Which one of the following gives yellow precipitate with iodine and $\mathrm{NaOH}$ solution?

Which one of the following gives yellow precipitate with iodine and $\mathrm{NaOH}$ solution?
  1. $\mathrm{CH}_3-\mathrm{CHO}$
  2. $\mathrm{C}_6 \mathrm{H}_5 \mathrm{COC}_6 \mathrm{H}_5$
  3. $\mathrm{HCHO}$
  4. $\mathrm{CH}_3 \mathrm{OH}$

Solution

Alcohols containing $\mathrm{CH}_3 \mathrm{CH}(\mathrm{OH})-$ group and carbonyl compounds containing $\mathrm{CH}_3 \mathrm{CO}-$ group give yellow precipitate with iodine and $\mathrm{NaOH}$ solution. This reaction is called iodoform test. Thus, $\mathrm{CH}_3 \mathrm{CHO}$ due to presence of $\mathrm{CH}_3 \mathrm{CO}$ group gives yellow ppt with $\mathrm{I}_2$ and $\mathrm{NaOH}$. $\mathrm{CH}_3 \mathrm{CHO}+3 \mathrm{I}_2+4 \mathrm{NaOH} \longrightarrow \mathrm{CHI}_3 +\mathrm{HCOONa}+3 \mathrm{NaI}+2 \mathrm{H}_2 \mathrm{O}$

Asked in: AP EAMCET 2010

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