Q2. Explain why alkyl halides,though polar,are immiscible with water?

1. Q1. Haloalkanes undergo nucleophilic substitution whereas haloarenes undergo electrophilicsubstitution.WHY?

2. Ans 1. Haloalkanes are more polar than haloarenes.Consequently the carbon atom carrying the halogen in haloalkane is more electron deficient than that in haloarenes.As the result halalkane undergo nucleophilic substitution more readily than haloarenes. In contrast haloarenes contain a benzene ring.since the typical reaction of benzene are electrophilicsubstitution,thereforehaloarenes undergo electrophilic substitution.

3. Q2. Explain why alkyl halides,though polar,are immiscible with water?

4. Ans2. Alkyl halide are polar molecules, therefore,their molecules are held together by dipole-dipole attraction.The molecules of water are held together by H bonds.Since the new force of attraction between water and alkyl halide molecules are weaker than the force of attraction already existing between alkyl halide-alkyl halide molecule and water-water molecules.Thus alkyl halide are immiscible with water.

5. Q3. What happens when chlorine is passed through boiling toluene in the presence of sunlight?

6. Ans 3. Free radical substitution occurs at the METHYL group giving first benzyl chloride, then benzal dichloride and finally benzotrichloride.

7. Q4. Which is stronger acid phenol or cresol?

8. Ans 4. Due to the I+ effect of CH3grp electron –density in the O-H bond increases.Thus it becomes difficult to break the O-H bond as compared to that of the phenol & hence cresol is more acidic than phenol.

9. Q5. Why gridnard reagent soluble in ether but not in benzene?

10. Ans 5. Gridnard reagent form coordination complexes with ether but not with benzene since the ether has lone pairs of electron but the benzene does not.

11. Q6. What makes ethanoic acid more stronger than ethanol.

12. Ans 6. Ethanoate ion obtained by loss of proton from ethanoic acid is stabilized by resonance but ethoxide ion obtained by loss of proton from ethanol is not.This makes ethanoic acid more stronger than ethanol.

13. Q7. Aniline gets coloured on standing in air for a long time WHY?

14. Ans 7. Due to the strong electron donating effect (+R-effect) of NH2grp, the electron density on benzene ring increases. As a result, aniline is easily oxidised on standing in air for long time to form coloured products.

15. Q8. Why do nitro compounds have high boiling point in comparison with other compounds of same molecular mass?

16. Ans 8. Nitro compounds have high dipole moments and thus have strong dipole-dipole interaction.Therefore ,their boiling points are much higher as compared to hydrocarbons or other compounds of same molecular mass.

17. Q9. Why is carbon nitro bond length in aromatic amines shorter than the aliphatic amines?

18. Ans 9. Due to the electron donating resonance effect of NH2 grp the carbon nitrogen bond in the aromatic amines has some double bond character. In contrast in the aliphatic amines the carbon –nitrogen bond has only single bond character therefore carbon-nitrogen bond length in aromatic amines is shorter than in aliphatic amines.

19. Q10. Why do amines act as nucleophiles?

20. Ans 10. Due to the presence of lone pair of electron on the N-atom,amines can easily donate this lone pair to the carbon species and hence act as nucleophiles.

21. Q.11) p-Dichlorobenzene has highmelting point & lower solubility than ortho and meta isomers. Discuss.

22. ANS: The p-isomers being more symmetrical fits closely in the crystal lattice & thus ha stronger intermolecular forces of attraction than o- & m- isomers. As during melting or dissolution, the crystal lattice breaks. therefore, large amount of energy is needed to melt or dissolve the p-isomer than the corresponding o-and m-isomers. In other words, the melting point of the p-isomer is higher and its solubility lower than the corresponding o-and m-isomers.

23. Q.12)The treatment of alkyl chlorides with aqueous KOH leads to the formation of alcohols but in presence of alcoholic KOH, alkenes are the major products. Explain why?

24. ANS: In aqueous solution, KOH is almost completely ionized to give OH- ions which being a strong nucleophile brings about a substitution reaction on alkyl halides to form alcohols. In the aqueous solution, OH- ions are highly solvated (hydrated). This reduces the basic character of OH- ions which fail to abstract hydrogen from the β-carbon of the alkyl chloride to form an alkenes.On the other hand, and alcoholic solution of KOH contains alkoxide (RO-) ions which being a much stronger base than OH- ions preferentially eliminates a molecule of HCI from an alkyl chloride to form alkenes.

25. Q13. Explain the following in one or two sentences :(i)Allyl chloride is hydrolysed more readily than n-propyl chloride.(ii)Vinyl chloride is hydrolysed more slowly than ethyl chloride.

26. ANS : (i)Allyl chloride readily undergoes ionization to produce resonance stabilized allyl carbocation. Since carbocations are reactive species, therefore, allyl cation readily combines with OH- ions to form allyl alcohol. CH2=CH-CH2-Cl CH2=CH-CH2+ + Cl- CH2=CH-CH2-OH CH2+-CH= CH2 On the other hand, n-propyl chloride does not undergo ioization to produce n-propyl carbocation and hence allyl chloride is hydrolysed more readily that n-propyl chloride. (ii)Vinyl chloride may be represented as a resonance hybrid of the following two structures: CH2=CH-Cl CH2--CH=Cl+ As a result of resonance, the carbon-chlorine bond acquires some double bond character. On the other hand, in ethyl chloride, the carbon-chlorine bond is a pure single bond. Thus, vinyl chloride undergoes hydrolysis more slowly that ethyl chloride.

27. Q14. Haloalkanes react with KCN to form alkyl cyanides as main product while AgCN form isocyanides as the chief product. Explain.

28. ANS : KCN is predominantly ionic and provides cyanide ions in solution. However both carbon and nitrogen atoms are in a position to donate electron pairs, the attack takes place mainly through carbon atom and not through nitrogen atom since C-C bond is more stable the C-N bond. However, AgCN is mainly covalent in nature and nitrogen is free to donate electron pair forming isocyanide as the main product.

29. Q15. Account for the following:Haloalkanes undergo nucleophilic substitution whereas haloarenes undergo electrophilic substitutions.

30. ANS : Haloalkanes are more polar than haloarenes. As a result the carbon atom carrying the halogen in halolkanes is more electron-deficient than that in haloarenes. Consequently, haloalkanes undergo nucleophilic substitutions more readily than haloarenes. On the other hand, haloarenes contain a benzene ring. Since the typical reaction of benzene are electrophilic substitutions, therefore, haloarenes undergo electrophilic substitutions while haloalkanes which do not contain a benzene ring to not undergo electrophilic substitutions.

31. Q16. Explainwhy propanol has higher boiling point than that of the hydrocarbon, butane ?

32. ANS : The molecules of butane are held together by weak van der Waal’s forces of attraction while those of propanol are held together by stronger intermolecular hydrogen bonding.Hence, the boiling point of propanol is much higher that that of butane.

33. Q17. Alcohols are comparatively more soluble in water than hydrocarbons of comparable molecular masses. Explain this fact.

34. ANS : Alcohols can form H-bonds with water and break the H-Bonds already existing between water molecules. Hence, they are soluble in water.In contrast, hydrocarbons cannot form H-bonds with water and hence are insoluble in water.

35. Q18.Lower alcohols are soluble in water, higher alcohols are not, why?

36. ANS :Lower alcohols can form H-bonds with water whereas higher alcohols cannot due to large hydrocarbons part.

37. Q19. Ortho-nirophenol is more acidic than ortho-methoxyphenol. Why?

38. ANS : It is because Nitro group is electron withdrawing which increases the acidic character whereas –OCH3 group is electron releasing which decreases acidic character.

39. Q20. Phenol has smaller dipole moment than methanol.ORWhy are dipole moments of phenols smaller that dipole moments of alcohols.

40. ANS : Due to electron withdrawing effect of the benzene ring the C-O bond in phenol is less polar but in case of methanol due to electron-donating effect of CH3 group, C-O bond is more polar.

41. Q21]Why does H2 is liberated at cathode during kolbes electrolytic reaction?

42. Ans]During Kolbes electrolytic reaction hydrogen gas is liberated at cathode because discharge potential of H+ ions is lower than that of Na+ ions, therefore H+ ions are discharged to produce H2 while Na+ ions remains in the solution .

43. Q22]why does alkynes are less reactive than alkanes towards electrophilic addition reaction?

44. Ans]Inspite of having two π- bonds ,alkynes are less reactive than alkenes having one π- bond towards electrophilic addition reaction because(1)Carbon atom in alkynes is sp hybridised which is more electronegative than the sp2hybridised carbon atom incase of alkene as a a result , π- electrons are more tightly held by the C- atoms of alkynes as compared to the π-electrons of alkene . Thus it is clear that electrons are easily available for the reaction with electrophilies in case of alkene.

45. Q23]Optically active 2-iodobutane on treatment with NaI in acetone give a product which does not show optical activity. why?

46. Ans]Optically active 2-iodobutane on treatment with NaI in acetone undergoes racemization and hence the product does not show optical activity .when 2-iodobutane (I)is treated with I- ion , it undergoes Walden inversion SN2 reaction to give 2-iodobutane(II)which is the enantiomer of (I).

47. Q4]Why CHF3 is less reactive than CHCl3 though fluorine is more electronegative than the chlorine?

48. Ans] CCl3 by the removal of proton from CHCl3 is stabilized by resonance due to presence of d-orbitals in chlorine while CF3 is not stabilized by resonance due to absence of d- orbitals in fluorine.

49. Q5]Haloarenes are insoluble in water but soluble in benzene. Why?

50. Ans]Haloarenes are insoluble in water as it cannot break the H-bonding already In water or it cannot form H –bonding with water . It is known that like dissolves like ,haloarenes due to presence of a large hydrocarbon part are soluble In hydrocarbon solvents such as benzene , ether etc.