Acidity Orders, "Why" Questions & Reasoning

CHEExam Practice & Reasoning

This subtopic gathers the reasoning-heavy questions — ranking acidity across alcohols, phenol, water and carboxylic acids, and explaining reactivity using resonance and inductive effects. The marks come from a clear, correct "why", not just the order.

Overall acidity order

CH_3COOH > C_6H_5OH > H_2O > C_2H_5OH

Acidity of alcohols

1^\circ > 2^\circ > 3^\circ

alkyl

+I

destabilises the alkoxide

RO^-

Phenol ionisation

C_6H_5OH \rightleftharpoons C_6H_5O^- + H^+ \quad (\text{resonance-stabilised anion})

Acidity order: $CH_3COOH > C_6H_5OH > H_2O > C_2H_5OH$ ; the carboxylate is most resonance-stabilised, phenoxide is ring-stabilised, and the alkyl $+I$ effect makes ethanol the weakest.
Phenol is more acidic than alcohols and water because $C_6H_5O^-$ delocalises its charge into the ring; it is less acidic than $CH_3COOH$ because carboxylate spreads charge over two equivalent oxygens.
Phenol reacts with NaOH (giving phenoxide) but not with $NaHCO_3$ , proving it is a weaker acid than carbonic acid — a direct comparison of acid strengths.
Phenol is far more reactive than benzene toward electrophilic substitution because $\text{-}OH$ donates electrons by resonance, activating the ring and directing $o,p\text{-}$ .
Yet phenol is less reactive than free phenoxide ion: in neutral phenol the oxygen lone pair is partly tied up, whereas $C_6H_5O^-$ is even more electron-rich and activated.
Hydroboration–oxidation is preferred over acid hydration to make propan-1-ol from propene because it is anti-Markovnikov and avoids carbocation rearrangements that hydration would cause.
Acidity of alcohols themselves is $1^\circ > 2^\circ > 3^\circ$ , since more alkyl groups ( $+I$ ) destabilise the alkoxide by intensifying its negative charge.
The 'why' for phenol > ethanol: in phenoxide the negative charge is delocalised onto ortho/para carbons (several resonance structures), so the ion is stabilised and the equilibrium of ionisation lies further right.
Substituent effect on phenol acidity: $\text{-}NO_2$ (electron-withdrawing) at o-/p- increases acidity (p-nitrophenol > phenol); $\text{-}CH_3$ / $\text{-}OCH_3$ (electron-donating) decrease it (cresol < phenol).
Carboxylic acid beats phenol because in carboxylate both oxygens are equivalent and bear equal charge (strong, symmetric resonance), whereas in phenoxide most charge stays on oxygen with only partial ring delocalisation.
Water is more acidic than ethanol because the $+I$ effect of the ethyl group makes ethoxide ( $C_2H_5O^-$ ) less stable than hydroxide; hence alcohols need Na metal, not aqueous NaOH, to form alkoxides.
Electron-withdrawing substituents also speed up acid-type behaviour and slow electrophilic substitution; electron-donating ones do the reverse — the same resonance/induction logic answers most 'why' questions in this chapter.

Where the marks go

Ranking phenol above carboxylic acids — carboxylic acids are stronger (carboxylate has two equivalent oxygens sharing the charge); phenol comes below them.
Saying ethanol is more acidic than water — it is LESS acidic; the alkyl $+I$ effect destabilises ethoxide relative to hydroxide.
Explaining phenol's ring reactivity and its acidity with the same direction of electron flow — for acidity the ring pulls charge OFF the anion (stabilising it), but as a neutral molecule the $\text{-}OH$ pushes electrons INTO the ring (activating it); keep the two arguments distinct.
Forgetting to mention resonance when justifying an order — stating only the order without the resonance/inductive reason earns no reasoning marks.
Treating all positions of a substituent alike — only o-/p- electron-withdrawing groups strongly raise phenol acidity by resonance; a meta group acts mainly through weaker induction.

How the board asks it

Give reasonsthe overall acidity order $CH_3COOH > C_6H_5OH > H_2O > C_2H_5OH$
Account for the fact that phenol is a stronger acid than ethanol but a weaker acid than acetic acid. Support your answer with resonance structures of the conjugate bases.
Give reasons $+I$ effect destabilising ethoxide relative to hydroxide
Give reasons: Water is a stronger acid than ethanol.
Give reasonssubstituent effect on phenol acidity
Arrange the following in increasing order of acid strength and justify your answer: phenol, $p$ -nitrophenol and $p$ -cresol.
Distinguishphenol reacts with $NaOH$ but not with $NaHCO_3$
How will you distinguish chemically between phenol and ethanol? State the reagent used and the observation in each case.
Give reasonsalkoxide stability and $1^\circ > 2^\circ > 3^\circ$ alcohol acidity
Why is ethanol a weaker acid than water, and why does the acidity of alcohols decrease in the order $1^\circ > 2^\circ > 3^\circ$ ? Explain using the inductive effect.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.