Carboxylic Acids Reactions

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Carboxylic acids can be found in many places, including your home. In this lesson we will look at some of the reactions that carboxylic acids can undergo and how they occur.

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.

The reaction between vinegar (acetic acid) and baking soda (sodium bicarbonate)
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) .

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