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UNIT ─ 13 AMINES GIVE REASONS:Q1) In the reduction of nitroalkanes to corresponding alkanamines iron scrap and HCl is preferred as reducing reagent. Ans) This is because FeCl2 formed gets hydrolysed to release HCl in the reaction. Thus, only a small amount of HCl is required to initiate the reaction. Q2) Ammonlysis of R-X is not a preferred method for preparing amines Ans) This method yields a mixture of primary, secondary, tertiary a mines and also quaternary ammonium salt which need to be separated Q3) Aromatic primary amines cannot be prepared by Gabriel phthalimide synthesis Ans) Because aryl halides do not undergo nucleophilic substitution since C-X bond has partial double character due to resonance and is difficult to break. Q4) Lower aliphatic amines are soluble in water. Ans) because they can form hydrogen bands with water molecules Q5) Higher amines are essentially insoluble in water Ans) Due to hydrophobic interactions of larger alkyl gramp Q6) Butan-1-ol is more soluble in water than butan-1-amine Ans) Butan-1-ol is more polar due to higher electronegitivity of oxygen companed to nitrogen Q7) The order of boiling points of isomeric amines is Primary > Secondary > Tertiary Ans) (last paragraph page 387) Q8) Boiling points n-C4H9OH is higher than of n-C4H9NH2. Ans) Though both have intermolecular H bond .The H bond between alcohol molecules are stronger owing to the higher polarity of O – H bond compared to N – H bond. Q9) Aliphatic amines are stronger bases than NH3. Ans) In both NH3 and amines there is an unshared pair of electrons on N atom due to which they behave as Lewis base. Due to electron donating inductive effect (+I) effect of alkyl groups electron density on N increases making amines stronger Lewis base than NH3. R NH 2 Q10) Aromatic amines are less basic than NH3 Ans) Election withdrawing aryl group (benzene ring) reduce elections density on N. Q11) The order of basicity of amines in a gaseous phase is 3o > 2o > 1o. Ans) Electron releasing alkyl group (+I effect) pushes electrons towards N and thus makes the unshared electron pair more available for sharing with the proton of the acid. So, basic nature increases with increase in number of alkyl groups. Q12) The order of basicity of amines in aqueous solutions is in the order 2o > 1o > 3o>NH3. Ans) In aqueous phase besides +I effect of alkyl groups hydration of substituted ammonium cations and steric factors affect base strength. Order of stability of ammonium cation by hydration (solvation)is 1o> 2o > 3o because RNH3+ has more -Hydrogen than R2NH2+, which has more than R3NH+ .Moreover larger and more number of alkyl groups also cause steric hindrance to formation of H bonds with water. Q13) The order of base strength in aqueous solution for ethyl substituted amines is in this order (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3 while for methyl substituted amines it is (CH3)2NH > CH3NH2 > (CH3)3N > NH3 Ans) Due to higher +I effect of Ethyl group Q14) The order of base strength in aromatic amines is : 2,4-Dimethoxyaniline > p-toluidine > aniline > p-Nitroaniline Ans) Electron releasing groups like –OCH3 , CH3 etc increase electron density on N and increase base strength while electron withdrawing groups like – NO2 , -COOH , -X etc decrease base strength . Moreover, more the number of electron releasing groups stronger is the base. Q15) Acylation reaction of amines is carried out in the presence of a base like pyridine which is a stronger base than amine. Ans) The base removes HCl formed and shifts the equilibrium of the reaction to the right side (LeChattelier’s Principle). pyridine C2H5NH2 + CH3COCl C2H5NHCOCH3 + HCl Q16) The product obtained when benzenesulphonyl chloride (Hinsberg’s reagent) reacts with a primary amine is soluble in alkali. But the product obtained when secondary amine reacts in a similar manner is not. Ans) The H attached to N is acidic due to the presence of strong e withdrawing sulphonyl group. So it forms salt with alkali and dissolves. In the case of sec-amines there is no H attached to N so the product does not dissolve in alkali. Q17) pKb of aniline is more than that of methylamine Ans) Higher the pKb value, weaker the base. Aniline is a weaker base because e withdrawing benzene ring reduces e density on N so the lone pair of electrons on N is less readily available for protonation while CH3 group in methyl amine is election releasing and it increase e density on N. Q18) Ethyl amine is soluble in water whereas aniline is not Ans) Ethyl amine can form H- bonds with water but the larger phenyl group causes hydrophobic interactions. Q19) Methyl amine in water reacts with ferric chloride to precipitate ferric hydroxide Fe(OH)3 Ans) Methyl amine being basic liberates OH─ ions in water which causes precipitation of Fe(OH)3 CH3NH2(aq) + H2O(l) → CH3NH3+ (aq) + OH─(aq) FeCl3(aq) + OH─ (aq) → Fe(OH)3 (s) + Cl─(aq) Q20) Amines are less acidic than alcohols of comparable molecular masses Ans) Due lower electro-negativity of N compared to oxygen N-H bond is less polar than O-H bond. Q21) Gabriel phthalimide synthesis is preferred for synthesizing only primary amines. Ans) This method yields only primary amines and not 2o & 3o amines. Moreover phthalic acid obtained can be used to prepare phthalimide. Q22) Anilines react with bromine water to form 2,4,6-tribromoaniline. Ans) -NH2 is a highly activating group; so electrophilic substitutions occur at all ortho & para positions. Q23) Aniline is first acetylated before bromination to yield 4-Bromoaniline. Ans) This is to reduce the activity of -NH2 group by acetylation. The lone pair of electrons on N of acetanilide interacts with oxygen atom due to resonance. So activating effect of ─ NHCOCH3 Group is less than that of ─NH2 group Q24) Although amino group is o,p-directing in aromatic electrophilic substitution reactions, aniline on nitration gives a substantial amount of m-Nitroaniline also. Ans) During nitration aniline being basic is protonated to form anilinium ion which is a deactivating group and hence m-directing. Q25) Aniline does not undergo Friedel-Craft’s reaction. Ans) Aniline being basic forms a salt with AlCl3, which is a Lewis acid. Due to this, nitrogen of aniline acquires positive charge and hence acts a strong deactivating group. Q26) Diazonium salts of aromatic amines are more stable than those of aliphatic amines. Ans) This because aromatic diazonium salts are resonance stabilized. Q27) Precipitate of AgCl dissolves in aqueous solution of amines. Ans. Due to the presence of lone pair of electrons on the N atom the amines can act as ligands and form soluble complexes. AgCl(s) + CH3NH2(aq) → [Ag(CH3NH2)2]+ Cl─ ******************************* CONVERSIONS 1. CH3COOH to CH3CH2COOH 2O / H CH 3COOH LiAlH 4 CH 3CH 2OH SOCl 2 CH 3CH 2Cl KCN CH 3CH 2CN H CH 3CH 2COOH 2. Methanamine to Ethanamine 2 / HCl / H 2O 2 / Ni CH 3 NH 2 NaNO CH 3OH SOCl 2 CH 3Cl KCN CH 3CN H CH 3CH 2 NH 2 3. Nitromethane to Dimethylamine 3Cl CH 3 NO2 Fe / HCl CH 3 NH 2 CH CH 3 NHCH 3 4. Propanoic acid to Ethanoic acid NH 3 / NaOH / Br2 CH 3CH 2COOH CH 3CH 2CONH 2 CH 3CH 2 NH 2 KMnO4 ( O ) NaNO2 / HCl / H 2O CH 3COOH CH 3CH 2OH 5. Nitrobenzene to Benzoic acid 6. Benzene to 3-Bromophenol 7. Benzoic acid to aniline 8. Aniline to 2,4,6-tribromofluorobenzene 9. Benzyl chloride to 2-Phenylethanamine 10. Chlorobenzene to 4-Chloroaniline 11. Aniline to 4-Bromoaniline 12. Benzamide to toluene 13. Aniline to Benzyl alcohol 14. Aniline to 1,3,5-Tribromobenzene 15. Ethanoic acid to Methanamine NH 3 / KOH / Br2 , NaOH / Br2 CH 3COOH CH 3CONH 2 CH 3 NH 2 16. CH3(CH2)4CN to CH3(CH2)4NH2 / Br2 3O CH 3 (CH 2 ) 4 CN H CH 3 (CH 2 ) 4 COOH NH 3 CH 3 (CH 2 ) 4 CONH 2 NaOH / CH 3 (CH 2 ) 4 NH 2 17. Methanol to Ethanoic acid KCN H 2O / H CH 3OH SOCl 2 CH 3Cl CH 3CN CH 3COOH 18. Ethanamine to Methanamine / Br2 2 / HCl / HONO 4 (O ) CH 3CH 2 NH 2 NaNO CH 3CH 2OH KMnO CH 3COOH NH 3 CH 3CONH 2 NaOH / CH 3 NH 2