Aldehydes and Ketones — Revision Notes

⚡ 30-Second Revision

Functional Groups: — Aldehyde ($R-CHO$), Ketone ($R-CO-R'$)
Carbonyl Group: — Planar, $sp^2$ hybridized, polar ($delta^+C=delta^-O$)
Reactivity: — Aldehydes > Ketones (Nucleophilic Addition) due to steric and electronic effects.
Preparation:

* $1^circ$ Alcohol $\xrightarrow{PCC}$ Aldehyde * $2^circ$ Alcohol $\xrightarrow{Oxidation}$ Ketone * Alkene $\xrightarrow{O_3, Zn/H_2O}$ Aldehyde/Ketone * Alkyne $\xrightarrow{HgSO_4, H_2SO_4}$ Ketone (except ethyne $\rightarrow$ ethanal) * $RCOCl \xrightarrow{H_2, Pd/BaSO_4}$ $RCHO$ (Rosenmund) * $RCN \xrightarrow{SnCl_2/HCl, H_3O^+}$ $RCHO$ (Stephen) * $RCN/RCOOR' \xrightarrow{DIBAL-H}$ $RCHO$

Nucleophilic Addition: — $HCN \rightarrow$ Cyanohydrins; $RMgX \rightarrow$ Alcohols; Alcohols $\rightarrow$ Acetals/Ketals; $NH_2Z \rightarrow$ Imines, Oximes, Hydrazones.
Reduction:

* To Alcohols: $RCHO \rightarrow 1^circ ROH$, $R_2CO \rightarrow 2^circ ROH$ (using $LiAlH_4, NaBH_4$) * To Hydrocarbons: $C=O \rightarrow CH_2$ (Clemmensen: $Zn-Hg/HCl$; Wolff-Kishner: $NH_2NH_2/KOH$)

Oxidation:

* Aldehydes: $\xrightarrow{Tollens'/Fehling's/KMnO_4}$ Carboxylic Acids * Ketones: Resistant to mild oxidation; strong oxidation causes C-C cleavage.

Alpha-Hydrogen Reactions:

* Aldol Condensation: Aldehydes/Ketones with $\alpha$-H $\xrightarrow{Dilute Base}$ $\beta$-hydroxy carbonyl $\xrightarrow{\Delta}$ $\alpha,\beta$-unsaturated carbonyl. * Cannizzaro Reaction: Aldehydes *without* $\alpha$-H $\xrightarrow{Conc. Base}$ Alcohol + Carboxylate salt.

Haloform Reaction: — $CH_3CO-$ or $CH_3CH(OH)-$ group $\xrightarrow{X_2/NaOH}$ $CHX_3$ (haloform) + Carboxylate salt.

2-Minute Revision

Aldehydes and ketones are defined by the carbonyl group ($C=O$). Aldehydes ($R-CHO$) have at least one hydrogen on the carbonyl carbon, making them easily oxidizable and more reactive towards nucleophilic addition than ketones ($R-CO-R'$), which have two alkyl/aryl groups.

The carbonyl carbon is electrophilic due to its polarity. Key preparation methods include oxidation of primary/secondary alcohols, ozonolysis of alkenes, and hydration of alkynes. Specific named reactions like Rosenmund (acyl chloride to aldehyde) and Stephen (nitrile to aldehyde) are important.

Their characteristic reactions are nucleophilic additions (e.g., with HCN, Grignard reagents, alcohols, ammonia derivatives). Reduction to alcohols uses $LiAlH_4$ or $NaBH_4$, while reduction to hydrocarbons uses Clemmensen ($Zn-Hg/HCl$) or Wolff-Kishner ($NH_2NH_2/KOH$).

Aldehydes are distinguished from ketones by Tollens' and Fehling's tests (positive for aldehydes). The acidity of alpha-hydrogens leads to Aldol condensation, while aldehydes lacking alpha-hydrogens undergo Cannizzaro reaction.

Methyl ketones give a positive Haloform test. Remember physical properties like higher boiling points than hydrocarbons but lower than alcohols, and water solubility for lower members.

5-Minute Revision

Aldehydes and ketones are organic compounds featuring the $sp^2$ hybridized, planar, and highly polar carbonyl group ($C=O$). This polarity makes the carbonyl carbon electrophilic, driving their primary reaction type: nucleophilic addition. Aldehydes ($R-CHO$) are more reactive than ketones ($R-CO-R'$) in these reactions due to less steric hindrance and greater electrophilicity of the carbonyl carbon. For example, formaldehyde is the most reactive, followed by other aldehydes, then ketones.

Preparation methods are diverse: oxidation of primary alcohols yields aldehydes (e.g., with PCC), while secondary alcohols yield ketones (e.g., with $K_2Cr_2O_7$). Ozonolysis of alkenes and hydration of alkynes are also crucial. Named reactions like Rosenmund reduction ($RCOCl \rightarrow RCHO$) and Stephen reaction ($RCN \rightarrow RCHO$) are specific for aldehydes. Friedel-Crafts acylation is key for aromatic ketones.

Nucleophilic addition reactions include addition of HCN to form cyanohydrins, Grignard reagents to form alcohols (primary from formaldehyde, secondary from other aldehydes, tertiary from ketones), and alcohols to form acetals/ketals (useful protecting groups). Ammonia derivatives ($NH_2Z$) add to form imines, oximes, etc.

Reduction reactions convert carbonyls to alcohols ($LiAlH_4$, $NaBH_4$) or to hydrocarbons ($CH_2$) via Clemmensen ($Zn-Hg/HCl$) or Wolff-Kishner ($NH_2NH_2/KOH$) reductions. The choice depends on acid/base sensitivity.

Oxidation reactions are critical for distinguishing aldehydes from ketones. Aldehydes are easily oxidized to carboxylic acids by mild agents like Tollens' reagent (silver mirror) and Fehling's solution (red precipitate). Ketones are resistant to mild oxidation. The Haloform reaction ($X_2/NaOH$) is specific for methyl ketones ($CH_3CO-R$) and acetaldehyde, producing a haloform ($CHX_3$).

Reactions due to alpha-hydrogens are vital. Alpha-hydrogens are acidic due to the carbonyl group's electron-withdrawing effect. This acidity allows for Aldol condensation, where aldehydes/ketones with alpha-hydrogens react in the presence of dilute base to form $\beta$-hydroxy carbonyl compounds, which can dehydrate to $\alpha,\beta$-unsaturated carbonyls.

Aldehydes *without* alpha-hydrogens (e.g., formaldehyde, benzaldehyde) undergo Cannizzaro reaction in concentrated base, disproportionating into an alcohol and a carboxylic acid salt.

Physical properties include higher boiling points than hydrocarbons (due to dipole-dipole interactions) but lower than alcohols (due to lack of intermolecular H-bonding among themselves). Lower members are water-soluble due to H-bonding with water. Focus on understanding the 'why' behind each reaction and property for NEET success.

Prelims Revision Notes

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Functional Groups: — Aldehyde ($R-CHO$), Ketone ($R-CO-R'$). Carbonyl carbon is $sp^2$ hybridized, planar, and electrophilic. Oxygen is nucleophilic.
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Nomenclature: — Aldehydes: '-al' suffix, carbonyl carbon is C1. Ketones: '-one' suffix, carbonyl carbon lowest number.
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Reactivity Order (Nucleophilic Addition): — Formaldehyde > other Aldehydes > Ketones. Due to steric hindrance and electronic effects (electron-donating alkyl groups reduce electrophilicity).
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Preparation Methods:

* Oxidation of Alcohols: $1^circ ROH \xrightarrow{PCC}$ Aldehyde; $2^circ ROH \xrightarrow{K_2Cr_2O_7/H_2SO_4}$ Ketone. * From Hydrocarbons: Ozonolysis of alkenes ($R_2C=CR_2 \xrightarrow{O_3, Zn/H_2O}$ Ketone/Aldehyde).

Hydration of alkynes ($R-C\equiv CH \xrightarrow{HgSO_4, H_2SO_4}$ Ketone; $HC\equiv CH \rightarrow CH_3CHO$). * Named Reactions: * Rosenmund: $RCOCl \xrightarrow{H_2, Pd/BaSO_4}$ $RCHO$. * Stephen: $RCN \xrightarrow{SnCl_2/HCl, H_3O^+}$ $RCHO$.

* DIBAL-H: $RCN/RCOOR' \xrightarrow{DIBAL-H, -78^circ C}$ $RCHO$. * Friedel-Crafts Acylation: Benzene $\xrightarrow{RCOCl/AlCl_3}$ Aromatic Ketone. * Gattermann-Koch: Benzene $\xrightarrow{CO, HCl/AlCl_3, CuCl}$ Benzaldehyde.

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Nucleophilic Addition Reactions:

* HCN: Forms cyanohydrins ($R-CH(OH)CN$). * **Grignard Reagents ($R'MgX$):** $HCHO \rightarrow 1^circ ROH$; $RCHO \rightarrow 2^circ ROH$; $R_2CO \rightarrow 3^circ ROH$. * Alcohols: $RCHO \xrightarrow{ROH, H^+}$ Acetals (protecting group). * **Ammonia Derivatives ($NH_2Z$):** Forms imines ($Z=R$), oximes ($Z=OH$), hydrazones ($Z=NH_2$), semicarbazones ($Z=NHCONH_2$).

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Reduction Reactions:

* To Alcohols: $LiAlH_4$ or $NaBH_4$ (Aldehyde $\rightarrow 1^circ ROH$, Ketone $\rightarrow 2^circ ROH$). * **To Hydrocarbons ($C=O \rightarrow CH_2$):** * Clemmensen: $Zn-Hg/Conc. HCl$ (acid-stable compounds). * Wolff-Kishner: $NH_2NH_2/KOH$, ethylene glycol (base-stable compounds).

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Oxidation Reactions:

* Aldehydes: Easily oxidized to carboxylic acids. * Tollens' Test: Positive (silver mirror) with ammoniacal $AgNO_3$. * Fehling's Test: Positive (red $Cu_2O$ ppt) with Fehling's solution. * Ketones: Resistant to mild oxidation. Strong oxidation causes C-C bond cleavage. * Haloform Reaction: For $CH_3CO-$ or $CH_3CH(OH)-$ groups. $RCOCH_3 \xrightarrow{X_2/NaOH}$ $RCOONa + CHX_3$ (yellow ppt for $CHI_3$).

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Reactions due to Alpha-Hydrogens:

* **Acidity of $\alpha$-H:** Due to resonance stabilization of enolate anion. * Aldol Condensation: Aldehydes/Ketones *with* $\alpha$-H $\xrightarrow{Dilute Base}$ $\beta$-hydroxy carbonyl $\xrightarrow{\Delta}$ $\alpha,\beta$-unsaturated carbonyl. * Cannizzaro Reaction: Aldehydes *without* $\alpha$-H $\xrightarrow{Conc. Base}$ Alcohol + Carboxylate salt.

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Physical Properties: — Higher BP than hydrocarbons, lower BP than alcohols. Lower members are water-soluble. Cannot form intermolecular H-bonds with each other.

Vyyuha Quick Recall

All Ketones Never Oxidize Readily, Aldehydes Can Convert Always.

All Ketones: Ketones
Never Oxidize Readily: Are resistant to mild oxidation (e.g., Tollens', Fehling's).
Aldehydes: Aldehydes
Can Convert Always: Are easily oxidized (Can Convert Always to Carboxylic Acids).