What is the decreasing order of strength of the following bases?
(I) $OH^{-}$ (II) $NH_{2}^{-}$
(III) $HC \equiv C^{-}$ (IV) $CH_{3} CH_{2}^{-}$

Question

What is the decreasing order of strength of the following bases? (I) OH − (II) NH 2 − ​ (III) HC ≡ C − (IV) CH 3 ​ CH 2 − ​

TestHub · Accepted Answer

Solution:

The strength of a base is inversely proportional to the acidity of its conjugate acid. Stronger conjugate acids yield weaker conjugate bases, and weaker conjugate acids yield stronger conjugate bases.

The conjugate acids for the given bases are:

(I) OH⁻: H₂O (pKa ~15.7)

(II) NH₂⁻: NH₃ (pKa ~38)

(III) HC≡C⁻: HC≡CH (pKa ~25)

(IV) CH₃CH₂⁻: CH₃CH₃ (pKa ~50)

Order of acidity of conjugate acids: H₂O > HC≡CH > NH₃ > CH₃CH₃

Order of basicity of bases (decreasing): CH₃CH₂⁻ > NH₂⁻ > HC≡C⁻ > OH⁻

Therefore, the decreasing order of strength of bases is (IV) > (II) > (III) > (I).

The final answer is $A$

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