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

Solution :

Step 1: Theoretical Basis for Differentiation

To distinguish between Pentan-2-one ($CH_3COCH_2CH_2CH_3$) and Pentan-3-one ($CH_3CH_2COCH_2CH_3$), we must identify a structural difference that allows for a selective chemical reaction. [Per the Haloform Reaction theory]:

  • Pentan-2-one: Contains a methyl ketone group ($CH_3CO-$), which is susceptible to the Iodoform test.
  • Pentan-3-one: Does not contain a methyl ketone group; the carbonyl carbon is flanked by two ethyl groups ($C_2H_5COC_2H_5$).

Step 2: The Iodoform Test Procedure

The Iodoform test involves the reaction of a methyl ketone with sodium hypoiodite ($NaOI$), which is generated in situ from iodine ($I_2$) and sodium hydroxide ($NaOH$).

Step 3: Chemical Reactions and Observations

Compound Reaction with $I_2/NaOH$ Observation
Pentan-2-one $CH_3COCH_2CH_2CH_3 + 3NaOI \rightarrow CH_3CH_2CH_2COONa + CHI_3 \downarrow + 2NaOH$ Formation of a yellow precipitate of iodoform ($CHI_3$).
Pentan-3-one No reaction with $NaOI$ No yellow precipitate formed.

Step 4: Visualizing the Structural Distinction

Pentan-2-one (Methyl Ketone) CH3-C(=O)-CH2-CH2-CH3 Contains CH3-CO group: Positive Iodoform Test Pentan-3-one CH3-CH2-C(=O)-CH2-CH3 No CH3-CO group: Negative Iodoform Test

Step 5: Justification of Result

The reaction proceeds via the exhaustive iodination of the alpha-carbon of the methyl group followed by base-promoted cleavage [By the Mechanism of Alpha-substitution and Nucleophilic Acyl Substitution]. Since Pentan-3-one lacks the specific methyl ketone moiety required to form the tri-iodo intermediate, it remains chemically inert under these specific test conditions.

Final Solution: The pair can be distinguished using the Iodoform Test. Pentan-2-one gives a yellow precipitate of $CHI_3$ (iodoform) when treated with $I_2$ and $NaOH$, whereas Pentan-3-one shows no reaction and does not form the yellow precipitate.


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