Carboxylic Acid
A carboxylic acid is an organic compound containing a carboxyl group (–COOH), which consists of both a carbonyl (C=O) and a hydroxyl (–OH) group attached to the same carbon. Carboxylic acids are weak acids that partially dissociate in water to donate a proton (H⁺) from the –OH group, with the resulting carboxylate anion stabilized by resonance. They are widespread in biology (amino acids, fatty acids, citric acid cycle intermediates) and industry (acetic acid for vinegar, benzoic acid as a preservative).
Key Formula
R-COOH <=> R-COO(-) + H(+); Ka = [RCOO-][H+] / [RCOOH]
LaTeX: R\text{-}COOH \rightleftharpoons R\text{-}COO^- + H^+, \quad K_a = \frac{[R\text{-}COO^-][H^+]}{[R\text{-}COOH]}
| Symbol | Meaning | Unit |
|---|---|---|
| R | alkyl or aryl group | none |
| Ka | acid dissociation constant | mol/L |
| [R-COO⁻] | carboxylate ion concentration | mol/L |
| [H⁺] | proton concentration | mol/L |
Worked Example
Problem
Acetic acid (CH₃COOH) has Ka = 1.8 × 10⁻⁵. Find the pH of a 0.10 M acetic acid solution.
Solution
Step 1: Set up ICE table. Initial: [CH₃COOH] = 0.10 M, [CH₃COO⁻] = 0, [H⁺] = 0. Step 2: At equilibrium: [CH₃COO⁻] = [H⁺] = x; [CH₃COOH] = 0.10 − x. Step 3: Ka = x² / (0.10 − x). Since Ka is small, approximate 0.10 − x ≈ 0.10. Step 4: x² = 1.8 × 10⁻⁵ × 0.10 = 1.8 × 10⁻⁶. Step 5: x = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ M. Step 6: pH = −log(1.34 × 10⁻³) = 2.87.
Answer
pH ≈ 2.87
Common Carboxylic Acids and Their Properties
| Name | Formula | Ka (25°C) | pKa | Occurrence/Use |
|---|---|---|---|---|
| Formic acid | HCOOH | 1.77 × 10⁻⁴ | 3.75 | Ant venom, leather tanning |
| Acetic acid | CH₃COOH | 1.8 × 10⁻⁵ | 4.74 | Vinegar, solvent |
| Propionic acid | C₂H₅COOH | 1.34 × 10⁻⁵ | 4.87 | Food preservative |
| Benzoic acid | C₆H₅COOH | 6.5 × 10⁻⁵ | 4.19 | Food preservative (E210) |
| Lactic acid | CH₃CHOHCOOH | 1.4 × 10⁻⁴ | 3.86 | Muscle metabolism, yogurt |
Interactive Tools
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Related Terms
Ester (organic)
An ester is an organic compound formed when a carboxylic acid reacts with an alcohol in a condensation reaction (esterification), with the loss of a water molecule, producing the functional group –COO– (or –RCOOR'). Esters are responsible for many characteristic fruit aromas and flavors, and they occur naturally in fats and oils (triglycerides are esters of glycerol and fatty acids). Industrially, esters are used as plasticizers, solvents, perfumes, and biodiesel components.
Alcohol (organic)
In organic chemistry, an alcohol is a compound in which a hydroxyl group (–OH) is bonded to a saturated carbon atom. Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°) based on how many carbon atoms are attached to the carbon bearing the –OH group. They are among the most versatile compounds in synthesis and commerce, serving as solvents, antiseptics, fuels, and precursors to aldehydes, ketones, and carboxylic acids.
Functional Group
A functional group is a specific atom or group of atoms within an organic molecule that determines its characteristic chemical reactions and physical properties. Functional groups are the reactive centers of organic molecules; the carbon skeleton (backbone) largely determines size and shape, while the functional group dictates behavior. Identifying functional groups is the foundation of systematically predicting and classifying organic reactions.
From Latin "carbo" (carbon/coal) + "oxy-" (oxygen, from Greek "oxys," sharp/acid) + "-lic" (adjectival suffix). The carboxyl group name was coined in the 19th century to reflect the carbon-oxygen-oxygen (C(=O)OH) composition. Hermann Kolbe and others systematized carboxylic acid chemistry in the 1840s–60s.