Consider the reaction of the cyclopentanone derivative shown below. i) Na
+
OCH
2
?
CH
3
?
,CH
3
?
CH
2
?
OH,25
°
C ? ii) CH
3
?
! Draw the structure for the major product formed in this reaction. The cyclopentanone starting material has been provided as a starting point.
The correct answer and explanation is:
The reaction involves the cyclopentanone derivative undergoing an aldol condensation reaction in the presence of a strong base (sodium methoxide, NaOCH3\text{NaOCH}_3) in methanol (CH3CH2OH\text{CH}_3\text{CH}_2\text{OH}) as the solvent. After that, a methylation step occurs (CH3I\text{CH}_3I).
Reaction Steps:
- Deprotonation:
- Sodium methoxide (NaOCH3\text{NaOCH}_3) acts as a strong base, abstracting an alpha proton from the cyclopentanone derivative. This generates an enolate ion stabilized by resonance with the ketone group.
- Aldol Addition:
- The enolate ion acts as a nucleophile and attacks another molecule of the cyclopentanone derivative at its electrophilic carbonyl carbon. This results in the formation of a β-hydroxy ketone intermediate.
- Aldol Condensation:
- Under the reaction conditions (heat, basic medium), the β-hydroxy ketone intermediate undergoes dehydration, forming an α,β-unsaturated ketone. This product contains a conjugated double bond system, which is thermodynamically stable.
- Methylation:
- In the second step, methyl iodide (CH3I\text{CH}_3I) reacts with the α,β-unsaturated ketone. The methylation likely occurs at the enolate ion, formed by deprotonating the α-hydrogen of the conjugated system. This leads to the final product: a methylated α,β-unsaturated ketone.
Major Product:
The major product is an α-methyl-α,β-unsaturated ketone derivative.
Explanation:
This reaction illustrates typical aldol condensation followed by an alkylation step. The first step, catalyzed by a strong base, generates an enolate ion. The subsequent aldol addition and condensation form the conjugated double bond system. The final methylation step introduces a methyl group to the alpha position of the unsaturated ketone.
This sequence is driven by the reactivity of the enolate ion and the electrophilic carbon of the carbonyl group, making aldol condensation a key step in organic synthesis for forming C–C bonds and generating conjugated systems.