Carboxylation - Online Tutor, Practice Problems & Exam Prep

Grignard reagents, strong nucleophiles, can synthesize carboxylic acids through carbonation with dry ice (frozen CO₂). When a Grignard reagent is added to dry ice, the carbon dioxide acts as an electrophile, leading to the formation of a carboxylate ion. This reaction involves the nucleophilic attack on the carbonyl carbon, resulting in a carboxylate that can be protonated to yield the desired carboxylic acid. This method highlights the utility of Grignard reagents in organic synthesis, particularly in carboxylic acid formation.

Carbonation of Grignard Reagents

In this video, I'm going to quickly discuss a really cool way to make carboxylic acids, and that's through carbonation of Grignards. It turns out that Grignard reagents are really great nucleophiles. They have a very strong negative charge on the R. When they're exposed to dry ice, what is dry ice? It's cold; it's not real ice. It's frozen CO₂ gas. When you literally pour a Grignard on frozen CO₂ gas, what's going to happen? It turns out that CO₂ acts like an electrophile, just like a carbonyl does. In fact, it's an even stronger electrophile because it has these two extremely strong dipoles. When the Grignard sees the carbon dioxide that's about evaporating because it's frozen, and you're pouring a liquid on it, the R is going to attack the carbon and push electrons down. What we're going to get is a molecule that now looks like this. I've got my carbonyl. I've got my O negative, and I've got my R. Check that out. In just one step, I went from something that looks nothing like a carboxylic acid to a carboxylate. That's what this is. I get a carboxylate. Now all I need is a protonation step to take care of the rest. I would use whatever I want to protonate my carboxylate, and I just made carboxylic acid from dry ice. Pretty cool, right? If you put dry ice in your pool, then it makes that dry ice vapor that hangs over the pool. I'm not sure if you guys have seen that. Or you could just pour a Grignard on it, and you can make carboxylic acids. I think those are two pretty cool choices. Go science. Let's move on to the next video.

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Here’s what students ask on this topic:

What is the role of Grignard reagents in carboxylation reactions?

Grignard reagents play a crucial role in carboxylation reactions due to their strong nucleophilic properties. When a Grignard reagent (R-MgX) is exposed to dry ice (frozen CO₂), the carbon dioxide acts as an electrophile. The nucleophilic R group attacks the carbonyl carbon of CO₂, forming a carboxylate ion. This intermediate can then be protonated to yield a carboxylic acid. This method is particularly useful in organic synthesis for creating carboxylic acids from simpler starting materials.

How does dry ice facilitate the formation of carboxylic acids in Grignard reactions?

Dry ice, which is solid CO₂, acts as an electrophile in Grignard reactions. When a Grignard reagent is poured onto dry ice, the nucleophilic R group of the Grignard attacks the carbonyl carbon of the CO₂. This results in the formation of a carboxylate ion. The carboxylate ion can then be protonated in a subsequent step to form the desired carboxylic acid. The use of dry ice is advantageous because it provides a readily available and reactive source of CO₂.

What are the steps involved in converting a Grignard reagent to a carboxylic acid using dry ice?

The conversion of a Grignard reagent to a carboxylic acid using dry ice involves two main steps. First, the Grignard reagent (R-MgX) is added to dry ice (solid CO₂). The nucleophilic R group attacks the electrophilic carbonyl carbon of CO₂, forming a carboxylate ion (R-COO^-). Second, the carboxylate ion is protonated using an acid (such as H₂O or HCl) to yield the final carboxylic acid (R-COOH).

Why is CO₂ considered a strong electrophile in carboxylation reactions with Grignard reagents?

CO₂ is considered a strong electrophile in carboxylation reactions with Grignard reagents because it has two strong dipoles due to the double bonds between carbon and oxygen. These dipoles create a significant partial positive charge on the carbon atom, making it highly susceptible to nucleophilic attack. When a Grignard reagent is introduced, the nucleophilic R group readily attacks the electrophilic carbon of CO₂, facilitating the formation of a carboxylate ion.

What are the advantages of using Grignard reagents for synthesizing carboxylic acids?

Using Grignard reagents for synthesizing carboxylic acids offers several advantages. Firstly, Grignard reagents are strong nucleophiles, making them highly reactive and efficient in forming carboxylate ions. Secondly, the reaction with dry ice (CO₂) is straightforward and typically yields high purity carboxylic acids. Additionally, this method allows for the synthesis of carboxylic acids from simpler and readily available starting materials, making it a valuable tool in organic synthesis.