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Q8.13(v):
Give simple chemical tests to distinguish between the following pairs of compounds. (v) Pentan-2-one and Pentan-3-one

Solution :

Given Variables & Chemical Foundations

To distinguish between Pentan-2-one ($CH_3COCH_2CH_2CH_3$) and Pentan-3-one ($CH_3CH_2COCH_2CH_3$), we utilize the Iodoform Test. This test is specific for methyl ketones ($R-CO-CH_3$) or compounds that yield methyl ketones upon oxidation [Per the Haloform Reaction mechanism].

Step 1: Analyzing the Molecular Structure

  • Pentan-2-one: Contains a terminal acetyl group ($-COCH_3$). [As per the requirement for Iodoform test, presence of the methyl ketone group is mandatory].
  • Pentan-3-one: Does not contain a methyl ketone group; it is a symmetric diethyl ketone. [As per structural analysis, no $CH_3CO-$ unit exists].

Step 2: The Iodoform Test Procedure

The sample is treated with Sodium Hydroxide ($NaOH$) and Iodine ($I_2$) solution. The reaction proceeds as follows:

$R-CO-CH_3 + 3I_2 + 4NaOH \rightarrow R-COONa + CHI_3 \downarrow + 3NaI + 3H_2O$

Step 3: Analytical Observation & Results

Compound Reaction with $I_2/NaOH$ Observation
Pentan-2-one Positive Test Formation of a yellow precipitate of Iodoform ($CHI_3$)
Pentan-3-one Negative Test No yellow precipitate formed

Technical Visual Representation

Pentan-2-one: CH3-CO-CH2-CH2-CH3 + I2/NaOH → CH3I (Yellow Ppt) Pentan-3-one: CH3-CH2-CO-CH2-CH3 + I2/NaOH → No Reaction

Theoretical Justification

The Iodoform test is chemically selective. The $\alpha$-hydrogens of the methyl group in Pentan-2-one are acidic and undergo successive iodination in the presence of an alkali [By the mechanism of base-catalyzed enolization]. Pentan-3-one lacks this methyl ketone moiety, preventing the cleavage of the C-C bond required to form the triiodomethyl anion ($CI_3^-$), which is the precursor to the yellow $CHI_3$ precipitate. [Per the Haloform Reaction principle].

Final Solution: Pentan-2-one gives a positive iodoform test (yellow precipitate), while Pentan-3-one does not respond to the test, providing a clear chemical distinction.


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