Question

Explain with the help of chemical reactions: (I) Acetone is treated with semicarbazide.

Hint:

Acidity increases with electron-withdrawing groups, especially when closer to –COOH.

Answer 1

(I) Acetone reacts with semicarbazide (H\(_2\)N–NH–CONH\(_2\)) to form a semicarbazone derivative via nucleophilic addition followed by elimination of water: \[ \text{CH}_3\text{COCH}_3 + \text{H}_2\text{N–NH–CONH}_2 \rightarrow \text{CH}_3\text{C=NNHCONH}_2 + \text{H}_2\text{O} \] \[ \boxed{\text{Acetone} + \text{Semicarbazide} \rightarrow \text{Semicarbazone}} \] (II) Two molecules of benzaldehyde are treated with conc. NaOH.
Solution:
(II) This is the **Cannizzaro reaction** (disproportionation of aldehydes with no \(\alpha\)-hydrogen): \[ 2\text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{NaOH}} \text{C}_6\text{H}_5\text{COO}^- + \text{C}_6\text{H}_5\text{CH}_2\text{OH} \] \[ \boxed{\text{One aldehyde is oxidised, other is reduced}} \] (III) Butan-2-one is treated with Zn–Hg and conc. HCl.
Solution:
(III) This is the **Clemmensen reduction**, converting ketones to alkanes: \[ \text{CH}_3\text{COCH}_2\text{CH}_3 \xrightarrow{\text{Zn–Hg}/\text{HCl}} \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3 \] \[ \boxed{\text{Butan-2-one} \rightarrow \text{Butane}} \] (a)(ii) Arrange the following in increasing order of acidic strength: (I) CH\(_3\)CH\(_2\)CH\(_2\)COOH, BrCH\(_2\)CH\(_2\)CH\(_2\)COOH, CH\(_3\)CHBrCH\(_2\)COOH, CH\(_3\)CH\(_2\)CHBrCOOH
Solution:
(I) Acidic strength increases with electron-withdrawing groups closer to –COOH: \[ \text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}<\text{BrCH}_2\text{CH}_2\text{CH}_2\text{COOH}<\text{CH}_3\text{CH}_2\text{CHBrCOOH}<\text{CH}_3\text{CHBrCH}_2\text{COOH} \] \[ \boxed{\text{Increasing acidity:} \text{no Br}<\gamma\text{-Br}<\beta\text{-Br}<\alpha\text{-Br}} \] (II) Benzoic acid, 4-Methoxybenzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid
Solution:
(II) Electron withdrawing groups increase acidity, donating groups decrease it. \[ \text{4-Methoxy}<\text{Benzoic acid}<\text{4-Nitro}<\text{3,4-Dinitro} \] \[ \boxed{\text{Increasing acidity:} \text{OMe}<\text{H}<\text{NO}_2<\text{NO}_2,\text{NO}_2} \]