Chapter 8 Solutions — Organic Chemistry — Some Basic Principles and Techniques

Exercises

1. 1.
   What are the hybridisation states of each carbon atom in the following compounds? CH2=C=O, CH3CH=CH2, (CH3)2CO, CH2=CHCN, C6H6
2. 2.
   Indicate the sigma and pi bonds in the following molecules: C6H6, C6H12, CH2Cl2, CH2=C=CH2, CH3NO2, HCONHCH3
3. 3.
   Write bond line formulas for: Isopropyl alcohol, 2,3-Dimethylbutanal, Heptan-4-one.
4. 4.
   Give the IUPAC names of the following compounds shown as bond-line formulas:

   • (a) phenyl attached to a 3-carbon chain
   • (b) a nitrile with a methyl branch
   • (c) a branched alkane with two methyl branches
   • (d) a 6-carbon chain bearing Cl and Br on the same (middle) carbon
   • (e) a chloro-aldehyde chain
   • (f) Cl2CHCH2OH.
5. 5.
   Which of the following represents the correct IUPAC name for the compounds concerned?

   • (a) 2,2-Dimethylpentane or 2-Dimethylpentane
   • (b) 2,4,7-Trimethyloctane or 2,5,7-Trimethyloctane
   • (c) 2-Chloro-4-methylpentane or 4-Chloro-2-methylpentane
   • (d) But-3-yn-1-ol or But-4-ol-1-yne
6. 6.
   Draw the formulas for the first five members of each homologous series beginning with the following compounds:

   • (a) H-COOH
   • (b) CH3COCH3
   • (c) H-CH=CH2.
7. 7.
   Give condensed and bond-line structural formulas and identify the functional groups present, if any, for:

   • (a) 2,2,4-Trimethylpentane
   • (b) 2-Hydroxy-1,2,3-propanetricarboxylic acid
   • (c) Hexanedial
8. 8.
   Identify the functional groups in the following compounds:

   • (a) a benzene ring bearing -CHO, -OCH3 (OMe) and -OH
   • (b) a benzene ring bearing -NH2 and -COOCH2CH2N(C2H5)2
   • (c) a benzene ring with a -CH=CH-NO2 substituent.
9. 9.
   Which of the two: O2N-CH2-CH2-O- or CH3-CH2-O- is expected to be more stable and why?
10. 10.
    Explain why alkyl groups act as electron donors when attached to a pi system.
11. 11.
    Draw the resonance structures for the following compounds and show the electron shift using curved-arrow notation:

    • (a) C6H5OH
    • (b) C6H5NO2
    • (c) CH3CH=CHCHO
    • (d) C6H5-CHO
    • (e) C6H5-CH2+ (benzyl cation)
    • (f) CH3CH=CH-CH2+ (allyl cation).
12. 12.
    What are electrophiles and nucleophiles? Explain with examples.
13. 13.
    Identify the reagents shown in bold in the following equations as nucleophiles or electrophiles:

    • (a) CH3COOH + HO- → CH3COO- + H2O
    • (b) CH3COCH3 + CN- → (CH3)2C(CN)OH
    • (c) C6H5 + CH3CO+ → C6H5COCH3
14. 14.
    Classify the following reactions in one of the reaction types studied in this unit:

    • (a) CH3CH2Br + HS^- → CH3CH2SH + Br^-
    • (b) (CH3)2C=CH2 + HCl → (CH3)2ClC-CH3
    • (c) CH3CH2Br + HO^- → CH2=CH2 + H2O + Br^-
    • (d) (CH3)3C-CH2OH + HBr → (CH3)2CBrCH2CH2CH3 + H2O
15. 15.
    What is the relationship between the members of the following pairs of structures? Are they structural or geometrical isomers or resonance contributors?

    • (a) two ketones — one with the C=O at the middle of a 5-carbon chain and one with the C=O next to the end
    • (b) two C=C structures differing only in the D/H arrangement about the double bond
    • (c) H-C(=O+H)-OH and H-C+(OH)-OH.
16. 16.
    For the following bond cleavages, use curved-arrows to show the electron flow and classify each as homolysis or heterolysis. Identify the reactive intermediate produced as free radical, carbocation, or carbanion:

    • (a) CH3-O-CH3 → CH3-O^ + ^CH3
    • (b) (CH3)2C=O + HO^- →
    • (c) (CH3)3C-Br → (CH3)3C^+ + Br^-
    • (d) benzene ring + E^+ →
17. 17.
    Explain the terms inductive and electromeric effects. Which electron displacement effect explains the following correct orders of acidity of the carboxylic acids:

    • (a) Cl3CCOOH > Cl2CHCOOH > ClCH2COOH
    • (b) CH3CH2COOH > (CH3)2CHCOOH > (CH3)3C.COOH
18. 18.
    Give a brief description of the principles of the following techniques taking an example in each case:

    • (a) Crystallisation
    • (b) Distillation
    • (c) Chromatography
19. 19.
    Describe the method, which can be used to separate two compounds with different solubilities in a solvent S.
20. 20.
    What is the difference between distillation, distillation under reduced pressure, and steam distillation?
21. 21.
    Discuss the chemistry of Lassaigne's test.
22. 22.
    Differentiate between the principle of estimation of nitrogen in an organic compound by

    • (i) Dumas method and
    • (ii) Kjeldahl's method.
23. 23.
    Discuss the principle of estimation of halogens, sulphur, and phosphorus present in an organic compound.
24. 24.
    Explain the principle of paper chromatography.
25. 25.
    Why is nitric acid added to sodium extract before adding silver nitrate for testing halogens?
26. 26.
    Explain the reason for the fusion of an organic compound with metallic sodium for testing nitrogen, sulphur, and halogens.
27. 27.
    Name a suitable technique of separation of the components from a mixture of calcium sulphate and camphor.
28. 28.
    Explain why an organic liquid vaporises at a temperature below its boiling point in its steam distillation.
29. 29.
    Will CCl4 give a white precipitate of AgCl on heating with AgNO3? Hint: CCl4 does not give silver chloride precipitate with AgNO3 whereas CH3Cl does. Give reason for your answer.
30. 30.
    Why is a solution of potassium hydroxide used to absorb carbon dioxide evolved during the estimation of carbon present in an organic compound?
31. 31.
    Why is it necessary to use acetic acid and not sulphuric acid for acidification of sodium extract for testing sulphur by lead acetate test?
32. 32.
    An organic compound contains 69% carbon and 4.8% hydrogen, the remainder being oxygen. Calculate the masses of carbon dioxide and water produced when 0.20 g of this substance is subjected to complete combustion.
33. 33.
    A sample of 0.50 g of an organic compound was treated according to Kjeldahl's method. The ammonia evolved was absorbed in 50 mL of 0.5 M H2SO4. The residual acid required 60 mL of 0.5 M NaOH solution for neutralisation. Find the percentage composition of nitrogen in the compound.
34. 34.
    0.3780 g of an organic chloro compound gave 0.5740 g of silver chloride in Carius estimation. Calculate the percentage of chlorine in the compound.
35. 35.
    In the Carius method, 0.468 g of an organic sulphur compound afforded 0.668 g of barium sulphate. Find out the percentage of sulphur in the given compound.
36. 36.
    In the organic compound CH2=CH-CH2-CN, the pair of hybridised orbitals involved in the formation of the C2-C3 bond is:

    • (a) sp-sp2
    • (b) sp2-sp3
    • (c) sp3-sp2
    • (d) sp3-sp
37. 37.
    In the Lassaigne's test for an organic compound, the Prussian blue colour is obtained due to the formation of:

    • (a) Na4[Fe(CN)6]
    • (b) Fe4[Fe(CN)6]3
    • (c) Fe3[Fe(CN)6]
    • (d) Fe4[Fe(CN)6]
38. 38.
    Which of the following carbocations is most stable?

    • (a) (CH3)3C^+
    • (b) CH3CH2^+
    • (c) CH3CH2CH2^+
    • (d) (CH3)2CH^+
39. 39.
    The best and latest technique for isolation, purification, and separation of organic compounds is:

    • (a) Crystallisation
    • (b) Distillation
    • (c) Sublimation
    • (d) Chromatography
40. 40.
    The reaction: CH3CH2I + OH^- → CH3CH2OH + I^- is classified as:

    • (a) electrophilic substitution
    • (b) nucleophilic substitution
    • (c) elimination
    • (d) addition