Methods of Preparation — Revision Notes

⚡ 30-Second Revision

Primary Alcohol to Aldehyde: — $R-CH_2OH \xrightarrow{PCC}$ or $R-CH_2OH \xrightarrow{DMP}$
Secondary Alcohol to Ketone: — $R_2CHOH \xrightarrow{PCC}$ or $R_2CHOH \xrightarrow{K_2Cr_2O_7/H_2SO_4}$
Ozonolysis of Alkene: — $R_2C=CR_2' \xrightarrow{1. O_3, 2. Zn/H_2O} R_2C=O + R_2'C=O$
Hydration of Ethyne: — $HC equiv CH \xrightarrow{HgSO_4/H_2SO_4} CH_3CHO$
Hydration of Terminal Alkyne: — $R-C equiv CH \xrightarrow{HgSO_4/H_2SO_4} R-CO-CH_3$
Rosenmund Reduction: — $R-COCl + H_2 \xrightarrow{Pd/BaSO_4} R-CHO$
Stephen Reaction: — $R-C equiv N \xrightarrow{1. SnCl_2/HCl, 2. H_3O^+} R-CHO$
DIBAL-H Reduction (Nitrile/Ester): — $R-C equiv N$ or $R-COOR' \xrightarrow{1. DIBAL-H, -78^circ C, 2. H_3O^+} R-CHO$
Grignard with Nitrile: — $R-C equiv N + R'-MgX \xrightarrow{H_3O^+} R-CO-R'$
Friedel-Crafts Acylation: — $Ar-H + R-COCl \xrightarrow{Anhydrous AlCl_3} Ar-CO-R$
Etard Reaction: — $C_6H_5-CH_3 \xrightarrow{1. CrO_2Cl_2, 2. H_3O^+} C_6H_5-CHO$
Gattermann-Koch Reaction: — $C_6H_6 + CO + HCl \xrightarrow{Anhydrous AlCl_3/CuCl} C_6H_5-CHO$

2-Minute Revision

Quickly review the main pathways to synthesize aldehydes and ketones. For aldehydes, remember mild oxidation of primary alcohols using PCC or DMP. From carboxylic acid derivatives, Rosenmund reduction of acyl chlorides and Stephen reaction or DIBAL-H reduction of nitriles (and esters) are key.

Aromatic aldehydes like benzaldehyde can be made from toluene via Etard reaction or from benzene via Gattermann-Koch. For ketones, secondary alcohols are oxidized using various agents including PCC or acidified dichromate.

Ozonolysis of alkenes is a versatile method for both, depending on alkene substitution. Hydration of alkynes (except ethyne) yields ketones. From carboxylic acid derivatives, Grignard reagents with nitriles or dialkylcadmium with acyl chlorides are important.

Aromatic ketones are typically formed by Friedel-Crafts acylation. Always pay attention to the specific reagents and conditions, as they dictate the selectivity and the final product.

5-Minute Revision

To master the preparation methods, categorize them by starting material.

1. From Alcohols: Primary alcohols ($R-CH_2OH$) yield aldehydes ($R-CHO$) with mild oxidants like PCC or Dess-Martin Periodinane (DMP) in anhydrous conditions. This prevents over-oxidation to carboxylic acids. Secondary alcohols ($R_2CHOH$) yield ketones ($R_2C=O$) with both mild oxidants (PCC, DMP) and stronger ones like acidified $K_2Cr_2O_7$ or Jones reagent. Tertiary alcohols are resistant to oxidation.

2. From Hydrocarbons: Ozonolysis of alkenes ($R_2C=CR_2'$) followed by reductive workup ($Zn/H_2O$ or $Me_2S$) cleaves the double bond to form aldehydes and/or ketones. The products depend on the substitution of the alkene. Hydration of alkynes ($R-C equiv C-R'$) with $HgSO_4/H_2SO_4$ forms enols that tautomerize to carbonyls. Ethyne gives acetaldehyde, while other terminal alkynes give methyl ketones following Markovnikov's rule.

3. From Carboxylic Acid Derivatives: Acyl chlorides ($R-COCl$) can be reduced to aldehydes ($R-CHO$) via Rosenmund reduction ($H_2/Pd/BaSO_4$, where $BaSO_4$ poisons the catalyst). They react with dialkylcadmium ($R_2'Cd$) to form ketones ($R-CO-R'$).

Nitriles ($R-C equiv N$) are reduced to aldehydes ($R-CHO$) by the Stephen reaction ($SnCl_2/HCl$ followed by hydrolysis) or by DIBAL-H at low temperatures. Nitriles react with Grignard reagents ($R'-MgX$) followed by hydrolysis to yield ketones ($R-CO-R'$).

Esters ($R-COOR'$) can also be reduced to aldehydes ($R-CHO$) by DIBAL-H at low temperatures.

4. From Aromatic Compounds: Aromatic aldehydes like benzaldehyde ($C_6H_5-CHO$) are prepared from toluene ($C_6H_5-CH_3$) via the Etard reaction ($CrO_2Cl_2$) or from benzene ($C_6H_6$) via the Gattermann-Koch reaction ($CO/HCl/AlCl_3/CuCl$). Aromatic ketones ($Ar-CO-R$) are synthesized using Friedel-Crafts acylation (aromatic compound + acyl chloride/anhydride + $AlCl_3$).

Focus on the unique reagents and conditions for each method, especially those that ensure selectivity (e.g., PCC, poisoned catalysts, DIBAL-H at low temp). Practice predicting products and identifying reagents for various transformations.

Prelims Revision Notes

For NEET, 'Methods of Preparation' is a high-yield topic. Focus on factual recall of reagents, conditions, and products for each reaction.

I. From Alcohols:

Primary Alcohol to Aldehyde: — Reagents: PCC (Pyridinium Chlorochromate) in $CH_2Cl_2$, Dess-Martin Periodinane (DMP). These are mild oxidants to prevent over-oxidation to carboxylic acids.
Secondary Alcohol to Ketone: — Reagents: PCC, DMP, or stronger oxidants like acidified $K_2Cr_2O_7$, Jones reagent ($CrO_3/H_2SO_4$ in acetone). Tertiary alcohols do not oxidize.

II. From Hydrocarbons:

Ozonolysis of Alkenes: — Reagents: 1. $O_3$, 2. $Zn/H_2O$ (reductive workup). Cleaves $C=C$ bond. Products: Aldehydes and/or ketones. Example: $R_2C=CR_2' \rightarrow R_2C=O + R_2'C=O$.
Hydration of Alkynes: — Reagents: $HgSO_4/H_2SO_4$. Ethyne ($HC equiv CH$) $ightarrow$ Acetaldehyde ($CH_3CHO$). Other terminal alkynes ($R-C equiv CH$) $ightarrow$ Methyl ketones ($R-CO-CH_3$) (Markovnikov's rule, enol-keto tautomerism).

III. From Carboxylic Acid Derivatives:

Rosenmund Reduction (Aldehydes from Acyl Chlorides): — Reagents: $H_2/Pd/BaSO_4$. $BaSO_4$ poisons Pd to stop at aldehyde stage. $R-COCl \rightarrow R-CHO$.
Stephen Reaction (Aldehydes from Nitriles): — Reagents: 1. $SnCl_2/HCl$, 2. $H_3O^+$. $R-C equiv N \rightarrow R-CHO$.
DIBAL-H Reduction (Aldehydes from Nitriles/Esters): — Reagents: 1. DIBAL-H (Diisobutylaluminium hydride) at $-78^circ C$, 2. $H_3O^+$. $R-C equiv N \rightarrow R-CHO$; $R-COOR' \rightarrow R-CHO$.
Ketones from Nitriles: — Reagents: 1. Grignard reagent ($R'-MgX$), 2. $H_3O^+$. $R-C equiv N \rightarrow R-CO-R'$.
Ketones from Acyl Chlorides: — Reagents: Dialkylcadmium ($R_2'Cd$). $R-COCl \rightarrow R-CO-R'$.

IV. From Aromatic Compounds:

Etard Reaction (Benzaldehyde from Toluene): — Reagents: 1. $CrO_2Cl_2$ (chromyl chloride), 2. $H_3O^+$. $C_6H_5-CH_3 \rightarrow C_6H_5-CHO$.
Gattermann-Koch Reaction (Benzaldehyde from Benzene): — Reagents: $CO/HCl$ in presence of anhydrous $AlCl_3/CuCl$. $C_6H_6 \rightarrow C_6H_5-CHO$.
Friedel-Crafts Acylation (Aromatic Ketones): — Reagents: Acyl chloride ($R-COCl$) or acid anhydride, anhydrous $AlCl_3$. $Ar-H \rightarrow Ar-CO-R$.

Memorize these key transformations and their specific reagents. Practice identifying the type of carbonyl product (aldehyde or ketone) based on the starting material and reagents.

Vyyuha Quick Recall

To remember the aldehyde-specific preparations from carboxylic acid derivatives and aromatic compounds:

'Really Smart Doctors Eat Grapes'

Rosenmund (Acyl chloride $ightarrow$ Aldehyde)
Stephen (Nitrile $ightarrow$ Aldehyde)
DIBAL-H (Nitrile/Ester $ightarrow$ Aldehyde)
Etard (Toluene $ightarrow$ Benzaldehyde)
Gattermann-Koch (Benzene $ightarrow$ Benzaldehyde)