The Chemistry of Carboxylic Acids and their Derivatives
Comparison of OH group chemistry in alcohols, phenols & carboxylic acids
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phenols and carboxylic acids titrations with alkali for quantitative
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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
Comparison of the hydroxyl group (-OH) in alcohols, phenols & carboxylic
acids - structure & acidity
functional group tests for carboxylic acids and their derivatives
of carboxylic acids
6.15.1 Comparison of the
hydroxyl group (-OH) in alcohols, phenols and carboxylic acids
Comparing their structure and relative
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:
Ka = [R-O-(aq)]
[H3O+(aq)] / [R-OH(aq)]
and often expressed as:
pKa = -log10Ka
The smaller pKa,
the stronger the acid and
Ka = 10-pKa
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
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
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.
(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)
+ 2Na ===> 2CH3CH2O-Na+
(i) on adding sodium ethoxide to
water, ethanol is re-formed by hydrolysis and an alkaline solution.
+ H2O(l) ===> CH3CH2OH(aq)
Therefore no reaction with sodium
hydrogencarbonate would be expected, not strong enough an acid.
The pKa of
ethanol is ~16, Ka = ~ 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
compounds in which the -OH group is directly attached to a benzene ring
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
+ NaOH(aq) ===> C6H5O-Na+(aq)
and the salt can be crystallised
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 pKa of phenol
is 10.0, Ka = 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.
an even more detailed discussion of the acidity of phenols see
in Part 7.5
The physical and chemical
properties of phenols
(c) Carboxylic acids
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
NaOH(aq) ===> CH3COO-Na+(aq)
and the salt can be crystallised
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
NaHCO3(aq) ===> CH3COO-Na+(aq)
+ H2O(l) + CO2(g)
You can test the gas with
limewater, which should give a milky white precipitate.
The pKa of ethanoic
acid is 4.76, Ka = 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
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!)
with blue litmus
Effect of sodium hydroxide (aq)
(c) Effect on sodium hydrogencarbonate
(d) Add sodium
(e) Add to iron(III) chloride
Phenylmethanol - an aliphatic alcohol
No colour change
Phenol - a phenol
Benzoic acid - a carboxylic acid
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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functional group tests for an alcohol, a phenol and carboxylic acids and
CHEMISTRY, deductions and comments
group R–OH chemical test
in alcohols and phenols (in 'dry'
The first 3 tests (i) – (iii)
given on the right are quite general for most alcohols
AND other substances too!
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!).
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.
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.
+ CH3COCl ==> CH3COOR + HCl
An ester and hydrogen chloride are
(ii) R–OH + PCl5 ==> R–Cl +
POCl3 + HCl
a chloro compound and hydrogen chloride
(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
(OH group is attached directly to aromatic ring)
chemical test. R–OH, where R is aryl
a few drops of iron(III) chloride solution to a little of the phenol in
gives a purple colour.
test for primary aromatic amines
– use it in reverse starting with
a known primary aromatic amine!)
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.
acids chemical test
the carboxylic acid with water and add a little sodium hydrogencarbonate solid or solution.
colourless gas gives white precipitate with limewater
+ 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
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
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).
or Acyl Chloride chemical
Fumes in air forming HCl(g)
(i) Add a few drops to water, test with litmus and
silver nitrate solution.
a little ethanol and pour the mixture into water.
Litmus turns red and a white precipitate with silver nitrate.
As above and you may detect a 'pleasant' ester
+ H2O ==> RCOOH + HCl
The acid chloride is hydrolysed to
form HCl acid (chloride ions) and the original carboxylic acid.
+ RCOCl ==> RCOOCH2CH3 + HCl
an ethyl ester and hydrogen chloride are
Amide chemical test
the suspected amide with dilute sodium hydroxide solution,
see in inorganic
evolved on boiling (no heat required to form ammonia, if it was an ammonium salt)
+ NaOH ==> RCOONa + NH3
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'.
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)
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)
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
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