Sarah has a doctorate in chemistry, and 12 years of experience teaching high school chemistry & biology, as well as college level chemistry.
What Is Monocarboxylic Acid?
Imagine you're biting into a juicy hamburger and crunch into a pickle. The tell-tale taste of vinegar fills your mouth. Have you ever wondered what vinegar actually is? The scientific name for vinegar is acetic acid, and the molecule is a monocarboxylic acid.
A carboxylic acid is an organic molecule that has mostly carbon and hydrogen atoms but also contains a carbon that has a double bond to an oxygen. The carbon that's doubly bound to the oxygen is attached to another oxygen, which is bound to a hydrogen. This functional group is the part of the molecule that changes in chemical reactions and is sometimes shown as -COOH or CO2 H.
Molecules can have several carboxylic acid functional groups. Citric acid, the carboxylic acids that adds a citrus taste to your soda, has three carboxylic acid groups. When a molecule has just has one carboxylic acid group, it's defined as a monocarboxylic acid.
Carboxylic acids are weak acids. This means that the COOH group gives its proton to another chemical, or is deprotonated. When the COOH loses an H, a carboxylate ion (COO-) forms. Acids have sour tastes, which is why citric acid is added to candy to give it a slightly sour taste.
The solubility of carboxylic acid molecules depends on the number of carbon atoms in the entire molecule. If the molecule has fewer than five carbons, it's soluble in water. If the molecule has more than five carbons, it is not soluble in water.
Carboxylic acid molecules tend to form hydrogen bonds with each other in solution. They form dimers, or two molecules that are associated with each other. Because it takes so much energy to break these bonds, the boiling point of carboxylic acids is greater than water.
Carboxylic acids can be synthesized in many ways. These are generally used in organic chemistry synthesis and teaching laboratories. If you have taken a chemistry lab, you may have synthesized a carboxylic acid. Let's look at three ways we can make carboxylic acids.
1. Oxidation of Alcohols
A primary alcohol can react with an oxidizer to produce carboxylic acids. Primary alcohols are OH groups attached to a carbon with two hydrogens. A common chemical that can oxidize alcohols is chromic acid (H2 CrO4).
2. Hydration of Nitriles
Nitriles are molecules with a carbon that has a triple bond to a nitrogen. A nitrile can react with an acid and water to form carboxylic acids. Whenever water reacts with a compound, the process is called hydration, or hydrolysis.
3. Acyl Halide Hydration
An acyl halide (a halogen attached to a molecule of just carbon and hydrogen) can react with water to form carboxylic acids. Water is reacting here, so this is also called hydration.
Monocarboxylic acids are used as medicines. For example, aspirin, the common pain reliever, has a carboxylic acid functional group. Cetirizine, an antihistamine that you may use for hay fever, is a monocarboxylic acid.
Soap is composed of deprotonated carboxylic acids, or carboxylate ions. These carboxylate ions have a polar region (COO-) that interacts with water and a nonpolar region that interacts with dirt and oil. The nonpolar region grabs the dirt and oil from your body and the polar region interacts with water to wash it off.
All right, let's take a moment or two to review. Monocarboxylic acids are molecules with one COOH functional group. And remember that a carboxylic acid is an organic molecule that has mostly carbon and hydrogen atoms, but also contains a carbon that has a double bond to an oxygen. They're weak acids, which can lose a proton, or deprotonate, from carboxylate ions. They're soluble in water if they have fewer than five carbons. They have a higher boiling point than water because they form dimers, which are two molecules that are associated with each other.
There are many ways to synthesize carboxylic acids. Here are three common ways:
- Primary Alcohol + Chromic Acid
- Nitrile + Water (remember that nitriles are molecules with a carbon that has a triple bond to a nitrogen and that whenever water reacts with a compound, the process is called hydration or hydrolosis)
- Acyl Chloride + Water (remember that acyl halide is a halogen attached to a molecule of just carbon and hydrogen)
We then learned that monocarboxylic acids are used as medicines (aspirin and cetirizine), and that carboxylate ions are used in soap.
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