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Q8.9:
Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as nucleophile and which as electrophile.

Solution :

Initial Parameters & Reaction Mechanism Basis

The aldol condensation between propanal ($CH_3CH_2CHO$) and butanal ($CH_3CH_2CH_2CHO$) involves the nucleophilic attack of an enolate ion (formed from an $\alpha$-hydrogen-containing carbonyl) onto the electrophilic carbonyl carbon of another molecule. Since both species contain $\alpha$-hydrogens, four distinct crossed-aldol products are theoretically possible through self-condensation and cross-condensation.

Step 1: Identification of Reactants and Potential Enolates

  • Propanal (A): $CH_3CH_2CHO$. Forms enolate at the $\alpha$-carbon: $CH_3\bar{C}HCHO$.
  • Butanal (B): $CH_3CH_2CH_2CHO$. Forms enolate at the $\alpha$-carbon: $CH_3CH_2\bar{C}HCHO$.

Step 2: Structural Derivations of Crossed Aldol Products

Product Nucleophile (Enolate Source) Electrophile (Carbonyl Source) Structural Formula
1 Propanal Butanal $CH_3CH_2CH_2CH(OH)CH(CH_3)CHO$
2 Butanal Propanal $CH_3CH_2CH(OH)CH(CH_2CH_3)CHO$
3 Propanal Propanal $CH_3CH_2CH(OH)CH(CH_3)CHO$
4 Butanal Butanal $CH_3CH_2CH_2CH(OH)CH(CH_2CH_3)CHO$

Step 3: IUPAC Nomenclature

  • Product 1: 2-methyl-3-hydroxyhexanal
  • Product 2: 2-ethyl-3-hydroxypentanal
  • Product 3: 2-methyl-3-hydroxypentanal
  • Product 4: 2-ethyl-3-hydroxyhexanal

Step 4: Molecular Visualization of Aldol Formation

Nucleophilic Attack Mechanism R-CH-CHO (Nucleophile) R'-CHO (Electrophile) [Carbon-Carbon Bond Formation]

Step 5: Justification of Mechanisms

According to [Nucleophilic Addition Mechanism], the base abstracts an $\alpha$-proton from the nucleophile to generate a resonance-stabilized enolate anion. This anion subsequently attacks the electrophilic carbonyl carbon of the second molecule. The resulting alkoxide is then protonated to form the $\beta$-hydroxy aldehyde [Per the Aldol Condensation definition].

Final Solution: The four products are 2-methyl-3-hydroxyhexanal (Propanal nucleophile/Butanal electrophile), 2-ethyl-3-hydroxypentanal (Butanal nucleophile/Propanal electrophile), 2-methyl-3-hydroxypentanal (Propanal self-condensation), and 2-ethyl-3-hydroxyhexanal (Butanal self-condensation).


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