Aldehydes and Ketones: Comprehensive Notes

Aldehydes and Ketones

The Carbonyl Functional Group

  • The carbonyl functional group is a carbon atom double-bonded to an oxygen atom (C=O) and single-bonded to two other atoms.
  • It is characteristic of aldehydes, ketones, carboxylic acids, and their derivatives.
  • In an aldehyde, the carbonyl carbon is directly bonded to at least one hydrogen atom.
  • In a ketone, the carbonyl carbon is directly bonded to two carbon atoms.
  • General formulas:
    • Aldehyde: R-CHO
    • Ketone: R-CO-R'

Nomenclature

  1. IUPAC Naming Conventions

    • Select the longest carbon chain containing the functional group as the parent chain.

  2. Aldehydes

    • Change the suffix -e of the parent alkane to -al.
    • The carbonyl group of an aldehyde is always at the beginning of the parent chain (carbon 1), so no number is needed to locate it.

  3. Ketones

    • Change the suffix -e of the parent alkane to -one.

  4. Numbering

    • Number the parent chain so the carbonyl carbon has the lowest possible number.

  5. Substituents

    • Identify and locate all other substituents.
    • Write the location number with the substituent (alphabetically) in front of the parent chain.
    • Use prefixes, commas, and dashes as usual.

  6. Higher-Priority Groups

    • If there is a higher-priority group, the carbonyl oxygen atom is treated as a substituent described by the prefix "oxo-" and given a number.

Acyl Groups

  • Acyl groups can be substituents.
  • Common acyl groups:

Physical Properties

  • Small molecule carbonyl compounds are water-soluble.
  • Acetone and acetaldehyde are miscible with water.
  • Carbonyl compounds are polar, forming dipole-dipole interactions.
  • They have higher boiling points than hydrocarbons with the same number of carbons (but lower than alcohols).

Reactions of Aldehydes and Ketones

  • Aldehydes and ketones react with nucleophiles.
  • Reactivity decreases as the number of R groups around the carbonyl carbon increases.
  • Order of reactivity: Formaldehyde > Aldehydes > Ketones

  1. Preparation of aldehydes and ketones

  2. Reactions of carbonyl compounds

    • Reactions at the carbonyl carbon: a nucleophile is added to the carbonyl group.
    • Reactions at the α-carbon.

Reactions at the Carbonyl Carbon with Nucleophiles

  • In the presence of hydroxide ions (basic conditions) or under acidic conditions, ketones and aldehydes form hydrates.
  • Basic conditions:
  • Acidic conditions:
  • Other nucleophiles can also add to the carbonyl carbon of aldehydes and ketones.
  • Nucleophilic trends in carbonyl attack differ from those in SN1 and SN2 reactions on sp^3 hybridized carbon atoms.
  • Cl^-, Br^-, and I^- are excellent nucleophiles in SN1 and SN2 reactions but ineffective for attacks on the carbonyl carbon.
  • Effective nucleophiles for carbonyl addition include cyanide.

Addition of Cyanide to Carbonyl Compounds

The Wittig Reaction

  • Reagent:

    • Ylide (phosphorus ylide) also known as Wittig reagent.

  • Mechanism:

    • Formation of ylide from triphenylphosphine and alkyl halide via S_N2 reaction.

    (Ph)3P: + CH3X \rightarrow (Ph)3P^+ - CH3 X^-

    • Deprotonation of phosphonium salt to form ylide.

    (Ph)3P^+ - CH3 + BuLi \rightarrow (Ph)3P=CH2 + BuH + LiX

  • Wittig reaction converts a carbonyl compound (aldehyde or ketone) to an alkene.

    (Ph)3P=CH2 + R2C=O \rightarrow R2C=CH2 + (Ph)3P=O

  • Goes through an oxaphosphetane intermediate.

  • Specific Examples

  • General interconversions

  • Why we would use the wittig reaction

Amines and Imines

  • Amines are classified as primary (1^o), secondary (2^o), or tertiary (3^o) based on the number of alkyl groups bonded to the nitrogen atom.

  • Treatment of an aldehyde or ketone with a primary amine yields an imine (also known as a Schiff base).

  • The imine nitrogen is sp^2 hybridized.

  • Imine formation is fastest under weakly acidic conditions.

  • Reaction

    R2C=O + R'NH2 \rightarrow R2C=NR' + H2O

Enamine Formation

  • A secondary amine reacts with an aldehyde or ketone to give an enamine.

  • Enamines have a nitrogen atom bonded to a C=C double bond.

  • Reaction

    R2C=O + R'2NH \rightarrow R2C=CHR' + H2O

Imine and Enamine Formation

  • With a primary amine, the intermediate iminium ion still has a proton on the N atom that may be removed to form a C=N (imine).
  • With a secondary amine, the intermediate iminium ion has no proton on the N atom. Therefore, a proton must be removed from an adjacent C—H bond, forming a C=C (enamine).

Reactions of Aldehydes and Ketones with Alcohols

  • Aldehydes and ketones react with alcohols to form hemiacetals and acetals.

  • Hemiacetals are formed when one molecule of alcohol adds to the carbonyl carbon.

  • Acetals are formed when two molecules of alcohol add to the carbonyl carbon.

  • Reaction

    R2C=O + ROH \rightleftharpoons R2C(OH)OR

    • Hemiacetal formation

    R2C(OH)OR + ROH \rightleftharpoons R2C(OR)2 + H2O

    • Acetal formation

  • Hemiacetals are generally unstable and exist as minor components of an equilibrium mixture, except when a hydroxyl group is part of the same molecule that contains the carbonyl group and a 5- or 6-membered ring can form.

Acetals as Protecting Groups for Aldehydes and Ketones in Synthesis

  • Acetals can be used as protecting groups for aldehydes and ketones in synthesis.
  • Using a protecting group for the ketone allows the reduction of the ester to take place without affecting the ketone. This is because acetals are unreactive towards many reagents, such as reducing agents.