Which one of the following orders of acid strength is correct?

Which one of the following orders of acid strength is correct?
  1. $\mathrm{RCOOH} > \mathrm{ROH} > \mathrm{HOH} > \mathrm{HC} \equiv \mathrm{HC}$
  2. $\mathrm{RCOOH} > \mathrm{HOH} > \mathrm{ROH} > \mathrm{HC} \equiv \mathrm{CH}$
  3. $\mathrm{RCOOH} > \mathrm{HOH} > \mathrm{HC} \equiv \mathrm{CH} > \mathrm{ROH}$
  4. $\mathrm{RCOOH} > \mathrm{HC} \equiv \mathrm{CH} > \mathrm{HOH} > \mathrm{ROH}$

Solution

Carboxylic acids are most acidic and alcohols are least acidic. Phenols are more acidic than alcohols but less acidic than carboxylic acids or carbonic acid. Thus, acidic strength order is \(\mathrm{R}-\underset{\underset{}{||} \atop \LARGE {\mathrm{O}}}{\mathrm{C}}-\mathrm{OH} > \mathrm{H}-\mathrm{O}-\mathrm{H} > \mathrm{R}-\mathrm{O}-\mathrm{H} > \mathrm{H}-\mathrm{C} \equiv \mathrm{C}-\mathrm{H}\) Related Theory Acid strength increases with : 1. increasing electronegativity of the central atom: The more electronegative the central atom, the more electron drift occurs, polarizing the $\mathrm{O}-\mathrm{H}$ bond which makes it easier for the molecule to dissociate. 2. increasing oxidation number of the central atom: The higher the oxidation state of the central atom, the greater the number of oxygens attached to the central atom. Oxygen is a highly electronegative element, contributing to greater electron drift away from the $\mathrm{O}-\mathrm{H}$ bond.

Asked in: NEET 2003

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