Arrange the following compounds as asked:
(a) In increasing order of pKb values: \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_6\text{H}_5\text{NHCH}_3 \), \( \text{C}_6\text{H}_5\text{NH}_2 \)
(b) In decreasing order of boiling point: \( \text{C}_2\text{H}_5\text{OH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{CH}_3)_2\text{NH} \)
(c) In decreasing order of solubility in water: \( \text{C}_6\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \)
Arrange the following compounds as asked:
(a) In increasing order of pKb values: \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_6\text{H}_5\text{NHCH}_3 \), \( \text{C}_6\text{H}_5\text{NH}_2 \)
(b) In decreasing order of boiling point: \( \text{C}_2\text{H}_5\text{OH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{CH}_3)_2\text{NH} \)
(c) In decreasing order of solubility in water: \( \text{C}_6\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \)
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Solution and Explanation
(a) In increasing order of pKb values: \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_6\text{H}_5\text{NHCH}_3 \), \( \text{C}_6\text{H}_5\text{NH}_2 \)
Solution:
Lower pKb implies stronger base. Order of increasing pKb (i.e., decreasing basicity):
\( (\text{C}_2\text{H}_5)_2\text{NH} < \text{C}_2\text{H}_5\text{NH}_2 < \text{C}_6\text{H}_5\text{NHCH}_3 < \text{C}_6\text{H}_5\text{NH}_2 \)
Quick Tip:
Aromatic amines are weaker bases due to resonance; aliphatic amines are stronger.
(b) In decreasing order of boiling point: \( \text{C}_2\text{H}_5\text{OH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \), \( (\text{CH}_3)_2\text{NH} \)
Solution:
Boiling point depends on hydrogen bonding and molecular weight. Order:
\( \text{C}_2\text{H}_5\text{OH} > \text{C}_2\text{H}_5\text{NH}_2 > (\text{CH}_3)_2\text{NH} \)
Quick Tip:
Alcohols have stronger H-bonding than amines, leading to higher boiling points.
(c) In decreasing order of solubility in water: \( \text{C}_6\text{H}_5\text{NH}_2 \), \( (\text{C}_2\text{H}_5)_2\text{NH} \), \( \text{C}_2\text{H}_5\text{NH}_2 \)
Solution:
Solubility depends on ability to form H-bonds and size of hydrophobic group. Order:
\( \text{C}_2\text{H}_5\text{NH}_2 > (\text{C}_2\text{H}_5)_2\text{NH} > \text{C}_6\text{H}_5\text{NH}_2 \)
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