Q2) Ammonlysis of R-X Is Not a Preferred Method for Preparing Amines

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 asmall 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) Becausearyl halides do not undergo nucleophilic substitution since C-Xbond has partial double characterdue 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 interactionsof largeralkyl 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 isPrimarySecondaryTertiary

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) effectof alkyl groups electron density on N increases making amines stronger Lewis base than NH3.

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 theunshared 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 > 3oNH3.

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 cationby hydration (solvation)is 1o> 2o > 3o because RNH3+ has more -Hydrogen than R2NH2+, whichhas more than R3NH+ .Moreover larger and more number of alkyl groups also cause sterichindrance 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).

C2H5NH2+CH3COCl pyridine 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 pKbvalue, weaker the base. Aniline is a weaker basebecause e withdrawing benzene ring reduces e density on N so the lone pair of electrons on N is less readily available for protonation while CH3group 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-NH2group 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─

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CONVERSIONS

CH3COOH to CH3CH2COOH

Methanamine to Ethanamine

Nitromethane to Dimethylamine

Propanoic acid to Ethanoic acid

Nitrobenzene to Benzoic acid

Benzene to 3-Bromophenol

Benzoic acid to aniline

Aniline to 2,4,6-tribromofluorobenzene

Benzyl chloride to 2-Phenylethanamine

Chlorobenzene to 4-Chloroaniline

Aniline to 4-Bromoaniline

Benzamide to toluene

Aniline to Benzyl alcohol

Aniline to 1,3,5-Tribromobenzene

Ethanoic acid to Methanamine

CH3(CH2)4CN to CH3(CH2)4NH2

Methanol to Ethanoic acid

Ethanamine to Methanamine