
In terms of reactivity, the carbonyl carbon of the aldehyde is electrophilic due to the polarization of the double bond between the carbon and oxygen atoms. Oxygen is more electronegative, and thus it draws more electron density towards itself, leaving the carbon atom electropositive.

• Electrophilic Center: The electropositive nature of the carbonyl carbon makes it susceptible to nucleophilic attack, resulting in a variety of organic reactions.
• Hydrogen Attachment: Its attachment to at least one hydrogen atom makes aldehydes more reactive than other carbonyl-containing compounds like ketones, where two carbon groups are attached to the carbonyl carbon.
• Polarity: This polarity influences the physical properties of aldehydes, such as boiling point and solubility, which are typically higher than hydrocarbons but lower than alcohols of similar molecular weight.

Recognition of this intrinsic reactivity helps to predict the behavior of aldehydes in various chemical reactions, including their susceptibility to oxidation to form carboxylic acids. Moreover, understanding this structure-reactivity relationship is fundamental in designing organic syntheses and in the field of medicinal chemistry where aldehydes are often part of drug molecules.