Ester to acid chloride transformation

In summary, the transformation of esters to acid chlorides involves the reaction of an ester with a chlorinating agent, typically thionyl chloride (SOCl2) or oxalyl chloride (COCl)2. This process proceeds through the nucleophilic substitution mechanism where the alcohol group of the ester is replaced by a chloride ion. The reaction results in the formation of an acid chloride and byproducts, such as alcohol and gaseous byproducts (SO2 and HCl in the case of thionyl chloride). This transformation is valuable in organic synthesis for preparing more reactive acyl chlorides that can participate in further reactions.
  • #1
jolly_math
51
5
Homework Statement
Which best accomplishes the following transformation?
Relevant Equations
ester to acid chloride
1712982333579.png


Here are 2 options:
A)
1712982413712.png
B)
1712982432602.png


For both syntheses, a protecting group is added to the alcohol, the ester is hydrolyzed to a carboxylic acid, then it is turned into an acid chloride using SOCl2. The difference is that A is base-catalyzed and B is acid-catalyzed.

The correct answer is A, but I don't understand why B doesn't work or is worse than A.

Thank you.
 
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  • #2
Eons passed since I worked with these things, but quick googling for stability of silyl ethers seems to be suggesting it is pH dependent.
 
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  • #3
Borek said:
Eons passed since I worked with these things, but quick googling for stability of silyl ethers seems to be suggesting it is pH dependent.
I've never seen this in my textbook or the resources I've used so far though. Is this a commonly-known fact?
 
  • #4
Even wiki states: "Reaction with acids (...) removes the silyl group when protection is no longer needed.", so my bet is yes, it is something quite well known.
 
  • #5
This protection group is acid sensitive. Step 2 in B negates step 1.
 

FAQ: Ester to acid chloride transformation

What is the general mechanism for the ester to acid chloride transformation?

The transformation of an ester to an acid chloride typically involves the reaction of the ester with a reagent that can facilitate the conversion, such as thionyl chloride (SOCl2) or oxalyl chloride (COCl)2. The mechanism generally includes the nucleophilic attack of the chloride ion on the carbonyl carbon of the ester, leading to the formation of an acyl chloride and byproducts such as alcohol and gases like SO2 or CO2.

What reagents are commonly used for this transformation?

Common reagents for converting esters to acid chlorides include thionyl chloride (SOCl2), oxalyl chloride ((COCl)2), and phosphorus pentachloride (PCl5). Each of these reagents has specific advantages, such as mild reaction conditions or the generation of gaseous byproducts that can drive the reaction to completion.

What are the advantages of converting esters to acid chlorides?

Converting esters to acid chlorides can be advantageous because acid chlorides are more reactive than esters, making them useful intermediates in organic synthesis. They can easily undergo nucleophilic acyl substitution reactions, allowing for the introduction of a variety of nucleophiles, which can lead to the synthesis of a wide range of compounds.

Are there any side reactions that can occur during this transformation?

Yes, side reactions can occur during the ester to acid chloride transformation. For example, if excess reagent is used, there may be overreaction leading to the formation of undesired byproducts. Additionally, the presence of moisture can hydrolyze the acid chloride back to the corresponding carboxylic acid. Therefore, it is essential to conduct the reaction under anhydrous conditions.

What precautions should be taken when performing this transformation?

Precautions include working in a fume hood due to the release of toxic gases such as SO2 or HCl during the reaction. It is also important to handle reagents like thionyl chloride and oxalyl chloride with care, as they can be corrosive and harmful. Proper personal protective equipment (PPE) such as gloves and goggles should be worn, and the reaction should be monitored to prevent overheating or excessive pressure buildup.

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