Preparation of Aldehydes & Ketones
Construct the polarized carbonyl core. Master selective reduction with DIBAL-H, the Dialkylcadmium trap for ketones, and essential Aromatic Name Reactions.
Module Focus: Controlled Oxidation & Reduction
Aldehydes ($-CHO$) and Ketones ($>C=O$) are intermediate oxidation states between alcohols and carboxylic acids. Therefore, their preparation relies heavily on controlled oxidation of alcohols (stopping before the acid stage) or controlled reduction of acid derivatives (stopping before the alcohol stage). Using the wrong reagent will cause you to overshoot your target product.
1. General Methods (Alcohols & Hydrocarbons)
- Primary ($1^\circ$) Alcohols: Yield Aldehydes.
Must use PCC or $CrO_3$ in anhydrous medium to prevent over-oxidation to carboxylic acid. - Secondary ($2^\circ$) Alcohols: Yield Ketones. Strong oxidizers like $K_2Cr_2O_7 / H^+$ or $KMnO_4$ can safely be used because ketones strongly resist further oxidation.
Addition of water to an alkyne in the presence of $Hg^{2+} / H_2SO_4$ (Heavy metal catalyst is strictly required).
NEET Exception: Ethyne yields Acetaldehyde. ALL other alkynes yield Ketones (due to Markovnikov addition).
Reductive ozonolysis ($O_3$, then $Zn/H_2O$) cleaves the double bond and caps both ends with Oxygen. Zinc prevents the newly formed aldehydes from being oxidized into acids by $H_2O_2$.
2. Preparation of Aldehydes Only (Name Reactions)
These methods selectively reduce carboxylic acid derivatives down to the aldehyde stage without continuing on to primary alcohols.
Acid chlorides (Acyl chlorides) are hydrogenated over a catalyst of Palladium deposited on Barium Sulfate ($Pd/BaSO_4$).
The Barium Sulfate Trap:
$BaSO_4$ acts as a catalytic poison. It deactivates the Palladium enough so that the highly reactive aldehyde is NOT reduced further into a primary alcohol. (Sometimes Quinoline or Sulfur is also added to enhance poisoning).
Nitriles (Cyanides) are reduced to corresponding imines using Stannous Chloride and HCl gas ($SnCl_2 / HCl$), which upon hydrolysis yield aldehydes.
Diisobutylaluminium hydride, $AlH(i-Bu)_2$, is a highly selective reducing agent used at very low temperatures ($-78^\circ\text{C}$).
- Reduces Nitriles ($R-CN$) strictly to Aldehydes.
- Reduces Esters ($R-COOR'$) strictly to Aldehydes.
If run at room temperature, it can over-reduce, which is why the $-78^\circ\text{C}$ condition is frequently specified in exams.
3. Preparation of Benzaldehyde
Controlled oxidation of Toluene. Chromyl Chloride ($CrO_2Cl_2$) in $CS_2$ forms a brown chromium complex, preventing further oxidation. Hydrolysis yields Benzaldehyde.
Direct formylation of Benzene. Uses Carbon Monoxide ($CO$) and $HCl$ gas in the presence of Anhydrous $AlCl_3/CuCl$.
4. Preparation of Ketones Only
The most heavily tested concept here involves the use of Organometallic reagents on acid chlorides and nitriles.
Grignard reagent attacks the triple bond of a nitrile to form an imine salt, which upon acid hydrolysis yields a ketone.
$\xrightarrow{H_3O^+} R-CO-R'$
Benzene reacts with an acid chloride in the presence of anhydrous $AlCl_3$ to form an aromatic ketone.
(Acetophenone)
NEET Grand Test: Carbonyl Prep
15 High-Yield Questions testing the Dialkylcadmium mechanism, specific Name Reaction reagents, and DIBAL-H rules.
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