# What is the order of acidic strength among phenol, catechol, resorcinol and hydroquinone?

This question is from a book that gives the answer as

$$\text{hydroquinone} < \text{resorcinol} < \text{phenol} < \text{catechol}$$

but does not provide any explanation for the answer. If I go by the stability of the conjugate base due to resonance, phenol seems to have the most number of resonance intermediates and seems to be most stable. Considerations about number of resonance intermediates of conjugate base gives an order that is nowhere close to that provided as an answers.

A quick search on says that resorcinol is more acidic that catechol, followed by phenol.

Hydroquinone - strong +M effect and weak -I effect

Phenol - the normal resonance you'd expect in all phenols

Resorcinol - -I effect

Catechol - +M effect, stronger -I effect and hydrogen bonding

Taking these factors into account, it is easy to see why hydroquinone is least acidic. Hydrogen bonding and the vicinity of the $\ce{-OH}$ to the anion (for the stronger -I effect) make cactechol more aciic than the phenol despite its +M contribution. Resorcinol has only -I effect and no counteracting electron-donating effects, which makes it the most acidic.

Catechol is less acidic than resorcinol and quinol because in case of Catechol present hydrogen bonding and do not give easily H+ ions.. In case of resorcinol - I work but in case of Quinol work +M and - I but +M is dominant hence the acidity order of. resorcinol) Quinol) Catechol.