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A carboxylic acid contains a carboxy (carboxyl) group, COOH, which has one oxygen atom doubly bonded to the carbon atom and a hydroxy (OH) group singly bonded to the carbon atom. The carbon atom of the carboxy group must also be attached to an organic group or a hydrogen. Examples of common carboxylic acids include:
Carboxylic acids that have long (12 to 24 carbon atom) unbranched aliphatic groups are called fatty acids.
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The hydrogen atom of the carboxyl group can dissociate (detach as H+) only to a very small extent, behavior that classifies carboxylic acids as weak acids. For example, we can say that the following equilibrium lies very far to the left side of the reaction (i.e. very little acetate anion forms in solution):
The names of carboxylic acids should end in "oic acid" according to official IUPAC nomenclature rules (see link below), however older common names that end in "ic acid" are still used. The "oic acid" naming scheme is superior because it unambiguously identifies the material as a carboxylic acid. In cases where another functional group takes naming precedence, "carboxy" is used as a prefix, although in certain cases the suffix "carboxylic acid" may also be used!
When a carboxylic acid is neutralized with a base it becomes a salt that is itself a weak base. The salt is named using the suffix "oate" instead of "oic acid". Sodium benzoate, shown below, is a common food preservative:
Carboxylic acids occur naturally in many organisms, and are primary components of your DNA. Examples of carboxylic acids you routinely encounter include ascorbic acid (vitamin C), acetic acid (vinegar is a 5% aqueous solution of vinegar), and citric acid (responsible for the sourness/tartness of citrus fruits).
Carboxylic acids are generally weak acids, but that does not mean that they can not be corrosive or pose other hazards. Always use proper personal protective equipment (PPE) such as goggles and gloves when working with carboxylic acids. Take care to avoid storage or use with incompatible chemicals such as bases. Avoid storing containers in metal cabinets, which can corrode. Caroxylic acids may also be flammable or combustible.
See also: acid, corrosive, ketone, pH.
Additional definitions from Google and OneLook.
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