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8.9.3. Reactions Involving –COOH Group
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Today, we are discussing the reduction of carboxylic acids. Can anyone tell me what reagents are typically used for this process?
Is it lithium aluminium hydride?
Correct! LiAlH4 is commonly used. For better selectivity, diborane can also be employed. Now, why do we use LiAlH4 over sodium borohydride?
Because sodium borohydride doesn’t reduce carboxylic acids?
Exactly! So remember, LiAlH4 is needed for reducing carboxylic acids effectively. Let's summarize this point: Carboxylic acids can be reduced to primary alcohols using LiAlH4 or diborane.
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Next, let's explore decarboxylation. Who can explain what happens during this reaction?
It's when a carboxylic acid loses carbon dioxide, right?
Exactly! What about the method used? Any ideas?
It involves heating the sodium salts of carboxylic acids with sodalime.
Correct! The reaction results in hydrocarbons with twice the number of carbon atoms present in the alkyl group of the acid.
What’s the name of the process that uses electrolysis for decarboxylation?
Great question! It's known as Kolbe electrolysis. Let’s recap: Decarboxylation results in hydrocarbons, typically through heating with sodalime or via electrolysis.
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Create a free accountNow, let's shift our focus to halogenation. What do we call the reaction where a carboxylic acid a-hydrogen is replaced by a halogen?
It’s the Hell-Volhard-Zelinsky reaction!
Exactly! Can anyone describe conditions for this reaction?
It involves using either chlorine or bromine, right, and requires red phosphorus?
Correct! This reaction leads to the formation of a-halocarboxylic acids. Remember, the presence of an a-hydrogen is crucial for this reaction to occur.
Overview
Short Summary
This section covers essential reactions involving carboxylic acids, including reduction, decarboxylation, and halogenation.
Reference YouTube Videos
Audio Book
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Create a free accountCarboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane. Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group.
Detailed Explanation
Carboxylic acids, which contain the –COOH group, can undergo reduction reactions. This means that their functional group can be converted into a less oxidized form. The most common way to reduce a carboxylic acid to a primary alcohol is to use lithium aluminium hydride (LiAlH4) or diborane (B2H6). These reducing agents facilitate the addition of hydrogen atoms to the carbon atom of the carboxylic acid, transforming the –COOH group into a –CH2OH group, which characterizes primary alcohols. However, other common reducing agents like sodium borohydride (NaBH4) are ineffective at reducing carboxylic acids because they are less potent than lithium aluminium hydride or diborane.
Examples & Analogies
Consider the process of baking bread. The process of converting flour (or the basic ingredients) into a full loaf is comparable to reducing a carboxylic acid into an alcohol. In the same way that stronger yeast helps the dough rise effectively, lithium aluminium hydride acts as a strong reducing agent to help transform carboxylic acids efficiently into their alcohol counterparts.
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Create a free accountCarboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO in the ratio of 3 : 1). The reaction is known as decarboxylation. Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid. The reaction is known as Kolbe electrolysis.
Detailed Explanation
Decarboxylation is a reaction where carbon dioxide (CO2) is removed from a carboxylic acid, resulting in the formation of a hydrocarbon. This often occurs when the sodium salts of carboxylic acids are heated with a mixture known as sodalime, which consists of sodium hydroxide (NaOH) and calcium oxide (CaO). The process can also occur through electrolysis of the aqueous solutions of alkali metal salts. In this case, the products are hydrocarbons that contain twice the number of carbon atoms as the original alkyl group of the acid.
Examples & Analogies
Imagine a balloon that is filled with air (representing the carbon dioxide in a carboxylic acid). When you let the air out of the balloon, you create a smaller object — a deflated balloon (analogous to the hydrocarbon). This process of releasing the air symbolizes decarboxylation, where CO2 is removed to yield a simpler hydrocarbon.
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
Reduction: Converting carboxylic acids to primary alcohols using LiAlH4 or diborane.
Decarboxylation: The process of losing CO2 from carboxylic acids, achievable via heating with sodalime.
Halogenation: The Hell-Volhard-
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