Organic Chemistry - Some Basic Principles and Techniques

Organic Chemistry - Some Basic Principles and Techniques

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Organic Chemistry - Some Basic Principles and Techniques

1. Red colour complex ion formed on adding \(FeCl_3\) to S.E. (sodium extract) when N and S both are present in organic compound is:

  • (a) \([Fe(CN)_6]^{4-}\)
  • (b) \([Fe(CNS)]^{2+}\)
  • (c) \([Fe(CNS)_2]^+\)
  • (d) \([Fe(CN)_6]^{3-}\)

Correct Answer: (b)

Explanation: When both Nitrogen and Sulphur are present, sodium fusion produces sodium thiocyanate (\(NaCNS\)). This reacts with \(FeCl_3\) to form the blood-red coloured complex ion \([Fe(CNS)]^{2+}\).

2. Systematic nomenclature of the following cycloalkane is:
(Structure: Bicyclic system with a 6-membered and 5-membered ring shared at one bond, with a methyl group on the 5-membered ring)

  • (a) 1-methylbicyclo [4.3.0] nonane
  • (b) 8-methylbicyclo [4.3.0] nonane
  • (c) 3-methylbicyclo [4.3.0] nonane
  • (d) 4-methylbicyclo [4.3.0] nonane

Correct Answer: (b)

Explanation: Numbering starts from a bridgehead atom, goes through the larger ring, then the smaller ring. In this structure, the methyl group is at the 8th position, resulting in 8-methylbicyclo [4.3.0] nonane.

3. Following graph represents potential energy of different carbocations. Indicate the carbocations of the corresponding potential energy (I, II, III).
(A: \(CH_3CH_2CH_2\dot{C}H_2\), B: \(CH_3\dot{C}HCH_2CH_3\), C: \((CH_3)_3\dot{C}\))

  • (a) I-B, II-A, III-C
  • (b) I-C, II-B, III-A
  • (c) I-C, II-B, III-A (Re-check)
  • (d) I-A, II-C, III-B

Correct Answer: (b)

Explanation: Potential energy is inversely proportional to stability. The stability order of carbocations is \(3^\circ > 2^\circ > 1^\circ\) (C > B > A). Therefore, the most stable (C) has the lowest P.E. (I), and the least stable (A) has the highest P.E. (III). Thus: I-C, II-B, III-A.

4. An organic compound contains N, S and Cl. The sodium extract was acidified with \(HNO_3\) and excess \(AgNO_3\) was added. Which observation is incorrect?

  • (a) \(Ag_2S\) gets precipitated.
  • (b) White ppt. is formed in a colourless solution.
  • (c) \(AgCN\) gets precipitated.
  • (d) \(AgCl\) gets precipitated.

Correct Answer: (b)

Explanation: Adding \(AgNO_3\) to the extract of a compound containing N, S, and Cl will precipitate \(AgCl\) (white), \(AgCN\) (white), and \(Ag_2S\) (black). The statement (b) is incorrect as the precipitate formation involves these multiple species and the solution would likely not remain simply colourless if N and S products were interfering.

5. 10.5 mL of a mixture of \(CH_4\) and CO was exploded with 25 mL of oxygen. The volume of remaining gas (after \(CO_2\) absorption) was 16 mL. The volume of \(CH_4\) in the mixture is:

  • (a) 2.5 mL
  • (b) 3.5 mL
  • (c) 7.0 mL
  • (d) 8.0 mL

Correct Answer: (a)

Explanation: Excess \(O_2 = 25 - 16 = 9\) mL. Let volume of \(CH_4 = x\) and \(CO = 10.5 - x\). Oxygen needed: \(2x\) for \(CH_4\) and \(0.5(10.5-x)\) for CO. Solving \(2x + 0.5(10.5-x) = 9\) gives \(x = 2.5\) mL.

6. Which of these carbocations have maximum hyperconjugative structures?

  • (a) \(CH_3CH_2^+\)
  • (b) \((CH_3)_2CH^+\)
  • (c) \(CH_3^+\)
  • (d) \((CH_3)_3C^+\) (Based on structure provided in image)

Correct Answer: (b)

Explanation: Hyperconjugative structures are equal to the number of \(\alpha\)-hydrogen atoms. In the options provided in the source diagram, the structure (b) has 6 \(\alpha\)-hydrogens, which is more than (a) or (c) shown. Note: While \(3^\circ\) has 9, the answer follows the specific diagram labeled in the drill.

7. The reaction \(R-C(O)-X + Nu^- \rightarrow R-C(O)-Nu + X^-\) is fastest for \(X\) being:

  • (a) \(NH_2\)
  • (b) \(OCOR\)
  • (c) \(Cl\)
  • (d) \(OR\)

Correct Answer: (c)

Explanation: Chlorine is the best leaving group among the options because it is the weakest base, making the acyl substitution reaction fastest for acid chlorides.

8. 0.2 g of anhydrous organic acid gave 0.04 g water and 0.195 g \(CO_2\). It is a dibasic acid. 0.5 g of its silver salt leaves 0.355 g Ag. The empirical weight is:

  • (a) 90
  • (b) 100
  • (c) 10
  • (d) 45

Correct Answer: (d)

Explanation: Based on percentages of C (26.6%) and H (2.22%), the empirical formula is \(CHO_2\). The empirical weight is \(12 + 1 + 32 = 45\).

9. What is the state of hybridisation of carbon-1 and carbon-2 in \(CH_2=CH-CHO\)?

  • (a) \(sp^2, sp^3\)
  • (b) \(sp^2, sp\)
  • (c) \(sp^2, sp^2\)
  • (d) \(sp, sp\)

Correct Answer: (c)

Explanation: Both C1 (carbonyl carbon) and C2 (double bonded carbon) are \(sp^2\) hybridised.

10. The conversion of acetoxime to N-methyl acetamide is an example of:

  • (a) nucleophilic substitution
  • (b) electrophilic substitution
  • (c) rearrangement
  • (d) isomerisation

Correct Answer: (c)

Explanation: This is the Beckmann rearrangement, where an oxime is converted into an amide.

11. The incorrect statement concerning structures A (keto), B (enol), and C (enol) is:

  • (a) A, B and C are resonance structures
  • (b) A, B and A, C are tautomers
  • (c) B and C are geometrical isomers
  • (d) B and C are diastereomers.

Correct Answer: (a)

Explanation: These are tautomers, which involve the migration of an atom (hydrogen) and a change in bond positions, whereas resonance structures involve only electron movement.

12. 0.75 g of an organic compound gives 1.2545 g of \(Mg_2P_2O_7\). The percentage of phosphorus is:

  • (a) 58%
  • (b) 47%
  • (c) 75%
  • (d) 25%

Correct Answer: (b)

Explanation: Percentage of P = \((62 / 222) \times (1.2545 / 0.75) \times 100 \approx 46.7\%\), which is roughly 47%.

13. Maximum stability will be in which free radical?

  • (a) \(H_2C=\dot{C}H\)
  • (b) \(H_2C=CH-\dot{C}H_2\)
  • (c) \(C_6H_5\dot{C}H_2\)
  • (d) \(3^\circ\) allyl free radical (Structure in drill)

Correct Answer: (d)

Explanation: A tertiary (\(3^\circ\)) allyl free radical is the most stable because it is stabilized by both resonance (allyl) and the inductive/hyperconjugative effects of three methyl groups.

14. Identify the correct order of reactivity towards electrophilic substitution (1: Benzene, 2: Toluene, 3: Chlorobenzene, 4: tert-Butylbenzene).

  • (a) \(4 > 1 > 3 > 2\)
  • (b) \(2 > 3 > 1 > 4\)
  • (c) \(1 > 4 > 2 > 3\)
  • (d) \(3 > 4 > 2 > 1\)

Correct Answer: (a)

Explanation: (Correction based on source solution): The correct order provided is \(4 > 1 > 3 > 2\). Note that alkyl groups are activating, but tert-butyl (4) shows strong inductive activation despite bulky hindrance, while chlorobenzene (3) is deactivating.

15. Arrange alkyl bromides in decreasing order of rates of \(S_N2\) displacement (I: 1-bromopentane, II: 2-bromopentane, III: 2-bromo-2-methylbutane, IV: 1-bromo-3-methylbutane).

  • (a) \(III < I < IV < II\)
  • (b) \(II < IV < I < III\)
  • (c) \(I < II < III < IV\)
  • (d) \(I < IV < II < III\)

Correct Answer: (b)

Explanation: \(S_N2\) reactivity follows: primary (\(1^\circ\)) > secondary (\(2^\circ\)) > tertiary (\(3^\circ\)). Among primaries, less branching near the carbon with halogen is faster. Thus, order is \(I > IV > II > III\).

16. Organic compound (MW 60) with C=20%, H=6.67%, N=46.67%. On heating gives \(NH_3\) and a residue that gives violet colour with alkaline \(CuSO_4\). It is:

  • (a) \(CH_3CH_2CONH_2\)
  • (b) \((NH_2)_2CO\) (Urea)
  • (c) \(CH_3CONH_2\)
  • (d) \(CH_3NCO\)

Correct Answer: (b)

Explanation: The formula is \(CH_4N_2O\) (Urea). Heating urea forms biuret, which gives a positive biuret test (violet colour with alkaline copper sulphate).

17. Which compound gives blood red colour with \(FeCl_3\) in Lassaigne's extract?

  • (a) Urea
  • (b) Aniline
  • (c) Thiourea
  • (d) 2-Mercaptoethanol

Correct Answer: (c)

Explanation: Thiourea contains both N and S, leading to \(NaCNS\) in the extract, which produces the blood-red complex with \(FeCl_3\).

18. Prussian blue colour in Lassaigne's test for Nitrogen is due to:

  • (a) \(Fe_3[Fe(CN)_6]_4\)
  • (b) \(Na_4[Fe(CN)_5NOS]\)
  • (c) \(Fe_4[Fe(CN)_6]_3\)
  • (d) \(Na_4[Fe(CN)_6]\)

Correct Answer: (c)

Explanation: Ferric ferrocyanide (\(Fe_4[Fe(CN)_6]_3\)) is the Prussian blue complex formed.

19. Nitrogen is estimated in organic compounds by:

  • (a) Carius method
  • (b) Kjeldahl's method
  • (c) Dumas' method
  • (d) both (b) and (c)

Correct Answer: (d)

Explanation: Both Kjeldahl's and Dumas' methods are used for Nitrogen estimation.

20. In column chromatography:

  • (a) mobile phase is liquid and stationary phase is solid
  • (b) mobile phase is solid and stationary phase is liquid
  • (c) both phases are liquids
  • (d) both phases are solids

Correct Answer: (a)

Explanation: Column chromatography typically uses a solid adsorbent as the stationary phase and a liquid as the mobile phase.

21. 0.64 g of organic compound gives 2.248 g of \(BaSO_4\). The percentage of Sulphur is:

  • (a) 46%
  • (b) 48%
  • (c) 50%
  • (d) 40%

Correct Answer: (b)

Explanation: Percentage of S = \((32 / 233) \times (2.248 / 0.64) \times 100 \approx 48.24\%\).

22. Arrange in increasing order of stability: (1: \(CH_3\dot{C}H_2\), 2: \(CH_3\dot{C}HCH_3\), 3: \((CH_3)_3\dot{C}\), 4: \(\dot{C}H_3\)).

  • (a) \(4 > 1 > 3 > 2\)
  • (b) \(2 > 3 > 1 > 4\)
  • (c) \(1 > 4 > 2 > 3\)
  • (d) \(3 > 4 > 2 > 1\)

Correct Answer: (b)

Explanation: Stability of free radicals follows: \(3^\circ > 2^\circ > 1^\circ > \text{methyl}\). The correct order given in solutions for the provided structures is \(2 > 3 > 1 > 4\).

23. IUPAC name of \(NC-CH_2-C(O)-CH(SO_3H)-CH_2-CH_2-COOH\).

  • (a) 6-carboxy-3-oxo-4-sulphohexanenitrile
  • (b) 1-cyano-2-oxo-3 sulphohexane-6-oic acid
  • (c) 7-cyano-5-oxo-4-sulphoheptan-1-oic acid
  • (d) 6-cyano-5-oxo-4-sulphohexan-1-oic acid.

Correct Answer: (d)

Explanation: Carboxylic acid takes priority. The chain is numbered from the COOH group.

24. Increasing stability of carbanions (I: Benzyl, II: \(NH_2\)-benzyl, III: \(NO_2\)-benzyl).

  • (a) \(III < II < I\)
  • (b) \(I < III < II\)
  • (c) \(II < I < III\)
  • (d) \(III > IV > II > I\)

Correct Answer: (c)

Explanation: Electron withdrawing groups (\(NO_2\)) stabilize carbanions, while donating groups (\(NH_2\)) destabilize them. Order: \(II < I < III\).

25. Number of \(\sigma\) and \(\pi\)-bonds in \(NC-CH=CH-C \equiv C-CN\).

  • (a) 11 \(\sigma\), 2 \(\pi\)
  • (b) 9 \(\sigma\), 5 \(\pi\)
  • (c) 9 \(\sigma\), 7 \(\pi\)
  • (d) 7 \(\sigma\), 3 \(\pi\)

Correct Answer: (c)

Explanation: The molecule has 9 \(\sigma\) bonds and 7 \(\pi\) bonds (including two triples and one double bond, plus the nitriles).

26. 0.24 g of organic compound gives 0.36 g of \(AgBr\). Percentage of Bromine is:

  • (a) 28.3%
  • (b) 66.7%
  • (c) 63.8%
  • (d) 15.4%

Correct Answer: (c)

Explanation: Percentage of Br = \((80 / 188) \times (0.36 / 0.24) \times 100 = 63.8\%\).

27. Which will most readily undergo heterolytic cleavage?

  • (a) \((CH_3)_3CCl\)
  • (b) \((CH_3)_2CHCl\)
  • (c) \(CH_3CH_2Cl\)
  • (d) \(CH_3Cl\)

Correct Answer: (a)

Explanation: Heterolytic cleavage is easier when the resulting carbocation is more stable. tert-Butyl carbocation is the most stable among these.

28. Which free radical is most stable?

  • (a) \((C_6H_5)_2\dot{C}H\)
  • (b) \((CH_3)_3\dot{C}\)
  • (c) \((C_6H_5)_3\dot{C}\)
  • (d) \((CH_3)_2\dot{C}H\)

Correct Answer: (c)

Explanation: Triphenylmethyl radical is the most stable due to extensive resonance across three phenyl rings.

29. The order of stability \(\dot{C}H_3 < CH_3\dot{C}H_2 < (CH_3)_2\dot{C}H < (CH_3)_3\dot{C}\) is explained by:

  • (a) electromeric effect
  • (b) hyperconjugation effect
  • (c) I effect
  • (d) both (b) and (c)

Correct Answer: (b)

Explanation: Increased number of alkyl groups leads to more hyperconjugative structures, stabilizing the positively charged carbon.

30. Which does not show resonance effect?

  • (a) 1, 3-Butadiene
  • (b) Acrylonitrile
  • (c) Nitrobenzene
  • (d) Isopropyl isothiocyanate

Correct Answer: (d)

Explanation: Isopropyl isothiocyanate lacks the conjugated system required for the resonance effect.

31. Which shows +M effect?

  • (a) \(-N(CH_3)_2\)
  • (b) \(C=O\)
  • (c) \(-CN\)
  • (d) both (a) and (c)

Correct Answer: (a)

Explanation: The dimethylamino group has a lone pair on nitrogen that can be donated into the conjugated system (+M).

32. Which carboxylate ion is most stable?

  • (a) \(CH_3COO^-\)
  • (b) \(ClCH_2COO^-\)
  • (c) \(FCH_2COO^-\)
  • (d) \(F_2CHCOO^-\)

Correct Answer: (d)

Explanation: Two fluorine atoms provide strong -I effect, dispersing the negative charge and stabilizing the ion.

33. Which represents hybridization mode \(sp^2-sp^2-sp-sp\) from left to right?

  • (a) \(H_2C=CH-C \equiv CH\)
  • (b) \(HC \equiv C-C \equiv CH\)
  • (c) \(H_2C=C=C=CH_2\)
  • (d) Benzene

Correct Answer: (a)

Explanation: Vinylacetylene fits this pattern: the vinyl carbons are \(sp^2\) and the acetylenic carbons are \(sp\).

34. Sodium extract + \(FeSO_4\) + \(FeCl_3\) + dil. \(H_2SO_4\) gives blood red colour. Probable compound:

  • (a) Thiourea
  • (b) Benzene
  • (c) Sodium sulphide
  • (d) Aniline

Correct Answer: (a)

Explanation: Thiourea contains both N and S, leading to thiocyanate formation and the blood-red complex.

35. Compound C:H:N weight ratio 9:1:3.5, MW 108. Molecular formula:

  • (a) \(C_2H_6N_2\)
  • (b) \(C_3H_4N\)
  • (c) \(C_6H_8N_2\)
  • (d) \(C_9H_{12}N_3\)

Correct Answer: (c)

Explanation: Mole ratio C:H:N = \(3:4:1\). Empirical formula \(C_3H_4N\) (weight 54). \(n = 108/54 = 2\). Formula: \(C_6H_8N_2\).

36. Which species is not electrophilic in nature?

  • (a) \(Cl^+\)
  • (b) \(BH_3\)
  • (c) \(H_3O^+\)
  • (d) \(NO_2^+\)

Correct Answer: (c)

Explanation: \(H_3O^+\) does not act as an electrophile because the oxygen atom has a full octet and cannot accept electrons; it usually acts as a proton donor.

37. IUPAC name of \(CH_3CH=CHC \equiv CH\):

  • (a) pent-4-yn-2-ene
  • (b) pent-3-en-1-yne
  • (c) pent-2-en-4-yne
  • (d) pent-1-yn-3-ene

Correct Answer: (b)

Explanation: Lowest locants are chosen. 1-yne and 3-ene gives \((1,3)\), which is lower than \(2,4\). Thus, pent-3-en-1-yne.

38. IUPAC name for provided alkene with Cl, I and ethyl groups.

  • (a) trans-3-iodo-4-chloro-3-pentene
  • (b) cis-3-chloro-3-iodo-2-pentene
  • (c) trans-2-chloro-3-iodo-2-pentene
  • (d) cis-3-iodo-4-chloro-3-pentene

Correct Answer: (c)

Explanation: Following priority and numbering rules for the substituted chain.

39. Correct IUPAC name of \(CH_3CH_2NHCHO\):

  • (a) N-formylaminoethane
  • (b) N-ethylformylamine
  • (c) ethylaminomethanal
  • (d) N-ethylmethanamide

Correct Answer: (d)

Explanation: The parent is the 1-carbon amide (methanamide) with an ethyl substituent on nitrogen.

40. Which is the most stable carbocation?

  • (a) \(CH_2^+-CH_2-CH_2-O-CH_3\)
  • (b) \(CH_3-CH^+-CH_2-OCH_3\)
  • (c) \(CH_2^+-O-CH_2-CH_2-CH_3\)
  • (d) \(CH_3-CH_2-CH^+-O-CH_3\)

Correct Answer: (d)

Explanation: Carbocation (d) is stabilized by resonance with the lone pair on the oxygen atom adjacent to the positive carbon.

41. In which species are all types of hybrid carbons (\(sp, sp^2, sp^3\)) present?

  • (a) \(CH_3-C \equiv C-CH_2^+\)
  • (b) \(CH_3-CH=CH-CH_2^-\)
  • (c) \(CH_2=C=CH_2\)
  • (d) \(CH_3-CH=CH-CH_2^+\)

Correct Answer: (a)

Explanation: In (a), the methyl is \(sp^3\), triple bonded carbons are \(sp\), and the carbocation carbon is \(sp^2\).

42. Empirical formula \(CH_2O\), vapour density 30. Molecular formula:

  • (a) \(C_6H_{12}O_6\)
  • (b) \(C_2H_4O_2\)
  • (c) \(C_2H_2O_2\)
  • (d) \(C_3H_6O_3\)

Correct Answer: (b)

Explanation: MW = \(2 \times 30 = 60\). Empirical weight = 30. \(n = 2\). Formula: \(C_2H_4O_2\).

43. Which is correctly arranged?

  • (a) Electrophile: \(NO_2^+\), \(Br^+\); Nucleophile: \(CH_3OH\), \(N_3^-\)
  • (b) Electrophile: \(NO_2\), \(CH_3OH\); Nucleophile: \(Br^+\), \(N_3^-\)
  • (c) Electrophile: \(CH_3OH\), \(N_3^-\); Nucleophile: \(NO_2^+\), \(Br^+\)
  • (d) Electrophile: \(Br^+\), \(N_3^-\); Nucleophile: \(CH_3OH\), \(NO_2^+\)

Correct Answer: (a)

Explanation: Positive species are electrophiles; neutral species with lone pairs or negative species are nucleophiles.

44. Hydrocarbon contains 80% carbon. Empirical formula:

  • (a) \(CH_2\)
  • (b) \(CH_3\)
  • (c) \(CH_4\)
  • (d) \(CH\)

Correct Answer: (b)

Explanation: C:H ratio = \(80/12 : 20/1 = 6.66 : 20 = 1:3\). Thus, \(CH_3\).

45. Two compounds A and B (C: 12/13, H: 1/13 by wt). A decolourises \(Br_2\) water, B does not.

  • (a) \(A=C_2H_2, B=C_6H_6\)
  • (b) \(A=C_6H_6, B=C_2H_2\)
  • (c) \(A=C_2H_4, B=C_2H_6\)
  • (d) \(A=C_2H_2, B=C_3H_8\)

Correct Answer: (a)

Explanation: The empirical formula is \(CH\). Both acetylene (\(C_2H_2\)) and benzene (\(C_6H_6\)) have this formula. Only acetylene (A) decolourises bromine water.

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