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Q8.1(iv):
What is meant by the following term? Give an example of the reaction. (iv) Aldol

Solution :

Definition: Aldol Reaction

The Aldol reaction refers to the base-catalyzed condensation of two molecules of aldehyde or ketone that possess at least one $\alpha$-hydrogen atom. The reaction results in the formation of a $\beta$-hydroxy aldehyde (known as "aldol") or a $\beta$-hydroxy ketone (known as "ketol"). [Per the mechanism of nucleophilic addition to carbonyl compounds].

Step 1: Structural Requirement

For a compound to undergo an Aldol reaction, it must contain an $\alpha$-hydrogen. The $\alpha$-hydrogen is acidic because the resulting enolate anion is resonance-stabilized.
Example: Ethanal ($CH_3CHO$) has three $\alpha$-hydrogens attached to the $CH_2$ group adjacent to the carbonyl carbon.

Step 2: Reaction Mechanism

The reaction proceeds in three primary phases:

  1. Deprotonation: A base (e.g., $OH^-$) removes an $\alpha$-hydrogen from the first molecule to form a nucleophilic enolate ion.
  2. Nucleophilic Attack: The enolate ion attacks the carbonyl carbon of a second neutral aldehyde molecule.
  3. Protonation: The resulting alkoxide ion abstracts a proton from water to yield the final $\beta$-hydroxy product.

Step 3: Chemical Equation

The reaction of two molecules of ethanal in the presence of dilute alkali ($NaOH$):

$2 CH_3CHO \xrightarrow{dil. NaOH} CH_3-CH(OH)-CH_2-CHO$

Visual Representation of the Aldol Product

CH₃ CH OH CH₂ CHO 3-hydroxybutanal (Aldol product)

Analytical Summary

Component Role in Reaction
Base ($OH^-$) Catalyst for enolate formation
$\alpha$-hydrogen Provides acidity required for enolate generation
$\beta$-hydroxy group Defining functional feature of the product

Final Solution: The Aldol reaction is the nucleophilic addition of an enolate ion derived from an aldehyde or ketone containing an $\alpha$-hydrogen to the carbonyl group of another molecule of the same or different aldehyde/ketone, resulting in a $\beta$-hydroxy carbonyl compound. Example: $2 CH_3CHO \xrightarrow{dil. NaOH} CH_3CH(OH)CH_2CHO$ (3-hydroxybutanal).


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