Reactions of Carboxylic Acids and Their Derivatives
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Fischer Esterification (Review)
The Fischer Esterification is the acid catalyzed reaction of an alcohol and a carboxylic acid to form an ester.
In order to drive the equilibrium of the Fischer Esterification to the ester product side, two things are commonly done. One trick is to run the reaction with an excess of starting materials. A large excess of either the alcohol or the carboxylic acid will work. The other trick is to remove the water product from the reaction using a dehydrating agent in the reaction flask that can absorb water, like molecular sieves. Also, water can be removed from a reaction using a Dean-Stark trap. This is a special attachment to a reflux apparatus that helps remove water from a reaction.
Once again, the mechanism for the esterification of an ester is a “protonate and go to town” mechanism. The carbonyl oxygen atom, with its negative lone pairs of electrons, is protonated as the mechanism’s first step.
Carboxylic acid to acid chloride (Review)
We learned in the alcohol chapter that OH groups of alcohols can be transformed into chlorides by using thionyl chloride (SOCl2). The same reaction can transform the OH group of a carboxylic acid into an acid chloride.
Acid chloride to ester
Another effective way to make an ester is to react an alcohol with an acid chloride. This is an exothermic reaction that is not an equilibrium reaction. The reaction usually proceeds well. Sometimes a base is put into the reaction flask to soak up the HCl that is produced. Acid chlorides can be made from reacting carboxylic acids with SOCl2.
One drawback for this reaction is that it is more water sensitive than a Fischer Esterification. Water will quench the acid chloride and turn it into a carboxylic acid.
Acid chloride to amide
Acid chlorides can react with primary amines to form amides. HCl, hydrochloric acid, is released in this reaction. A base is needed to soak up or react with this acid. One easy way to accomplish this is to use two equivalents of amine. One reacts with the acid chloride to make an amide and the other equivalent of amine is used to react with the HCl that is produced.
Ketones from Carboxylic Acids (Review)
Ketones can be synthesized from carboxylic acids. To do this, the proton of the carboxylic proton must first be removed using a base. This can be done by using lithium hydroxide (LiOH) as the base. Any alkyl lithium compound could also work. This deprotonation forms a carboxylate ion. A nucleophilic, negatively charged alkyl group can attack the carbonyl of this carboxylate. The electrons go up onto the oxygen atom. This forms a species with two negatively charged oxygen atoms. Following protonation using acid and water, a geminal diol is formed. Geminal diols like this are unstable, and easily dehydrate forming a carbonyl. In this case, the dehydration forms a ketone.
