Aldehydes and Ketones part 2

Common Aldehydes and Ketones

Formaldehyde (HCHO)

  • Colorless gas at room temperature; pungent, suffocating odor.

  • Low concentrations (0.1–1.1 ppm) can cause irritation in eyes, throat, bronchial system; higher concentrations trigger asthma.

  • Contact can lead to dermatitis.

  • Formed from incomplete combustion of hydrocarbons, contributing to smog irritation.

  • Causes kidney damage, coma, and can be lethal; breakdown product of methyl alcohol.

  • Sold as 37% aqueous solution (formalin); disinfects by reacting with amino groups in proteins.

  • Polymerizes into paraformaldehyde when standing.

Acetaldehyde (CH3CHO)

  • Sweet-smelling, flammable liquid; oxidation of ethyl alcohol.

  • Less toxic than formaldehyde; minor production during carbohydrate breakdown.

  • Formerly used to produce acetic acid; large doses can cause respiratory failure.

  • Used for making polymeric resins and in mirror silvering.

Acetone (CH3COCH3)

  • Widely used organic solvent; dissolves most organic compounds.

  • Miscible with water; volatile, poses fire/explosion hazard in closed spaces.

  • No chronic health risks with casual exposure; produced during fat/carbohydrate breakdown.

Benzaldehyde (PhCHO)

  • Colorless liquid with pleasant almond or cherry-like odor; extracted from bitter almonds.

  • Used as flavoring/fragrance in food, cosmetics, pharmaceuticals, and soap.

  • Generally regarded as safe by the FDA; industrial precursor for various compounds.

Oxidation of Aldehydes

Overview

  • Aldehydes oxidize to carboxylic acids; alcohols can oxidize to aldehydes or ketones.

  • Aldehyde oxidation involves replacing hydrogen on carbonyl carbon with an –OH group.

  • Ketones do not react cleanly with oxidizing agents.

Testing Methods

  • Tollens’ Reagent: Silver ion in ammonia; distinguishes aldehydes (forms carboxylic acid and silver) from ketones.

  • Benedict’s Reagent: Blue Cu(II) ions reduce to red Cu(I) oxide with aldehydes; less effective for ketones.Note: Once used for detecting sugars in urine.

Reduction of Aldehydes and Ketones

Overview

  • Aldehydes and ketones can be reduced to alcohols.

  • Reduction occurs via hydrogen addition to carbonyl group, producing –OH group.

  • Aldehydes form primary alcohols; ketones form secondary alcohols.

Mechanism

  • Hydride ion (H–) adds to carbonyl carbon, creating a negative charge on the carbonyl oxygen.

  • An acid (H+) bonds to oxygen, resulting in neutral alcohol.

Biological Systems

  • Coenzyme NAD+ acts as a reducing agent; cycles between forms (NADH/NAD+) by transferring hydride ions.

Worked Example 15.2

Reduction of Benzaldehyde

  • Steps to find product:

    1. Draw structure of benzaldehyde with a carbonyl double bond.

    2. Modify structure to show a single bond between C and O; include partial bonds.

    3. Add hydrogen atoms and rewrite product.

  • Product: Benzyl alcohol.