Aldehydes and Ketones I: Electrophilicity and Oxidation-Reduction

Aldehydes and Ketones I: Electrophilicity and Oxidation-Reduction section provides High Yield information for College Students, Medical Students to succeed in the MCAT exam and Medical School.

Aldehydes and Ketones I: Description and Properties

  • Ketone: two alkyl groups bonded to the carbonyl
  • Aldehyde: one alkyl group and one hydrogen
  • Characterized by strong smell (found in many spices)

  • Aldehydes are named by replacing the –e at the end of an alkane with the suffix –al
    • When named as substituents, use the prefix oxo-
  • Common names for first five aldehydes:
    • -formaldehyde
    • -acetaldehyde
    • -propionaldehyde
    • -butyraldehyde
    • -valeraldehyde
  • If an aldehyde is attached to a ring, the suffix –carbaldehyde is used
  • Ketones are named by replacing the –e with the suffix –on
    • When naming ketones by common name, the two alkyl groups are named alphabetically followed by –ketone.
    • When names as substituents, use either oxo- or keto-

Physical Properties

  •   Governed by the presence of the carbonyl group. Dipole is stronger than the dipole of an alcohol since the double-bonded oxygen is more electron-withdrawing.
  • Even though aldehydes and ketones have dipoles that are more polar, the boiling point of alcohols is higher due to the presence of hydrogen bonding in an alcohol.
  • Both usually act as electrophiles due to the electron withdrawing properties of the carbonyl oxygen, which leaves a partial positive charge on the oxygen.
  • Generally, aldehydes are more reactive towards nucleophiles since they have less steric hindrance and fewer electron donating groups than ketones.

  • An aldehyde can be formed from the partial oxidation of a primary alcohol, but only with PCC.
  • A ketone can be obtained from the oxidation of a secondary alcohol. Range of reagents can be used (from dichromate, chromium trioxide to PCC).
    • No concern for oxidizing too far since the reactants will stop at the ketone stage.

Nucleophilic Addition Reactions

  • Carbonyl carbon has a partial positive charge and carbonyl oxygen has a partial negative charge, which make it prime for a nucleophilic attack.
  • Nucleophile forms a covalent bond to the carbon which break the pi bond associated with carbonyl.
    • The electrons from the pi bond are pushed onto the oxygen atom.
    • This bond breaking forms a tetrahedral intermediate
  • Any time a carbonyl is opened, should ask: Can I reform the carbonyl?
    • Carbonyl will not reform if no good leaving group is present (aldehydes/ketones)
      • O will simply accept a proton form the solvent to form a hydroxyl group (alcohol)
    • Carbonyl double bond reforms if good leaving groups are present (carboxylic acid and its derivatives)
      • Double bond pushes off the leaving group

  • In presence of water, aldehydes/ketones react to form germinal diols (1,1- diols)
  • Nucleophilic oxygen in water attacks the electrophilic carbonyl carbon

Acetals and Hemiacetals
  • Hemiacetal: when one equivalent of alcohol (acts as the nucleophile) is added to an aldehyde
  • Hemiketal: when one equivalent of alcohol (acts as the nucleophile) is added to a ketone
    • Recognized by the retention of the hydroxyl group
  • Hemi is known as the halfway point and is the endpoint in basic conditions
  • When two equivalents of alcohol are added, the reaction proceeds to completion which results in the formation of an Acetal and a ketal.
    • Reaction proceeds by the substitution reaction SN1 and is catalyzed by anhydrous acid
    • Hydroxyl group of a hemiacetal or hemiketal is protonated under acidic conditions and lost as a molecule of water. Which leads to the formation of a carbocation.
    • Another equivalent of alcohol attacks the carbocation which results in the formation of an Acetal or ketal.
    • Are relatively inert, so they are often used as protecting groups for carbonyl functionalities.
      • Can be converted back to carbonyls with aqueous acid and heat

Imines and Enamines
  • Nitrogen and nitrogen based functional groups act as good nucleophiles since nitrogen has a lone pair of electrons
  • Imine: simplest case. Ammonia adds to a carbon atom and water is lost
    • Compound with a nitrogen atom double bonded to a carbon atom
    • Condensation reaction: small molecule is lost in the formation of the bond
    • Nucleophilic substitution since nitrogen replaces carbonyl oxygen
  • Common ammonia derivatives: hydroxylamine, hydrazine, semicarbazide
  • Enamines: contain both a double bond and a nitrogen-containing group
    • Imines can undergo tautomerization to form these

  • Hydrogen cyanide is a classic nucleophile on the MCAT
    • Has both triple bonds and an electronegative nitrogen atom which makes it relatively acidic (pKa of 9.2).
  • After the hydrogen atom dissociates, the nucleophilic CN can attack the carbonyl carbon atom.
  • Reaction with aldehydes and ketones produces stable compounds called cyanohydrins
    • Gains stability from newly formed C-C bond.

Oxidation-Reduction Reactions

  • Aldehydes are in the middle of the redox spectrum since they are more oxidized than alcohols but less oxidized than carboxylic acid
  • Ketones are as oxidized as secondary carbons can get.

Oxidation of Aldehydes
  • When aldehydes are oxidized further they from carboxylic acid.
    • Facilitated through any oxidizing agent that is stronger than PCC

Reduction by Hydride Reagents
  • Can also undergo reduction to form alcohols
  • Often performed by hydride reagents (lithium aluminum hydride & sodium borohydride)