Aldehydes and ketones Knowledge-Based Study

Study notes

1. Introduction to Carbonyl Compounds

   Both aldehydes and ketones are organic compounds characterized by the presence of a carbonyl group ($C=O$), in which a carbon atom is joined to an oxygen atom by a double bond.

   Structural Differences

   • Aldehydes: The carbonyl carbon is bonded to at least one hydrogen atom, placing the carbonyl group at the end of the carbon chain. Their general formula is $R-CHO$ (or $H-CHO$ for formaldehyde).
   • Ketones: The carbonyl carbon is bonded to two alkyl or aryl groups, positioning the carbonyl group within the carbon skeleton. Their general formula is $R-CO-R'$.

   The carbonyl carbon atom is $sp^2$-hybridized and adopts a trigonal planar geometry with bond angles of approximately 120°. Because oxygen is significantly more electronegative than carbon, the $C=O$ double bond is strongly polarized ($C^{\delta+}=O^{\delta-}$), rendering the carbonyl carbon electrophilic.

   Systematic Naming Rules

   IUPAC rules differentiate aldehydes and ketones based on principal functional group suffixes:

   Naming Aldehydes

   • Identify the longest continuous carbon chain that contains the $-CHO$ carbon.
   • Replace the terminal -e of the corresponding alkane name with the suffix -al.
   • The carbonyl carbon of an aldehyde is always assigned position 1, so a locant number for the carbonyl carbon is omitted (e.g., methanal, ethanal, propanal).

   Naming Ketones

   • Identify the longest chain containing the carbonyl carbon.
   • Replace the terminal -e of the parent alkane with the suffix -one.
   • Number the chain from the end that gives the carbonyl carbon the lowest possible locant (e.g., propan-2-one, pentan-3-one). Propan-2-one is commonly known as acetone.

   Intermolecular Forces, Boiling Points, and Solubility

   Boiling Points

   Due to the permanent dipole across the carbon-oxygen double bond, aldehydes and ketones interact via dipole-dipole attractions.

   • Their boiling points are higher than those of nonpolar hydrocarbons and weakly polar ethers of comparable molecular mass.
   • However, because neither aldehydes nor ketones have a hydrogen atom directly bonded to an oxygen atom, they cannot form intermolecular hydrogen bonds with themselves. Consequently, their boiling points are lower than those of isomeric or similar-sized alcohols and carboxylic acids.

   Solubility in Water

   Although carbonyl compounds cannot donate a hydrogen bond to themselves, the lone pairs on the electronegative oxygen atom can accept hydrogen bonds from water molecules. As a result, low-molecular-mass aldehydes and ketones (such as methanal, ethanal, and propan-2-one) are completely miscible with water. Solubility decreases sharply as the nonpolar hydrocarbon chain lengthens beyond four to five carbons.

   Differentiating Aldehydes and Ketones

   The presence of a hydrogen atom attached directly to the carbonyl carbon makes aldehydes substantially easier to oxidize than ketones.

   Oxidation Reactions

   • Aldehydes: Readily oxidized to carboxylic acids by common oxidizing agents such as acidified potassium dichromate ($K_2Cr_2O_7$) or potassium permanganate ($KMnO_4$).
   • Ketones: Resist mild oxidation because breaking a stable carbon-carbon bond is energetically unfavorable.

   Diagnostic Laboratory Tests

   • Tollens' Test (Silver Mirror Test): Aldehydes reduce ammoniacal silver nitrate ($[Ag(NH_3)_2]^+$) to metallic silver, forming a characteristic silver mirror on the reaction vessel. Ketones give a negative result.
   • Fehling's and Benedict's Tests: Aldehydes reduce deep blue copper(II) tartrate or citrate complexes to an insoluble red precipitate of copper(I) oxide ($Cu_2O$). Ketones typically do not react under these conditions.

   Both classes undergo characteristic nucleophilic addition reactions at the electrophilic carbonyl carbon, such as the addition of hydrogen cyanide ($HCN$) or sodium bisulfite ($NaHSO_3$).