Aldehydes & Ketones

• Bond Polarity: The oxygen atom is more electronegative than the carbon atom, creating a permanent dipole where the oxygen is $\delta-$ and the carbon is $\delta+$. This electronic arrangement makes the carbon atom an electrophilic center.

• Nucleophilic Susceptibility: Because the carbonyl carbon is electron-deficient, it is highly susceptible to attack by nucleophiles (species with a lone pair of electrons). This is the fundamental driving force for the majority of reactions involving aldehydes and ketones.

• Planar Geometry: The carbonyl group has a trigonal planar geometry around the carbon atom. This planarity is crucial for stereochemistry, as it allows nucleophiles to attack from either side of the plane with equal probability.

Oxidation of Alcohols

• Primary Alcohols to Aldehydes: Primary alcohols are oxidized to aldehydes using acidified potassium dichromate($VI$). To prevent further oxidation to carboxylic acids, the aldehyde must be immediately distilled out of the reaction mixture as it forms.
• Secondary Alcohols to Ketones: Secondary alcohols are oxidized to ketones under reflux with acidified potassium dichromate($VI$). Unlike aldehydes, ketones do not undergo further oxidation easily because they lack a hydrogen atom bonded to the carbonyl carbon.

Reduction of Carbonyls

• Reagents: Sodium tetrahydridoborate ($NaBH_4$) in aqueous solution or Lithium tetrahydridoaluminate ($LiAlH_4$) in dry ether are common reducing agents. These provide the hydride ion ($:H^-$) which acts as the nucleophile.
• Outcomes: Aldehydes are reduced back to primary alcohols, while ketones are reduced to secondary alcohols. This is a nucleophilic addition reaction.

• Oxidation Sensitivity: Aldehydes are easily oxidized to carboxylic acids, whereas ketones are resistant to oxidation. This difference is the basis for chemical tests used to distinguish between them.

• Tollens' Mechanism: Tollens' reagent contains $[Ag(NH_3)_2]^+$ ions; the aldehyde reduces $Ag^+$ to metallic silver while being oxidized to a carboxylate ion.

• Reaction Overview: Hydrogen cyanide ($HCN$) reacts with carbonyl compounds to form hydroxynitriles. This reaction is significant in organic synthesis because it increases the carbon chain length by one atom.

• Mechanism Step 1: The nucleophilic cyanide ion ($:CN^-$) attacks the $\delta+$ carbonyl carbon, breaking the $C=O$ pi bond and moving the electrons to the oxygen to form a negatively charged intermediate.

• Mechanism Step 2: The intermediate oxygen atom is protonated by an $H^+$ ion (from $HCN$ or water) to form the final hydroxyl ($-OH$) group.

• Formation of Chiral Centers: If the carbonyl carbon is bonded to two different groups (as in most aldehydes and unsymmetrical ketones), the addition of a nucleophile like $CN^-$ creates a chiral center.

• Racemic Mixtures: Because the carbonyl group is planar, the nucleophile has a 50% chance of attacking from above or below the plane. This results in an equal mixture of two enantiomers, known as a racemic mixture or racemate.

• Optical Activity: While individual enantiomers rotate plane-polarized light, a racemic mixture is optically inactive because the opposite rotations of the two enantiomers cancel each other out.

• Distillation vs. Reflux: Always specify 'distillation' when preparing an aldehyde from a primary alcohol to show you understand how to prevent over-oxidation. Use 'reflux' for ketones or carboxylic acids.

• Naming Nitriles: When naming hydroxynitriles, the carbon in the $-CN$ group is always carbon-1. The hydroxyl group is treated as a prefix ('hydroxy-') rather than a suffix.

• Identifying Unknowns: If a question asks how to distinguish an unknown carbonyl, Tollens' reagent is the 'gold standard' answer due to the distinct silver mirror observation.

• Mechanism Precision: In nucleophilic addition diagrams, ensure the curly arrow starts exactly from the lone pair of the nucleophile and points directly to the $\delta+$ carbon atom.