Study Notes on Aldehydes and Ketones

Chapter 19: The Chemistry of Aldehydes and Ketones

19.1 – Structure and Bonding in Aldehydes and Ketones

  • Aldehydes and ketones are defined as carbonyl compounds due to the presence of a carbonyl group (C=O).

19.2 – Nomenclature of Aldehydes and Ketones

  • Common Prefixes Used in Nomenclature:

    • R— In R—CH＝O or R—CO₂H

    • Notable examples include:

    • Acet: ext{H}_3 ext{C}—

    • Isobutyr: ( ext{CH}_3)_2 ext{CH}—

    • Butyr: ext{CH}_3 ext{CH}_2 ext{CH}_2—

    • Valer: ext{CH}_3 ext{CH}_2 ext{CH}_2 ext{CH}_2—

  • Aldehydes have the IUPAC name suffix -al.

    • For aldehydes bonded to a ring: use carbaldehyde.

  • Ketones receive the IUPAC name suffix -one.

    • For aromatic ketones: name the two alkyl groups in alphabetical order followed by "ketone".

  • Polyfunctional Compounds: If multiple functional groups are present, one functional group is prioritized as the principal functional group, while others are considered substituents. Aldehyde and ketone carbonyl groups have higher priority than alcohol (-OH) or thiol (-SH) groups.

19.3 – Physical Properties of Aldehydes and Ketones

  • Simple aldehydes and ketones are typically liquids.

  • Their boiling points are lower than those of corresponding alcohols due to not being hydrogen-bond donors.

  • They are polar compounds because of the C=O bond dipole.

  • Solubility in Water: Small aldehydes and ketones (four or fewer carbons) are water-soluble since they can act as hydrogen bond acceptors.

19.4 – Spectroscopy of Aldehydes and Ketones

  • Infrared (IR) Spectroscopy:

    • Strong C=O stretch observed at 1700 ext{ cm}^{-1} for both aldehydes and ketones.

    • Aldehydes exhibit C—H stretching near 2710 ext{ cm}^{-1}.

    • Conjugation with a π bond lowers absorption frequency.

  • NMR Spectroscopy:

    • 13C NMR: Carbonyl C=O appears at chemical shifts ext{δ}=190–220.

    • α-Carbons appear at ext{δ}=30–50.

    • The aldehydic protons display a large chemical shift due to the electronic environment.

  • UV-Vis Spectroscopy:

    • π → π* transitions occur around 150 ext{ nm} (below the operating range); n → π* transitions at 260–290 ext{ nm}, significantly weaker than π-π transitions.

    • Conjugation in aldehydes or ketones leads to different λmax values.

19.5 – Synthesis of Aldehydes and Ketones

  • Synthetic Methods:

    1. Oxidation of alcohols.

    2. Synthesis from alkynes.

    3. Friedel–Crafts acylation.

    4. Ozonolysis of alkenes.

    5. Periodate cleavage of glycols.

19.6-8 – Introduction to Aldehyde and Ketone Reactions

  • Basicity of Aldehydes and Ketones: The carbonyl oxygen displays weak basic properties. Comparison of pKa values of conjugate acids shows that protonated alcohols are less acidic than protonated carbonyls.

  • Reversible Addition Reactions: Aldehydes and ketones can undergo reversible addition reactions where a nucleophile adds across the C=O bond.

19.9 – Reduction of Aldehydes and Ketones to Alcohols

  • Aldehydes can be reduced to primary alcohols, while ketones are reduced to secondary alcohols via metal hydride reagents like LiAlH4 and NaBH4.

19.10 – Reactions with Grignard and Related Reagents

  • Grignard reagents (R—MgX) and organolithium reagents react as nucleophiles with carbonyl groups, forming irreversible addition products due to their basicity.

19.11 – Acetals and Their Use as Protecting Groups

  • Formation of Acetals: Acetals form when aldehydes or ketones react with excess alcohol in acidic conditions, yielding hemiacetals first, which can then convert to acetals.

  • Acetals are stable under strong reducing conditions and can revert back to aldehydes or ketones when treated with water in the presence of acid.

19.12 – Reactions with Amines

  • Imines Formation: Primary amines react with aldehydes or ketones to form imines, while secondary amines form enamines.

  • Mechanism: Involves nucleophilic addition followed by dehydration under acid catalysis.

19.13 – Reduction of Carbonyl Groups to Methylene Groups

  • Wolff–Kishner Reduction: Reduces carbonyl groups to methylene under basic conditions via imine intermediates.

  • Clemmensen Reduction: Another method to achieve a similar transformation.

19.14 – The Wittig Alkene Synthesis

  • This synthesis allows the preparation of alkenes from aldehydes and ketones via the addition-elimination sequence involving phosphorus ylides.

  • Regioselectivity: Highly regioselective; the starting alkyl halide influences the formation of the product alkene.

19.15 – Oxidation of Aldehydes to Carboxylic Acids

  • Common oxidants include Cr(VI) reagents, with aldehydes oxidizing more readily than ketones.

  • Tollen’s test indicates aldehyde presence via the formation of a metallic silver mirror.

19.16 – Aldehydes and Ketones in Organic Synthesis

  • Aldehydes and ketones serve as versatile intermediates in organic synthesis, often undergoing various transformations, including nucleophilic attacks, reductions, and oxidations.

Textbook Problems

  • Problems related to sections 19.1, 19.2, 19.3, and 19.4 recommended for practice.