Question Bank [2022] — Chemistry STD 12 Science — Question
Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryQuestion Bank [2022]3 Marks
Question
Explain haloform reaction with suitable example.
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Answer
This reaction is given by acetaldehyde, all methyl ketones $(CH_3–CO–R)$, and all alcohols containing $CH_3(CHOH)–$ group.
When an alcohol or methyl ketone is warmed with sodium hydroxide and iodine, a yellow precipitate is formed. Here the reagent sodium hypoiodite is produced in situ.
During the reaction, the sodium salt of the carboxylic acid is formed which contains one carbon atom less than the substrate.
The methyl group is converted into haloform $(CHX_3)$. e.g. Acetone is oxidized by sodium hypoiodite to give sodium salt of acetic acid and yellow precipitate of iodoform.
$\begin{array}{cc}
\phantom{..}\ce{O}\phantom{.......................................}\ce{O}\phantom{..........................}\\
\phantom{..}||\phantom{.......................................}||\phantom{..........................}\\
\ce{\underset{\text{Acetone}}{CH3 - C - CH3} + \underset{\text{Sodium hypoiodite}}{3NaOI} ->[NaOH/I2][\Delta] \underset{\text{Sodium acetate}}{CH3 - C - O- Na+} + \underset{\text{Iodoform}}{CHI3 ↓} + 2NaOH}
\end{array}$
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