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Notes on Carbonyl and Carboxyl Groups: Structure, Properties, and Forms

Understanding Carbonyl and Carboxyl Groups

The Carbonyl Group

  • Definition: A functional group where a carbon atom is double-bonded to an oxygen atom (C=O).

  • Carbon Valence: Carbon always has a valence of 4, meaning it forms four bonds. In a carbonyl, the carbon forms two bonds with oxygen and two other bonds with different atoms or groups (one to the right, one to the left).

  • Polarity: Due to the electronegativity difference between carbon and oxygen, the carbonyl group is polar and possesses a partial charge.

  • Solubility: Its polarity makes it soluble in water.

  • Occurrence: Carbonyl groups are found in various biological molecules, including ketones and sugars, among many other types of molecules.

The Carboxyl Group

  • Formation: A carboxyl group is formed when a hydroxyl group (-OH) is directly connected to a carbonyl group (C=O).

  • Nomenclature: The name "carboxyl" is a portmanteau: "carb" comes from "carbonyl," and "oxyl" comes from "hydroxyl."

  • Composition: It encompasses the entire structure of the carbonyl and the attached hydroxyl group. Visual aids might represent the whole carboxyl in a green box, the carbonyl part in yellow, and the hydroxyl part in red.

  • Commonality: Carboxyl groups are extremely common and are characteristic components of carboxylic acids, organic acids, and many other organic molecules.

Forms of Carboxyl Groups: Uncharged (Neutral) vs. Charged

  • Writing Conventions: There are different ways to represent a carboxyl group:

    • Neutral/Uncharged Form: Typically written as C=O with an attached OH group (C(=O)OH).

    • Textbook Abbreviation: Often abbreviated as COOH.

  • Acidic Property: An acid, by definition, donates hydrogen ions (H^+) when dissolved in a solution, particularly water.

  • Dissociation in Water: When a carboxylic acid dissolves in water, it loses its hydrogen ion from the hydroxyl group. This results in the oxygen atom carrying a negative charge (O^-).

    • The charged form is represented as C=O with an attached O^- group (C(=O)O^-).

    • Abbreviated Charged Form: Commonly written as COO^- (with the H^+ having dissociated).

  • Equilibrium: In any organic acid solution, there is an equilibrium, meaning a certain proportion always exists in the uncharged (neutral) form, and another proportion exists in the charged form.

  • Naming Convention for Uncharged Form: The name of the molecule in its neutral, uncharged form typically ends in "acid."

    • Examples: Lactic acid, Pyruvic acid, Citric acid.

  • Naming Convention for Charged Form: The name of the molecule in its charged form (after losing H^+ in solution) typically ends in "ate."

    • Examples: Lactate (from lactic acid), Pyruvate (from pyruvic acid), Citrate (from citric acid).

  • Key Distinction: The sole difference between the "acid" and "ate" forms is the presence or absence of the dissociable hydrogen ion.

  • Practical Relevance: Students should not be confused by these different writing conventions as they refer to the same chemical entity in different states. These forms will be encountered frequently in biological processes, such as cellular respiration.