Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.
Carboxylic Acids at Home
You can probably find a carboxylic acid in your home. It is called vinegar, or acetic acid. Have you ever mixed vinegar and baking soda together? It is a real cool experiment to watch, but it can also help clean out pipes and messy cooking pans. In this reaction the vinegar (acetic acid) reacts with the baking soda (sodium bicarbonate) to replace the hydrogen with an ionic bond with sodium. Carbon dioxide is formed in this reaction which is what causes the bubbling that will be seen.
A carboxylic acid is a functional group where the carbon is connected to an alcohol and double bonded to an oxygen. Since there are two oxygen atoms connected to the carbon if the hydrogen is removed from the alcohol the electrons can be shared between both oxygen atoms. This makes the carboxylic acid able to react in several different ways.
Replacing the Hydrogen
Carboxylic acids are, as implied by the name, very acidic. As such they have a very low pKa. This means that the hydrogen on the carboxy group can easily be removed and replaced with something else. That hydrogen can be so easily removed that most of the time carboxylic acids are written with a negative on the oxygen and no hydrogen.
This allows carboxylic acids to react with several different compounds. The most basic reaction is as a salt. The negative charge on the oxygen attracts positively charged ions to it. In these cases a covalent bond is not formed, but they are associated with each other due to the charges. This is what happened in the vinegar and baking soda reaction.
That hydrogen can be completely replaced with another molecule. In this case the molecule needs to be a strong electrophile. An electrophile is electron loving, it is looking for more electrons to fill all of its shells. The negative charge on the carboxylic acid is the perfect environment with extra electrons to fulfill that electrophile's needs. When there is a negative charge this means that there are extra electrons since the electrons carry a negative charge.
Replacing the Hydroxyl Group
If another hydrogen is available then the hydroxide (OH) can become a very good leaving group, as water. In this situation a new molecule can be formed where the hydroxyl group is replaced with another molecule. In these reactions alkyl halides, amides, and esters can be formed.
Alkyl halides are formed when the carboxylic acid reacts with a halide rich molecule, such as SOCl2. These reactions are a little different because the alcohol is actually removed in two different sections, instead of simply as water. The hydrogen forms hydrogen chloride (HCl) and the oxygen attaches to the molecule that the halide was originally on, forming sulfur dioxide, (SO2) .
Reactions with carboxylic acids that form esters are called Fischer Esterification. A carboxylic acid reacts with an alcohol forming an ester and water. It is important to follow each molecule because otherwise this could look like a simple substitution of the hydrogen with the 'R' group from the alcohol. But when each molecule is followed it can be seen that the blue oxygen on the carboxylic acid is replaced with the red oxygen from the alcohol. Remember an 'R' group is just a simple way to show any chain of carbons.
Carboxylic acids can also undergo oxidation and reduction reactions. If it is bombarded with enough hydrogens the carboxylic acid can be reduced into an alcohol. Typically the hydrogens are in the form of lithium aluminum hydride or diborane.
The carboxylic acid is already extremely oxidized. The only way the carbon in the carboxylic acid can become more oxidized is by breaking off from the rest of the molecule and forming carbon dioxide.
Carboxylic acids can react by replacing the hydrogen with another molecule or replacing the entire alcohol group with another molecule. When replacing the hydrogen a salt can form or a covalent bond can form with an electrophile. When replacing the hydroxyl group the alcohol typically leaves as water. When the hydroxyl group is replaced with an alcohol chain this reaction is called Fischer Esterification. Carboxylic acids can also undergo redox reactions to be oxidized (into carbon dioxide) or reduced (into an alcohol).
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