Practical Organic Chemistry flash cards

1. 1.What is the purpose of Lassaigne's test?

   To detect the presence of N, S, halogens (and P) in an organic compound by converting these covalently-bonded elements into ionic form (water-soluble Na salts) via fusion with sodium metal.

   Hint: Covalent → ionic conversion

2. 2.Why must covalently bonded elements be converted to ionic form before detection?

   Elements like N, S, X in organic compounds are covalently linked and do not give ionic tests. Fusion with Na converts them to ionic species ($NaCN$, $Na_2S$, $NaX$) which are water-soluble and give characteristic ionic reactions.

   Hint: Ionic tests need ions

3. 3.Write the reactions when C, N, S, X react with fusion sodium in Lassaigne's test.

   $Na + C + N \rightarrow NaCN$; $2Na + S \rightarrow Na_2S$; $Na + X \rightarrow NaX$; and if both N & S present: $Na + C + N + S \rightarrow NaSCN$ (sodium sulphocyanide).

   Hint: CN, S, X, SCN

4. 4.How is the sodium extract (Lassaigne's extract) prepared?

   Fuse the compound with a small piece of dry Na in an ignition tube till red hot, plunge the hot tube into distilled water, boil, and filter. The filtrate is the sodium extract, which must be alkaline.

   Hint: Fuse → plunge in water → filter

5. 5.Why must the sodium extract be made alkaline before the nitrogen test?

   Alkalinity ensures excess free Na has reacted and that $NaCN$ is not lost. Under alkaline conditions $Fe^{2+}$ forms and reacts with $CN^-$ to give ferrocyanide; acid is added only afterward to dissolve $Fe(OH)_2/Fe(OH)_3$.

   Hint: Preserve $CN^-$

6. 6.Describe the test for nitrogen in Lassaigne's extract and the observation.

   To the extract add $FeSO_4$, boil, then acidify with dilute $H_2SO_4/HCl$. A Prussian blue (or green) colouration confirms nitrogen.

   Hint: $FeSO_4$ → Prussian blue

7. 7.Write the reactions forming Prussian blue in the nitrogen test.

   $6NaCN + FeSO_4 \rightarrow Na_4[Fe(CN)_6] + Na_2SO_4$; then $3Na_4[Fe(CN)_6] + 4Fe^{3+} \rightarrow Fe_4[Fe(CN)_6]_3 + 12Na^+$. The $Fe^{3+}$ comes from aerial oxidation of $Fe^{2+}$.

   Hint: Ferrocyanide + $Fe^{3+}$

8. 8.What is the composition of Prussian blue?

   Ferric ferrocyanide, $Fe_4[Fe(CN)_6]_3$.

   Hint: $Fe(III)$ salt of ferrocyanide

9. 9.Describe the sodium nitroprusside test for sulphur and its observation.

   To a fresh portion of sodium extract add sodium nitroprusside solution; a violet/purple colouration confirms sulphur.

   Hint: Violet colour

10. 10.Write the reaction of the sodium nitroprusside test for sulphur.

    $Na_2S + Na_2[Fe(CN)_5NO] \rightarrow Na_4[Fe(CN)_5NOS]$ — a violet/purple complex.

    Hint: $Na_4[Fe(CN)_5NOS]$

11. 11.Describe the lead acetate test for sulphur.

    Acidify a portion of the extract with acetic acid and add lead acetate; a black precipitate of PbS confirms sulphur: $Na_2S + (CH_3COO)_2Pb \rightarrow PbS\downarrow + 2CH_3COONa$.

    Hint: Black PbS

12. 12.How is halogen detected in the sodium extract (silver nitrate test)?

    Acidify the extract with dilute $HNO_3$, boil (to expel HCN/$H_2S$), then add $AgNO_3$. A precipitate indicates halogen: white ($AgCl$), pale yellow ($AgBr$), yellow ($AgI$).

    Hint: $AgNO_3$ → coloured ppt

13. 13.Why must the sodium extract be boiled with dilute $HNO_3$ before adding $AgNO_3$ for halogen test?

    If N or S are present, $CN^-$ and $S^{2-}$ would also precipitate with $Ag^+$ (as AgCN, $Ag_2S$) giving a false halogen result. Boiling with $HNO_3$ decomposes them to HCN and $H_2S$ gases, removing the interference.

    Hint: Remove $CN^-$, $S^{2-}$ interference

14. 14.Distinguish $AgCl$, $AgBr$, $AgI$ by colour and solubility in ammonia.

    $AgCl$: white, fully soluble in dilute $NH_3$. $AgBr$: pale yellow, partially/sparingly soluble in $NH_3$. $AgI$: yellow, insoluble in $NH_3$.

    Hint: White→sol, yellow→insol in $NH_3$

15. 15.How can you confirm bromine and iodine specifically (layer test)?

    Acidify extract with dilute $HCl$, add $CCl_4/CHCl_3$ and chlorine water; shake. An orange/brown organic layer indicates $Br_2$, a violet/purple layer indicates $I_2$ liberated by $Cl_2$.

    Hint: Free halogen in organic layer

16. 16.When both N and S are present, what species forms and how does it affect tests?

    $NaSCN$ (sodium thiocyanate) forms. With $Fe^{3+}$ it gives a blood-red $[Fe(SCN)]^{2+}$ colouration (not Prussian blue). Using excess sodium decomposes $NaSCN$ into $NaCN + Na_2S$, restoring separate N and S tests.

    Hint: Blood-red; use excess Na

17. 17.Why does a blood-red colour sometimes appear instead of Prussian blue in the N test?

    When N and S coexist and insufficient Na was used, $SCN^-$ survives; with $Fe^{3+}$ it forms blood-red ferric thiocyanate $[Fe(SCN)]^{2+}$. Excess Na avoids this by cleaving $SCN^-$.

    Hint: $SCN^-$ + $Fe^{3+}$

18. 18.How is phosphorus detected in an organic compound?

    Heat compound with an oxidising agent (fused $Na_2O_2$ or $HNO_3$) to form phosphate, then add ammonium molybdate in $HNO_3$. A canary-yellow precipitate of ammonium phosphomolybdate confirms P.

    Hint: Yellow ammonium phosphomolybdate

19. 19.QUESTION: A sodium extract gives a Prussian blue colour with $FeSO_4$ but no violet with nitroprusside. What elements are present?

    Nitrogen is present (Prussian blue); sulphur is absent (no violet). So the compound contains N but not S.

    Hint: Blue=N, no violet=no S

20. 20.QUESTION: On acidifying the extract with $HNO_3$ and adding $AgNO_3$, a pale-yellow ppt sparingly soluble in $NH_3$ forms. Which halogen?

    Bromine — $AgBr$ is pale yellow and only sparingly soluble in ammonia.

    Hint: Pale yellow, partly $NH_3$-soluble

21. 21.Why can't fluorine usually be detected by the Lassaigne silver nitrate test?

    $AgF$ is soluble in water, so no precipitate forms. Fluoride is instead confirmed by other tests (e.g. the chromyl chloride test is not applicable; fluoride gives a precipitate with $CaCl_2$ as $CaF_2$).

    Hint: $AgF$ is soluble

22. 22.What is the general principle of purification of organic compounds?

    Exploit differences in physical properties (solubility, volatility, boiling point, adsorption, melting point) between the compound and its impurities to separate them.

    Hint: Difference in physical property

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