Water, alcohols and phenol all have an O–H bond that can lose H+, but they do it to very different extents. The stability of the anion left behind explains the order, and acyl chlorides give a fast, irreversible route from alcohols to esters.
An acid HA is strong when its anion A− is stable, so the order of acidity follows the stability of the anion. In an alkoxide ion, RO−, the alkyl group is electron-donating: it pushes extra electron density onto the already negative oxygen, which destabilises the ion. So alcohols (pKa about 16) are slightly weaker acids than water (pKa 15.7), and much weaker than phenol (pKa 10.0), whose charge is delocalised into the ring. The order is ethanol < water < phenol < ethanoic acid. Alcohols react with acyl chlorides at room temperature to give esters and HCl: unlike esterification with a carboxylic acid, this needs no catalyst and is not reversible.
Add sodium to water and to ethanol in the simulator and compare how fast hydrogen is given off. Then use the table to explain the difference in terms of the anion formed.
| Compound | pKa | Ka / mol dm−3 | Anion | Why the anion is (un)stable |
|---|---|---|---|---|
| ethanol | about 16 | about 1 × 10−16 | C2H5O− | ethyl group donates electrons onto O−: least stable |
| water | 15.7 | 2.0 × 10−16 | OH− | charge on one O, no alkyl push |
| phenol | 10.0 | 1.00 × 10−10 | C6H5O− | charge delocalised into the ring |
| ethanoic acid | 4.76 | 1.74 × 10−5 | CH3COO− | charge shared equally over two O atoms |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which list puts the compounds in order of increasing acid strength?
Why is ethanol a weaker acid than water?
What are the products when ethanoyl chloride reacts with ethanol?
Which condition is needed for an acyl chloride to react with an alcohol?
Make an ester in the simulator both ways, from an acid and from an acyl chloride, then compare the conditions in the table.
| Route | Equation | Conditions | Outcome |
|---|---|---|---|
| carboxylic acid + alcohol | CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O | conc. H2SO4 catalyst, heat | slow, reversible, incomplete |
| acyl chloride + alcohol | CH3COCl + C2H5OH → CH3COOC2H5 + HCl | room temperature, no catalyst | fast, goes to completion, steamy HCl fumes |
| acyl chloride + secondary alcohol | CH3COCl + CH3CH(OH)CH3 → CH3COOCH(CH3)2 + HCl | room temperature | 1-methylethyl ethanoate |
Alcohols are weaker acids than water, not stronger: the alkyl group is electron-donating and destabilises the alkoxide ion. Do not say the alkyl group ‘strengthens the O–H bond’ as the whole explanation; the mark is for the anion stability. With an acyl chloride the by-product is HCl, not water, and the reaction is not reversible and needs no acid catalyst.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
The pKa of phenol is 10.0 and that of ethanol is about 16. Approximately how many times larger is Ka for phenol than for ethanol?
Why is reacting ethanol with ethanoyl chloride preferred to reacting it with ethanoic acid when a high yield of ethyl ethanoate is needed?
Sodium ethoxide is dissolved in water. Which statement is correct?
Which ester forms when propan-1-ol reacts with benzoyl chloride, C6H5COCl?
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