Important Questions for Class 12 Chapter 13 - Amines

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Important questions based on NCERT syllabus for Chapter 13 - Amines:

Question-1: Write IUPAC names of the following compounds and classify them into primary, secondary and tertiary amines.
(i) (CH3)2 CHNH2 (ii) CH3(CH2)2NH2 (iii) CH3NHCH(CH3)2
(iv) (CH3)3 CNH2 (v) C6H5NHCH3(vi) (CH3CH2)2NCH3

Solution: (i) Propan-2-amine(1°)
(ii)Propan-1-amine (1°) ,
(iii)N-Methylpropan-2-amine (2°) .
(vi)N-Ethyl-N-methylethanamine (3°)
(vii)3-Bromobenzenamine or 3-bromoaniline (1°)

Question-2: Give plausible explanation for each of the following:
(i) Why are amines less acidic than alcohols of comparable molecular masses?
(ii) Why do primary amines have higher boiling point than tertiary amines?

Solution: (i) Loss of proton from an amine gives an amide ion while loss of a proton from alcohol give an alkoxide ion.
R—NH2—>R—NH– +H+
R—O —H—>R— O– +H+ .
Since O is more electronegative than N, so it wijl attract positive species more strongly in comparison to N. Thus, RO~ is more stable than RNH®. Thus, alcohols are more acidic than amines. Conversely, amines are less acidic than alcohols.
(ii) Due to the presence of two H-atoms on N-atom of primary amines, they undergo extensive intermolecular H-bonding while tertiary amines due to the absence of H-atom on the N-atom do not undergo H-bonding. As a result, primary amines have higher boiling points than tertiary amines of comparable molecular mass.

Question-3: Why are aliphatic amines stronger bases than aromatic amines?

Solution: Aromatic amines are far less basic than ammonia and aliphatic amines because of following reasons:
(a)Due to resonance in aniline and other aromatic amines, the lone pair of electrons on the nitrogen atom gets delocalised over the benzene ring and thus it is less easily available for protonation. Therefore, aromatic amines are weaker bases than ammonia and aliphatic amines.
(b)Aromatic amines arS more stable than corresponding protonated ion; Hence, they hag very less tendency to combine with a proton to form corresponding protonated ion, and thus they are less basic.