Question

The reaction of a Grignard reagent with a carboxylic acid does not give a secondary alcohol. This is because (a) Grignard reagents only react with the aldehydes, ketones, esters and epoxides. (b) The carboxylic acid is too sterically hindered to react. (c) The carboxylic acid is not electrophilic enough to react. (d) The Grignard reagent is a base, so an acid-base reaction occurs.

Short answer

The correct answer is (d): The Grignard reagent is a base, so an acid-base reaction occurs.

Step-by-step solution

Understanding Grignard Reagents

Grignard reagents ( R-MgX ) are nucleophiles that usually react with carbonyl compounds such as aldehydes, ketones, and esters to form alcohols. They typically donate a pair of electrons to an electrophilic carbon in these compounds.

Recognizing Carboxylic Acid Properties

Carboxylic acids contain a highly polar O-H bond, making them acidic. The carbon in the carboxyl group (C=O) is indeed electrophilic but also accompanied by the acidic hydrogen (H).

Reaction between Grignard Reagent and Carboxylic Acid

When a Grignard reagent reacts with a carboxylic acid, the base (Grignard) deprotonates the acid, resulting in the formation of a carboxylate ion and the corresponding alkane of the Grignard reagent, rather than adding to the carbonyl group.

Conclusion on Alcohol Formation

Due to the acid-base reaction, the nucleophilic addition of Grignard to the carbonyl group cannot occur. Thus, no alcohol formation happens.

Key concepts

Nucleophilic Addition

In organic chemistry, nucleophilic addition is a fundamental reaction where a nucleophile, a species rich in electrons, attacks an electrophile, a species that accepts electrons. This type of reaction is common with carbonyl compounds, which include aldehydes, ketones, and esters. Carbonyl carbons have a partial positive charge due to the electronegativity of oxygen, making them perfect targets for nucleophilic attack. The nucleophile donates its electrons to the carbon, breaking the double bond and forming a new bond with the carbonyl carbon.

Grignard reagents are excellent nucleophiles because they contain a carbon bonded to magnesium, which makes the carbon electron-rich. When these reagents react with carbonyl compounds, they usually convert them into alcohols through nucleophilic addition. However, the reactivity changes when Grignard reagents encounter carboxylic acids due to their distinct properties.

Carboxylic Acid Properties

Carboxylic acids are unique in their structure because they have a functional group consisting of a carbonyl (C=O) and a hydroxyl (O-H) group. This dual nature gives them both acidic and nucleophilic properties. The highly polar O-H bond is significant because it can lose a proton (H⁺) easily, which is why carboxylic acids are considered acidic.

Additionally, the presence of a carbonyl group means the carbon is electrophilic, ready to accept electrons. However, in the case of Grignard reagents, this opportunity is lessened by the acidic nature of carboxylic acids. These acids can engage in quicker acid-base reactions, which takes precedence over nucleophilic addition. The dual reactive capabilities make carboxylic acids an interesting challenge in synthetic chemistry.

Acid-Base Reaction

An acid-base reaction occurs between two chemical species where an acid donates a proton (H⁺) to a base. In the context of the Grignard reaction with carboxylic acids, the Grignard reagent acts as a base due to its highly polar carbon-magnesium bond. This allows it to readily accept protons from the carboxylic acid.

During such a reaction, instead of forming a secondary alcohol, the Grignard reagent converts to another compound, often creating a carboxylate salt and an alkane. The quick deprotonation stops the expected nucleophilic addition to the carbonyl group. This interaction highlights why Grignard reagents are not suitable for forming alcohols from carboxylic acids. Understanding this behavior is critical for predicting reaction outcomes.

Electrophilic Carbon

The term electrophilic refers to a carbon atom that seeks to attract electrons. In the Grignard reactions context, this generally refers to carbonyl carbon atoms that can attract the electron pairs from Grignard reagents, allowing for the formation of new bonds.

In carboxylic acids, while the carbonyl carbon is technically electrophilic due to the polar C=O bond, it is effectively shielded by the acidic nature of the molecule. The acidic proton (H⁺) in the carboxylic group presents a more immediate reaction site for bases like Grignard reagents. As a result, typical nucleophilic addition does not occur, and secondary alcohol formation is bypassed. The understanding of such dynamics is crucial for chemists planning synthetic routes.