Qualitative & Quantitative Analysis of Organic Compounds Why analyse organic compounds? In organic chemistry, we often need to answer two basic questions about an unknown compound: Which elements are present? And in what percentages? Qualitative tests tell us the presence of elements like nitrogen, sulfur, halogens, and phosphorus. Quantitative methods then measure %C, %H, %N, %S, %halogen, and sometimes %P. This is foundational for structure determination and commonly tested in NEET. Qualitative analysis: Which elements are present? Detection of carbon and hydrogen (simple combustion) Principle: On heating the organic compound with copper(II) oxide (CuO) in oxygen, carbon converts to carbon dioxide (CO2) and hydrogen converts to water (H2O). CO2 turns limewater milky; H2O turns anhydrous copper(II) sulfate from white to blue. Observations: - Limewater (aqueous Ca(OH)2) turns milky due to CaCO3 formation. - Anhydrous CuSO4 turns blue on hydration to CuSO4·5H2O. Examples used for illustration: ethanol (IUPAC: ethanol; SMILES: CCO), carbon dioxide (IUPAC: carbon dioxide; SMILES: O=C=O), water (IUPAC: water; SMILES: O). Organic compound + O2 (with CuO) → CO2 (limewater test) + H2O (anhydrous CuSO4 turns blue) Apparatus diagram: combustion tube with CuO and sample heated by burner; gas path through wash bottle of limewater, then drying tube with anhydrous CuSO4. Labels: CuO, sample, limewater, anhydrous CuSO4; arrows for gas flow; clean vector chemistry style. Simple combustion test for C and H: organic sample + CuO heated in a tube; gases bubbled through limewater (turns milky) and passed over anhydrous CuSO4 (turns blue). 2026-05-26T17:05:16.087Z gpt-image-2 Lassaigne’s test (Sodium fusion) for N, S, halogens Problem: N, S, and halogens are covalently bound in organic molecules and do not give ionic tests directly. Trick: Fuse the organic compound with freshly cut sodium metal at red heat to convert these elements into water-soluble ionic salts. Then test the aqueous extract (sodium fusion extract or Lassaigne’s extract) for each element. Organic-N/S/X → NaCN/Na2S/NaX Heat a small piece of clean, dry sodium in a fusion tube until molten (red heat). Add a small amount of the dry organic sample; heat strongly so sodium reacts with the sample. Red heat; use minimum quantities; perform behind safety shield Boil for a few minutes; then filter to obtain clear Lassaigne’s extract Water dissolution of salts Quench the hot tube in water, crush the fused mass, and boil with distilled water to extract the ionic salts into solution. Preparation of Lassaigne’s extract sodium metal sodium reductant/convertor to ionic salts sodium cyanide sodium cyanide product if N present sodium sulfide disodium sulfide product if S present product if halogen present (X = Cl, Br, I) sodium chloride/bromide/iodide sodium halide Convert covalently bound N, S, and halogens into NaCN, Na2S, and NaX respectively by fusing with sodium; then extract with water. gpt-image-2 2026-05-26T17:05:16.581Z Lassaigne’s fusion setup and key color outcomes: Prussian blue (N), black PbS (S), purple with sodium nitroprusside (S), and AgX precipitates (Cl white, Br pale yellow, I yellow). Two-part illustration: Left—fusion tube with sodium and sample at red heat, then quench into beaker of water (Step 1→2). Right—four small panels showing positive tests: (i) Prussian blue solution, (ii) black PbS precipitate with lead acetate, (iii) purple color with sodium nitroprusside, (iv) AgCl white, AgBr pale yellow, AgI yellow with labels. Clean vector style, arrows red. Element-wise tests on Lassaigne’s extract Interference removal for halogen test Before adding AgNO3, acidify the extract with dilute HNO3 and boil to expel CN− (as HCN) and S2− (as H2S) so they do not form AgCN or Ag2S false precipitates. Nitrogen (Prussian blue test sequence) Reagents: Lassaigne’s extract + freshly prepared FeSO4 solution; heat; then add dilute H2SO4; finally add a few drops of FeCl3. Observation: Deep Prussian blue color (ferriferrocyanide). Chemistry: CN− from extract forms sodium ferrocyanide Na4[Fe(CN)6] with Fe2+. On acidification and oxidation (Fe3+), Prussian blue Fe4[Fe(CN)6]3 forms. Sulfur (two tests) Lead acetate test: Acidified extract + lead(II) acetate → black precipitate of PbS (confirms S2−). Sodium nitroprusside test: Extract + Na2[Fe(CN)5NO] → purple/violet coloration due to formation of a thiocomplex with S2−. Note: Both tests are sensitive; lead acetate test is confirmatory (black PbS). Halogens (AgNO3 test) Procedure: Boil extract with dilute HNO3, cool, then add AgNO3 solution. Observations: AgCl—white curdy (soluble in dilute NH3); AgBr—pale yellow (sparingly soluble in NH3); AgI—yellow (insoluble in NH3). Underlying: X− + Ag+ → AgX(s), where X = Cl, Br, I. Nitrogen and sulfur together Fusion can yield thiocyanate (SCN−): Na + C + N + S → NaSCN. Test: With FeCl3, blood-red complex [Fe(SCN)]2+ appears (distinct from Prussian blue). Phosphorus Oxidation step: Fuse the sample with sodium peroxide (Na2O2) to convert P → Na3PO4. Test: Acidify with HNO3 and add ammonium molybdate solution → canary yellow precipitate of ammonium phosphomolybdate (presence of P). C, H Combustion with CuO Heat with CuO; pass gas through limewater and over anhydrous CuSO4 Limewater turns milky (CaCO3); CuSO4 turns blue (hydration) Lassaigne + Prussian blue FeSO4 + heat; then dil. H2SO4; then FeCl3 Deep Prussian blue color Lead acetate Acidified extract + Pb(OAc)2 Black PbS precipitate Sodium nitroprusside Na2[Fe(CN)5NO] Purple/violet coloration Cl/Br/I Silver nitrate (after HNO3 boil) Dilute HNO3 + AgNO3 AgCl white (soluble in NH3), AgBr pale yellow (sparingly soluble), AgI yellow (insoluble) N + S Thiocyanate formation FeCl3 on extract Blood red [Fe(SCN)]2+ complex Ammonium molybdate Na2O2 fusion; HNO3; (NH4)2MoO4 Canary yellow ammonium phosphomolybdate Element Test name Reagents Positive observation Qualitative tests on organic compounds (quick view) Row Quantitative analysis: How much of each element? Estimation of carbon and hydrogen (Liebig’s method) Principle: A known mass of the organic compound is burnt in excess oxygen over heated copper(II) oxide. The formed CO2 is absorbed in KOH (or soda-lime) and H2O is absorbed in anhydrous CaCl2 or Mg(ClO4)2 tubes. The gain in mass of each absorber gives m(CO2) and m(H2O). From these, calculate %C and %H. %C formula (Liebig) Use the mass of CO2 absorbed; 12/44 accounts for C in CO2. Use the mass of H2O absorbed; 2/18 accounts for H in H2O. %H formula (Liebig) Linear apparatus diagram with labels: O2 cylinder → combustion tube (CuO + sample) heated → CaCl2 tube (H2O) → KOH solution bottle (CO2) → vent. Show mass labels m1, m2. Clean vector, arrows for gas flow, red heat zone. Liebig combustion train: oxygen inlet, combustion tube with CuO, sequential absorbers (CaCl2 for H2O; KOH for CO2), and balances to measure mass gains. 2026-05-26T17:05:17.030Z gpt-image-2 Estimation of nitrogen: Dumas vs Kjeldahl Two classic methods are used: - Dumas method: Combust the sample in oxygen with a copper catalyst; nitrogen in the compound is ultimately obtained as N2 gas (NOx reduced over hot copper). Measure the volume of dry N2 (after removing CO2 and H2O) and compute %N. Works for most nitrogen types, including nitro and azo. - Kjeldahl method: Digest the sample with concentrated H2SO4 (with K2SO4 to raise boiling point and CuSO4 as catalyst) to convert organic N to NH4+ (as NH4HSO4). Make strongly alkaline with NaOH, distil NH3 into a known excess of standard H2SO4, and back-titrate the remaining acid with standard NaOH. Not applicable for nitro (−NO2), nitroso (−NO), azo (−N=N−), and many heterocyclic nitrogens that are not converted to NH3 under these conditions. Heating until clear; fume hood Digest sample with conc. H2SO4, K2SO4 (boiling point elevant), and CuSO4 catalyst until the solution becomes clear (organic N → NH4HSO4). Distillation with condenser; receiver kept cool Add excess NaOH to liberate NH3 from NH4+ and distil the liberated NH3 into a known volume of standard H2SO4 in the receiver. Back-titrate the excess acid in the receiver with standard NaOH to find acid consumed by NH3. Use appropriate indicator (e.g., methyl red) Kjeldahl method workflow sulfuric acid digestion medium sulfuric acid sodium hydroxide sodium hydroxide releases NH3 from NH4+ ammonia distilled into standard acid azane (ammonia) Organic nitrogen → NH4+ on digestion → NH3 on alkalisation → captured by standard acid and quantified by back-titration. Use volumes in mL and normalities as given in the problem statement. Follow your problem’s unit conventions consistently. %N (Kjeldahl) as mandated Kjeldahl method does not work for −NO2 (nitro), −NO (nitroso), −N=N− (azo), and many heterocyclic nitrogens because they are not converted to NH3 under digestion. remember gpt-image-2 2026-05-26T17:05:16.984Z Kjeldahl setup: digestion flask with H2SO4/K2SO4/CuSO4; distillation with addition of NaOH; condenser delivering NH3 into standard acid receiver; back-titration. Three-panel schematic: (1) digestion flask with acid and catalysts heating; (2) distillation head where NaOH is added, condenser to receiver with H2SO4; (3) titration of receiver with NaOH. Clear labels, arrows indicating NH3 path; vector style. Estimation of sulfur and halogens (Carius method) Principle: Heat a known mass of the organic compound with fuming nitric acid in a sealed hard glass (Carius) tube at elevated temperature. The element is oxidised to a stable inorganic anion. - Sulfur → sulfate ( SO4 2− ), precipitated and weighed as BaSO4. - Halogen (Cl/Br/I) → halide, precipitated and weighed as AgX. From the mass of the precipitate, compute %S or %X. Safety: Carius tubes are sealed and heated in protective iron jackets/furnaces. 233 g mol−1 is molar mass of BaSO4; 32 g mol−1 is sulfur. %S from BaSO4 Use molar mass ratio of X in AgX; m(AgX) is mass of the silver halide precipitate. %X from AgX (X = Cl, Br, I) gpt-image-2 Carius method apparatus: sealed thick-walled tube in protective iron jacket heated in a furnace, then contents treated to precipitate BaSO4 or AgX for weighing. Cutaway of a sealed Carius tube inside an iron jacket with screw cap; furnace indicated; after cooling, solution poured out to be treated with BaCl2 (for SO4 2− ) or contains AgNO3 for AgX. Labels and safety notes; clean vector style. 2026-05-26T17:05:17.249Z Estimation of phosphorus and oxygen Phosphorus: Oxidise the phosphorus to orthophosphoric acid (H3PO4), then form ammonium phosphomolybdate (yellow). Alternatively, precipitate with a magnesium mixture and ignite to constant mass of magnesium pyrophosphate (Mg2P2O7), which is weighed to compute %P. Oxygen: In classical analysis, percentage of oxygen is obtained by difference after determining all other elements: %O = 100 − (%C + %H + %N + %S + %halogens + %P). In exam sums, unless specified otherwise, %O is by difference. Do not try to ‘directly’ estimate oxygen from any single gravimetric step. neet-alert Industrial and modern methods Kjeldahl remains a workhorse for protein analysis in foods. The nitrogen percentage is multiplied by a factor to estimate crude protein: typically 6.25 for general foods, 5.83 for cereals. Modern CHN analysers automate microcombustion and measure CO2, H2O, and N2 using IR and thermal conductivity detectors for rapid %C, %H, %N. For trace elements, ICP-OES/ICP-MS and high-resolution mass spectrometry give very precise elemental compositions. gpt-image-2 2026-05-26T17:05:17.650Z Block diagram: autosampler → microcombustion furnace → gas separation → IR detectors for CO2/H2O → thermal conductivity detector for N2 → data system. Labels and simple icons; vector, clean palette. Modern CHN analyser: schematic of microcombustion furnace, separation columns, IR cell for CO2/H2O, and TCD for N2, with computer readout of %C, %H, %N. C, H Liebig combustion Combustion to CO2/H2O, absorb separately %C = (12/44)·m(CO2)/m(sample)·100; %H = (2/18)·m(H2O)/m(sample)·100 Dumas Combustion → N2 measured volumetrically Use STP molar volume 22.4 L mol−1 to compute %N Kjeldahl Digest → NH3, capture in standard acid; back-titrate %N via back-titration (use mandated formula) Carius (BaSO4) Oxidise to sulfate; weigh BaSO4 %S = (32/233)·m(BaSO4)/m(sample)·100 Cl/Br/I Carius (AgX) Oxidise → precipitate AgX and weigh %X = (atomic mass X / mass AgX)·m(AgX)/m(sample)·100 Gravimetry Oxidise → phosphomolybdate or Mg2P2O7 From mass of Mg2P2O7 or molybdate By difference 100 − sum of others %O = 100 − (%C + %H + %N + %S + %halogens + %P) Row Quantitative methods at a glance Element Method Principle Key formula Amines, amides, amino acids, proteins (convertible to NH4+ on digestion) Nitro (−NO2), nitroso (−NO), azo (−N=N−), many heterocyclic N (not converted to NH3) Works for (examples) Fails for (do not use) Kjeldahl applicability tip AgCl vs AgBr vs AgI: remember both color and ammonia solubility. This is a favourite NEET trap. Silver halides: “White—Cl, Light—Br, Bright—(deep) I” and Solubility in NH3 goes Cl > Br > I. False. Kjeldahl only measures nitrogen converted to NH3 during digestion. Nitro (−NO2), nitroso (−NO), azo (−N=N−), and many heterocyclic nitrogens are not measured. Kjeldahl gives total nitrogen including nitro groups. They differ: AgCl is white and dissolves in dilute NH3; AgBr is pale yellow and dissolves only partially; AgI is yellow and insoluble in NH3. AgCl, AgBr, and AgI all look the same. Percentage of oxygen is estimated directly by a single classical test. In classical organic analysis, %O is obtained by difference: 100 − (sum of other elemental percentages). Lassaigne’s test is used for N, S, halogens, and with peroxide pretreatment for P. It is not used to detect oxygen. Lassaigne fusion can detect any element including oxygen. High-yield NEET checkpoints Lassaigne products: N → NaCN; S → Na2S; halogen (X) → NaX; N+S together → NaSCN (blood red with Fe3+). Liebig calculations: Use absorber mass gains in the %C and %H formulas—watch units. Kjeldahl %N: Identify volumes, normalities, and sample mass correctly before applying the formula. AgX colors/solubility: AgCl white (soluble in NH3), AgBr pale yellow (sparingly soluble), AgI yellow (insoluble). Carius principle: Oxidise in sealed tube, then gravimetry as BaSO4 (S) or AgX (halogens). Sodium fusion of an organic compound to convert N, S, and halogens into water-soluble salts (NaCN, Na2S, NaX) for qualitative detection. Lassaigne’s test sodium fusion test Sodium fusion extract The aqueous extract obtained after fusing the sample with sodium and boiling with water; used for element tests. Deep blue pigment formed as Fe4[Fe(CN)6]3 in the nitrogen test sequence. Prussian blue Na2[Fe(CN)5NO], gives purple color with sulfide (S2−) in Lassaigne’s extract. Sodium nitroprusside Ammonium phosphomolybdate Canary yellow salt formed when phosphate reacts with ammonium molybdate in acidic medium; indicates phosphorus. Liebig combustion Classical combustion analysis where CO2 and H2O are trapped and weighed to compute %C and %H. Dumas method Nitrogen estimation by converting all nitrogen to N2 gas, measuring its volume. Kjeldahl method Nitrogen estimation via digestion to NH4+, distillation of NH3, and back-titration. Sealed-tube oxidation for determining S (as BaSO4) and halogens (as AgX) gravimetrically. Carius method CHN analyser Instrument that automates microcombustion and measures CO2/H2O/N2 to report %C, %H, %N quickly. Key terms All-in-one: mandated key equations Mandated nitrogen (Kjeldahl) equation Mandated halogen and sulfur equations remember When gas volumes are given for Dumas nitrogen at non-STP conditions, convert to STP using P1V1/T1 = P2V2/T2 (Kelvin). Use 22.4 L mol−1 (older NCERT/NEET) or 22.7 L mol−1 (IUPAC) as specified in the paper.