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A short class with Sarah and her students. The other ways are under More.
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I add sodium hydrogencarbonate solution to two test tubes, one with ethanoic acid and one with phenol. What do you expect?
Fizzing in the ethanoic acid tube, because carbon dioxide is released.
And in the phenol tube?
Nothing. Phenol is too weak an acid to release CO2 from a hydrogencarbonate.
Right. Now, why is chloroethanoic acid stronger than ethanoic acid?
Chlorine pulls electrons towards itself and spreads out the negative charge on the carboxylate ion, so the ion is more stable.
Excellent. Remember the rule: whatever stabilises the anion strengthens the acid.
Ethanoic acid is warmed with ethanol and a few drops of concentrated sulphuric acid. What forms?
Ethyl ethanoate and water. You can smell the fruity ester.
Which oxygen in the ester came from the alcohol?
The one in the middle, the –O–. The acid loses its whole –OH group.
Good. To make ethanoyl chloride, why do chemists prefer thionyl chloride to PCl5?
Because SO2 and HCl are gases and bubble away, so the product is easy to purify.
Exactly.
Sodium ethanoate is heated with soda lime. What gas comes off?
Methane. The salt loses CO2, so we lose one carbon.
And if we electrolyse a concentrated solution of sodium ethanoate instead?
Kolbe electrolysis gives ethane at the anode, so two methyl groups join up.
Which reagent turns ethanoic acid into ethanol?
Lithium aluminium hydride. Sodium borohydride is not strong enough.
Right. And chlorine with red phosphorus?
That is the HVZ reaction. It puts a chlorine on the α-carbon, giving chloroethanoic acid.
the carboxylate ion is stabilised by resonance, so acids react with Na, NaOH and NaHCO3.
electron-withdrawing groups raise acidity, electron-releasing groups lower it, and the effect fades with distance.
replacing the –OH group gives esters, acyl chlorides, anhydrides and amides.
LiAlH4 reduces –COOH to –CH2OH; soda lime and Kolbe electrolysis remove it as CO2.
a halogen and red phosphorus substitute the α-hydrogen.
it has an aldehyde-like hydrogen, so it reduces Tollens' reagent.
Ethanoic acid with sodium hydrogencarbonate: CH3COOH + NaHCO3 → CH3COONa + H2O + CO2, with brisk effervescence that phenol does not give.
Making ethyl ethanoate: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O, warmed with a little concentrated sulphuric acid; the product smells fruity.
Methane from sodium ethanoate: CH3COONa + NaOH → CH4 + Na2CO3, heated with soda lime.
Chloroethanoic acid by the HVZ reaction: CH3COOH + Cl2 → ClCH2COOH + HCl, in the presence of red phosphorus.
Soda lime takes one away; Kolbe joins two the electric way.
The –COOH group went to a market. It swapped its –OH for –Cl at the thionyl chloride stall, for –OR at the alcohol stall and for –NH2 at the ammonia stall, but at the LiAlH4 stall it was turned into a plain –CH2OH.
A, C, E, A for swapping the –OH: Anhydride, Chloride (acyl), Ester, Amide.
F.C.B. - Fluorine, Chlorine, Bromine: the order of strength of the halogenoethanoic acids.
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Test that distinguishes a carboxylic acid from a phenol
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Best reagent for making an acyl chloride, and why
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Reagent that reduces RCOOH to RCH2OH
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Products of Kolbe electrolysis of sodium ethanoate
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Which is stronger, ClCH2COOH or CH3COOH?
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The negative logarithm of the acid dissociation constant; the smaller the pKa, the stronger the acid.
The shift of electron density along a chain of σ bonds caused by an electronegative or an electron-releasing group.
The acid-catalysed reaction of a carboxylic acid with an alcohol to give an ester and water.
A compound RCOCl in which the –OH group of an acid has been replaced by –Cl.
The loss of carbon dioxide from a carboxylic acid or its salt, as when a sodium salt is heated with soda lime.
A mixture of sodium hydroxide and calcium oxide used in decarboxylation.
Electrolysis of a concentrated aqueous solution of the sodium or potassium salt of an acid, giving an alkane at the anode.
The Hell–Volhard–Zelinsky reaction, in which a halogen and red phosphorus replace an α-hydrogen of a carboxylic acid by a halogen atom.
Carboxylic acids react as acids, by replacing their –OH group, through their whole –COOH group and at the carbon chain or ring. This lesson covers each type of reaction with equations, and shows how substituents change acid strength.
Carboxylic acids turn blue litmus red, react with metals and alkalis, and release carbon dioxide from carbonates and hydrogencarbonates, because the carboxylate ion they form is stabilised by resonance. Electron-withdrawing groups make them stronger acids and electron-releasing groups make them weaker. Replacing the –OH group gives esters, acyl chlorides, anhydrides and amides; the whole –COOH group can be reduced to –CH2OH or removed as CO2 by soda lime or Kolbe electrolysis; and the α-carbon or the benzene ring can be substituted. Methanoic acid stands apart because it also behaves like an aldehyde.
The carboxyl group has a polar O–H bond, a C–OH bond that can break, a carbonyl group made less electrophilic by resonance, and a carbon chain or ring attached to it. Each of these parts gives a family of reactions: acidity (O–H breaks), substitution of –OH (C–OH breaks), reactions of the whole –COOH group, and substitution in the hydrocarbon part.
Methanoic acid, HCOOH, has a hydrogen atom on its carboxyl carbon, so part of the molecule looks like an aldehyde group. It therefore reduces Tollens' reagent (giving a silver mirror) and Fehling's solution, and it decolourises acidified potassium permanganate, being oxidised to carbon dioxide and water. Warmed with concentrated sulphuric acid, it loses water to give carbon monoxide: HCOOH → CO + H2O.