Question

Rank the following in decreasing order of basic strength:
(i) $C{H}_3-C{H}_2-C\equiv C^{-}$
(ii) $C{H}_3-C{H}_2-S^{-}$
(iii) ${C{H}_3-C{H}_2-C{O}_2}^{-}$
(iv) $C{H}_3-C{H}_2-O^{-}$.

• A. $\left(iv\right)>\left(i\right)>\left(ii\right)>\left(iii\right)$
• B. $\left(i\right)>\left(iv\right)>\left(ii\right)>\left(iii\right)$
• C. $\left(i\right)>\left(iv\right)>\left(iii\right)>\left(ii\right)$
• D. $\left(ii\right)>\left(i\right)>\left(iv\right)>\left(iii\right)$

Answer 1

The correct option is B $\left(i\right)>\left(iv\right)>\left(ii\right)>\left(iii\right)$

The gradation of acidic strength of organic compounds:

Acid $>SH$(Thiol) $>OH$(alcohol) $>$ Alkyne

Here OH has a smaller bond length than SH so it's a weaker acid.

In alkyne, C is less electronegative than O so less acidic.

Hence, by Bronsted theory, strong acids have weak conjugate bases.

So Basicity order is:

Alkyne > Alcohol > Thiol > Acid

(i) > (iv) > (ii) > (iii)

Option: B