Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

Comparison of OH group chemistry in alcohols, phenols & carboxylic acids

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry comparing acidity of alcohols, phenols and carboxylic acids titrations with alkali for quantitative analysis

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6.15 Qualitative tests for carboxylic acids and derivatives and a comparison of the acidity of alcohols, phenols and carboxylic acids, quantitative analysis of carboxylic acids

Sub-index for this page

6.15.1 Comparison of the hydroxyl group (-OH) in alcohols, phenols & carboxylic acids - structure & acidity

6.15.2 Qualitative organic functional group tests for carboxylic acids and their derivatives

6.15.3 Quantitative analysis of carboxylic acids

6.15.1 Comparison of the hydroxyl group (-OH) in alcohols, phenols and carboxylic acids

Comparing their structure and relative acidity

Introduction

Theoretically any organic compound with an -OH group can behave like an acid in water.

R-OH(aq)  +  H₂O(l)    R-O^-(aq)  +  H₃O⁺(aq)

(Here R is anything organic - alkyl, aryl or acyl R'-C=O)

The acid dissociation constant can be expressed as:

K_a = [R-O^-(aq)] [H₃O⁺(aq)] / [R-OH(aq)]  moldm^-3

and often expressed as: pK_a = -log₁₀K_a

The smaller pK_a, the stronger the acid and K_a = 10^-pKa moldm^-3

However, the nature of R greatly influences the acidic character so that you get a sequence of decreasing acidity of:

carboxylic acids > aromatic phenols > aliphatic alcohols

(a sequence of decreasing Ka values)

The differences in relative acid strength will now be described, discussed and explained, though should be born in mind that general equilibrium shown above is very much to the left in most case.

The most important point is the relative stability of the anion, since the hydrated proton is common to all the equilibria discussed.

If another atom or atoms can share the negative charge of the RO^- anion, the anion becomes more stable - it is a general rule in chemistry that 'delocalising' the charge lowers the potential energy of the system.

 

(a) Alcohols (primary, secondary or tertiary - all 'count' the same here!)

Aliphatic compounds in which the -OH group is attached directly to an alkyl group e.g. ethanol.

Alcohols have virtually no acidic character when dissolved in water.

Aqueous solutions of alcohols do NOT form salts with strong bases like sodium hydroxide.

In fact, you can make salts of alcohols by reacting directly with a metal, but on adding the salt to water, it is rapidly hydrolysed to the parent alcohol e.g.

(i) sodium dissolving in ethanol to form the salt sodium ethoxide (an alkoxide salt)

2CH₃CH₂OH  +  2Na  ===>  2CH₃CH₂O^-Na⁺  +  H₂

(i) on adding sodium ethoxide to water, ethanol is re-formed by hydrolysis and an alkaline solution.

CH₃CH₂O^-Na⁺(aq)  +  H₂O(l)   ===>  CH₃CH₂OH(aq)  +  NaOH(aq)

Therefore no reaction with sodium hydrogencarbonate would be expected, not strong enough an acid.

The pK_a of ethanol is ~16, K_a = ~ 1 x 10^-16 mol dm^-3

There is nothing in the structure of aliphatic alcohols (C-OH) that can stabilise the alkoxide anion.

For more see Part 4 Chemistry of Alcohols

 

(b) Phenols

Aromatic compounds in which the -OH group is directly attached to a benzene ring e.g. 2-methylphenol.

Phenols are very weakly acid when dissolved in water.

Aqueous solutions of phenols are neutralised by strong bases to give salts.

e.g. phenol forms the salt sodium phenoxide

C₆H₅OH(aq)  +  NaOH(aq)  ===>  C₆H₅O^-Na⁺(aq)  +  H₂O(l)

and the salt can be crystallised from solution.

There is usually no reaction with sodium hydrogencarbonate, although more strongly acidic than alcohols, phenols are still weaker than 'carbonic acid' and will not liberate carbon dioxide from carbonate or hydrogencarbonate ions.

The pK_a of phenol is 10.0, K_a = 1.0 x 10^-10 mol dm^-3

A million times more strongly acidic than ethanol.

The considerable increase in strength, albeit phenols are still very weak acids, is due to the stabilisation of the 'phenoxide' anion by the negative charge being delocalised to some extent by the pi orbitals benzene (aromatic ring) - illustrated below.

For an even more detailed discussion of the acidity of phenols see section 7.9.5

in Part 7.5 The physical and chemical properties of phenols

 

(c) Carboxylic acids

Here the -OH group is attached to a carbon atom which is also part of a carbonyl group (C=O) - being double bonded to oxygen e.g. ethanoic acid.

Carboxylic acids are weak acids when dissolved in water.

Aqueous solutions of carboxylic acids are neutralised by strong/weak bases to give salts.

e.g. ethanoic acid forms the salt sodium ethanoate

CH₃COOH(aq)  +  NaOH(aq)  ===> CH₃COO^-Na⁺(aq)  +  H₂O(l)

and the salt can be crystallised from solution.

There is usually a reaction with sodium hydrogencarbonate, because most carboxylic acids are stronger than 'carbonic acid' and will liberate carbon dioxide from carbonate or hydrogencarbonate ions.

CH₃COOH(aq)  +  NaHCO₃(aq)  ===> CH₃COO^-Na⁺(aq)  +  H₂O(l)  +  CO₂(g)

You can test the gas with limewater, which should give a milky white precipitate.

The pK_a of ethanoic acid is 4.76, K_a = 1.74 x 10^-5 mol dm^-3

Over a hundred thousand times more strongly acidic than phenol.

In the carboxylate anion, the negative charge is delocalise across the O-C-O bond system and seems to have a greater effect than delocalisation in a benzene ring

For more details see section 6.4 The weakly acidic nature and general reactions of carboxylic acids acting as acids - carboxylate salts

A summary of the reactions of alcohols, phenols and carboxylic acids with various reagents

(using a deliberate comparison with three molecules containing a benzene ring!)

Compound	Structure	(a) Test with blue litmus	(b) Effect of sodium hydroxide (aq)	(c) Effect on sodium hydrogencarbonate	(d) Add sodium	(e) Add to iron(III) chloride
Phenylmethanol - an aliphatic alcohol		No change	No reaction	No reaction	Effervescence H2	No colour change
Phenol - a phenol		Turns faintly pink	Will form a salt	No reaction	Effervescence H2	Violet colour
Benzoic acid - a carboxylic acid		Turns pink	Will form a salt	Effervescence CO2	Effervescence H2	No colour change

More detailed notes on the use of these reactions in performing qualitative tests is given in section 6.15.2

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6.15.2 Qualitative organic functional group tests for an alcohol, a phenol and carboxylic acids and their derivatives
CHEMICAL TEST FOR	TEST METHOD	OBSERVATIONS	TEST CHEMISTRY, deductions and comments
Hydroxy group R–OH chemical test in alcohols and phenols (in 'dry' conditions*)   The first 3 tests (i) – (iii) given on the right are quite general for most alcohols AND other substances too!	(i) Mix it with a few drops of ethanoyl chloride, test fumes with litmus and silver nitrate (* note ethanoyl chloride reacts with water, phenols and amines too!). (ii) Mix it with a little phosphorus(V) chloride and test as above. (iii) Warm with a little ethanoic acid and a few drops of conc. sulphuric acid. Pour into water and stir gently.	(i) Fumes turn litmus red and a white precipitate with silver nitrate(aq) (drop on end of glass rod), ammonia fumes give a white cloud of ammonium chloride, if the mixture is poured into water you may detect a 'pleasant' ester odour, can test for HCl but water and amines produce it too! (ii) as for (i) but no ester smell! (iii) You should get a 'pleasant' characteristic smell of an ester on the surface.	(i) R–OH + CH3COCl ==> CH3COOR + HCl An ester and hydrogen chloride are formed (ii) R–OH + PCl5 ==> R–Cl + POCl3 + HCl a chloro compound and hydrogen chloride are formed. (i) and (ii) Ag+(aq) + Cl–(aq) ==> AgCl(s) from the hydrogen chloride fumes dissolved in water. (iii) CH3COOH + ROH ==> CH3COOR + H2O Note: Alcohols are distinguished from carboxylic acids (pH <7) by being neutral pH 7 - alcohols do not change blue litmus red, or create effervescence with a pinch of sodium hydrogencarbonate.
Phenols (OH group is attached directly to aromatic ring) chemical test. R–OH, where R is aryl e.g. C6H5OH	Add a few drops of iron(III) chloride solution to a little of the phenol in water.	Usually gives a purple colour.	(see also test for primary aromatic amines – use it in reverse starting with a known primary aromatic amine!) Note: Phenols are also distinguished from carboxylic acids (pH <7) by not producing effervescence with a pinch of sodium hydrogencarbonate, BUT the pH of an aqueous solution of phenols is <7 and blue litmus will turn a faint pink - so take care.
Carboxylic acids  chemical test RCOOH	Mix the carboxylic acid with water and add a little sodium hydrogencarbonate solid or solution.	fizzing, colourless gas gives white precipitate with limewater	RCOOH + NaHCO3 ==> RCOONa + H2O + CO2 (see also salts of aliphatic carboxylic acids below)
Salts of aliphatic carboxylic acids e.g. RCOO–Na+ or (RCOO–)2Mg etc.	Add a little dilute hydrochloric/sulfuric acid to a suspected salt of an aliphatic carboxylic acid.	The solid or solution should have no strong odour, but after adding the mineral acid you should get a pungent odour of the original aliphatic acid. If its the salt of an aromatic carboxylic acid, you get little odour and maybe a white crystalline precipitate.	The stronger acid, HCl/H2SO4 displaces the weaker aliphatic carboxylic acid which have strong–pungent characteristic odours e.g. ethanoic acid from an ethanoate salt (smell of acetic acid, vinegar) and butanoates release butanoic acid (butyric acid, rancid odour).
Acid or Acyl Chloride chemical test RCOCl Fumes in air forming HCl(g)	(i) Add a few drops to water, test with litmus and silver nitrate solution. (ii) Add to a little ethanol and pour the mixture into water.	(i) Litmus turns red and a white precipitate with silver nitrate. (ii) As above and you may detect a 'pleasant' ester odour.	(i) RCOCl + H2O ==> RCOOH + HCl The acid chloride is hydrolysed to form HCl acid (chloride ions) and the original carboxylic acid. (ii) CH3CH2OH + RCOCl ==> RCOOCH2CH3 + HCl an ethyl ester and hydrogen chloride are formed
Acid Amide  chemical test RCONH2	Boil the suspected amide with dilute sodium hydroxide solution, see in inorganic for ammonia tests.	ammonia evolved on boiling (no heat required to form ammonia, if it was an ammonium salt)	RCONH2 + NaOH ==> RCOONa + NH3
Esters chemical test RCOOR' R = H, alkyl or aryl R' = alkyl or aryl There is no simple test for an ester. Usually a colourless liquid with a pleasant 'odour'.	The ester can be reacted with saturated ethanolic hydroxylamine hydrochloride + 20% methanolic KOH and gently heated until boiling. Then mixture acidified with 1M HCl(aq) and FeCl3(aq) added dropwise.	Deep red or purple colour formed. The test depends on the formation of a hydroxamic acid R–C(=NOH)OH which forms coloured salts with Fe3+(aq) ion.	The reaction is also given by acid chlorides and acid anhydrides, and phenols give a purple colour with iron(III) chloride, so frankly, the test is not that good. This test is not likely to be expected in your exams.

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