Grignard Reagents and CO2: Synthesizing Carboxylic Acids
We've already seen how Grignard Reagents (R-MgX) react with Aldehydes and Ketones to produce Alcohols. But what happens if you react a Grignard reagent with Carbon Dioxide (CO2)?
This specific reaction is one of the most important conversions in Organic Chemistry because it achieves two things simultaneously: It synthesizes a Carboxylic Acid, and it strictly "Steps Up" the carbon chain by exactly one carbon atom. Let's trace the mechanism!
Video Tutorial: The Reaction Mechanism
Watch Abhishek Sengar sir from CHEMCA expertly map out the nucleophilic attack of the Ethyl group on Carbon Dioxide, and the subsequent hydrolysis to form Propanoic Acid.
Step-by-Step Reaction Breakdown
Carbon Dioxide (O=C=O) has a highly electrophilic central carbon atom due to the two highly electronegative Oxygen atoms pulling electron density away from it. This makes it a perfect target for the nucleophilic Grignard reagent!
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Nucleophilic Addition (in Dry Ether):
Our Grignard reagent is Ethyl Magnesium Bromide (CH3CH2-MgBr). The Ethyl carbon attached to the Magnesium acts as a strong nucleophile.
It attacks the central electrophilic carbon of CO2. One of the C=O pi-bonds breaks, transferring electrons to the Oxygen. The Oxygen then grabs the MgBr+ fragment to form the intermediate adduct: a Magnesium Carboxylate salt (CH3CH2-C(=O)O-MgBr+). -
Acidic Hydrolysis:
The intermediate salt is then treated with acidic water (H2O / H+). The O-MgBr ionic bond is cleaved. The carboxylate oxygen is protonated to become a Carboxylic Acid (-COOH) group, yielding the final product.
Fig: The Carbon Dioxide molecule literally becomes the -COOH group, adding exactly 1 carbon to the chain.
Practice Questions for JEE & NEET
At the end of the video, Abhishek Sir emphasized that this reaction can be used as a "Step-Up" conversion method. Test your understanding of this below!
Question 1: Why is the reaction between a Grignard Reagent and CO2 formally called a "Step-Up" reaction?
Answer: Because the product contains exactly ONE more carbon atom than the original alkyl group.
Reasoning:
In Organic Conversions, a "step-up" reaction is any sequence that increases the length of the parent carbon chain.
In our example, we started with an Ethyl group (2 carbons) in the Grignard reagent. By reacting it with CO2, we physically attached a 3rd carbon atom to the end of the chain, converting the CO2 molecule itself into the -COOH group. The final product was Propanoic Acid (3 carbons). It perfectly stepped-up the chain by n + 1!
Question 2: What would be the final major product if we reacted Phenyl Magnesium Bromide (C6H5MgBr) with Carbon Dioxide followed by acidic hydrolysis?
Answer: Benzoic Acid (C6H5COOH).
Reasoning:
The mechanism works exactly the same way for aromatic Grignard reagents! The nucleophilic Phenyl ring attacks the electrophilic carbon of CO2 in dry ether. After hydrolysis, the CO2 molecule is transformed into a Carboxylic acid group directly attached to the benzene ring.
This is one of the most reliable and highly-tested methods for synthesizing Benzoic acid from Bromobenzene in JEE/NEET!
Grignard Reaction: Isopropyl Magnesium Bromide + Ethanal
Grignard Reagents (R-MgX) are the ultimate building blocks in Organic Chemistry. In our previous post, we saw how a Grignard reagent reacts with Formaldehyde to form a primary alcohol.
But what happens if we step it up and use a larger aldehyde? In this classic JEE/NEET conversion, we will react a branched Grignard reagent with Ethanal (Acetaldehyde) to synthesize a secondary alcohol. Let's trace the mechanism and master the IUPAC naming.
Video Tutorial: The Reaction Mechanism
Watch Abhishek Sengar sir from CHEMCA expertly map out the nucleophilic attack, the intermediate adduct, and the strict rules for IUPAC numbering of the final product.
Step-by-Step Reaction Breakdown
When drawing the reaction, look closely at your Grignard reagent. The specific carbon atom attached to the Magnesium is your nucleophile. That exact carbon will form the new bond with the carbonyl carbon!
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Nucleophilic Addition (in Dry Ether):
Our Grignard reagent is Isopropyl Magnesium Bromide ((CH3)2CH-MgBr). The central CH carbon attacks the electrophilic carbonyl carbon of Ethanal / Acetaldehyde (CH3-CHO).
The pi-bond breaks, and the oxygen grabs the MgBr+ fragment to form the intermediate adduct: CH3-CH(OMgBr)-CH(CH3)2. -
Acidic Hydrolysis:
The intermediate adduct is treated with acidic water (H2O / H+). The O-MgBr bond is cleaved. The Oxygen is protonated to become an Alcohol (-OH), yielding the final product.
Fig: Notice how numbering must prioritize the -OH group. Therefore, we number from left to right.
Mastering the IUPAC Naming
Let's follow Abhishek Sir's numbering methodology for the final product:
- Principal Functional Group: The Alcohol (-OH) takes top priority. Rule: You must number the chain from the end that gives the -OH group the lowest possible locant.
- Longest Chain: Numbering from the left gives the OH position 2. Numbering from the right gives it position 3. Therefore, we number Left to Right. The parent chain is 4 carbons long: Butan-2-ol.
- Substituents: The remaining methyl group is attached to Carbon-3.
Practice Questions for JEE & NEET
At the end of the video, Abhishek Sir mentioned a brilliant shortcut rule. Let's test your knowledge of it!
Question 1: In the video, Abhishek Sir stated a rule about Grignard reagents and the "Degree" of the alcohol formed. Based on this rule, what type of alcohol (1°, 2°, or 3°) would be formed if we reacted our Grignard reagent with Propanone (Acetone) instead of Ethanal?
Answer: A Tertiary (3°) Alcohol.
Reasoning:
This is the ultimate Grignard shortcut rule:
- Formaldehyde (H-CHO) has 0 alkyl groups attached. Adding the Grignard R-group gives it 1 alkyl group = Primary (1°) Alcohol.
- Any other Aldehyde (like Ethanal) has 1 alkyl group attached. Adding the Grignard R-group gives it 2 alkyl groups = Secondary (2°) Alcohol. (This is what happened in the video!)
- A Ketone (like Acetone) already has 2 alkyl groups attached. Adding the Grignard R-group gives it 3 alkyl groups = Tertiary (3°) Alcohol.
Question 2: Why is it absolutely necessary to use a solvent that "does not provide an active acidic hydrogen" (like Dry Ether) when reacting a Grignard Reagent? What would happen if we used Ethanol as the solvent instead?
Answer: The Grignard reagent would be destroyed, forming an alkane.
Reasoning:
Grignard Reagents (R-MgX) are essentially sources of carbanions (R-). This makes them not only great nucleophiles, but incredibly strong Bases.
If you use a solvent with an acidic hydrogen (like water, ethanol, or amines), the Grignard reagent will immediately undergo an acid-base reaction, ripping the proton off the solvent to form an Alkane (R-H). The intended nucleophilic addition to the aldehyde would completely fail. This is why strict anhydrous, non-acidic conditions (like Dry Ether) are mandatory!
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