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Advanced A/AS Level Organic Chemistry: Acidic nature and carboxylic acid reactions

Part 6. The Chemistry of  Carboxylic Acids and their Derivatives

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

Part 6.4 The weakly acidic nature and general reactions of carboxylic acids acting as acids

Sub-index for this page

6.4.1 Carboxylic acids are weak acids - ionisation and the structure of the carboxylate ion

6.4.2 Structure of carboxylic acids and the ionisation constant Ka (dissociation constant)

6.4.3 Observations - comparing the reactivity of weak carboxylic acids and strong mineral acids

6.4.4 Reactions of carboxylic acids with metals

6.4.5 Reactions of carboxylic acids with oxides & hydroxides (soluble/insoluble bases)

6.4.6 Reactions of carboxylic acids with hydrogencarbonates and carbonates

6.4.7 Reaction of carboxylic acids with ammonia

INDEX of all carboxylic acids and derivatives notes

All Advanced A Level Organic Chemistry Notes

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6.4.1 Carboxylic acids are weak acids - ionisation and the structure of the carboxylate ion

The general structure of a carboxylic acid and the corresponding carboxylate ion.

general structure carboxylic acid corresponding carboxylate ion resonance hybrids delocalised O-C-O bond system advanced organic chemistry

The displayed general formula for a monocarboxylic acid. R = H, alkyl (e.g. CH3CH2) or aryl (e.g. C6H5).

The 'traditional' displayed formula for the carboxylate anion (this negative ion is the conjugate base of the acid).

and are known as resonance hybrids.

This is a way of describing bonding in structures that can be better described by looking at multiple possible contributing structures - imagine an oscillation between them (blue arrows show the theoretical electron shifts).

The result is that the theoretical pi bond pair of the C=O group is actually a delocalised system across the two C-O bonds.

At a higher level you can think of it as sp2 hybridisation for the trigonal planar -C< bond arrangement and the delocalised '0.5' bond of the O-C-O formed from the overlap of the pz atomic orbitals.

and The true structure of the carboxylate ion in which both C-O bonds are equal and both equivalent to a bond order of 1.5.

The R-C-O and O-C-O bond angles are 120o - a trigonal planar sigma bond arrangement, plus the delocalised pi bond system in which two electrons reside in the pi orbitals above and below the plane of the O-C-O bonds.

Explanation: bond order = the equivalent pairs of bonding electrons in a covalent bond

e.g. C-H, O-H and C-O have a bond order of 1, C=C and C=O have a bond order of 2, but here it is 1.5.

The result of the possible 'resonance hybrids' is that the theoretical pi bond pair of electrons from the C=O group actually form a delocalised system across the two C-O bonds.

This means both of the C-O bonds is identical and equivalent to 0.5 of a pi bond plus 1.0 of a sigma bond, hence the bonder order of 1.5.

and This shows the ionisation of a carboxylic acid and the usual 'shorthand' formula in common use.

 

Expressing the ionisation, both chemically and mathematically

In aqueous solution, carboxylic acids are weak acids, that is only ionised or dissociated to a small extent.

RCOOH(aq)  +  H2O(l)    H3O+(aq)  +  RCOO-(aq)

RCOOH(aq)    H+(aq)  +  RCOO-(aq)   (useful when doing pH and Ka calculations)

The ionisation constant (dissociation constant, acidity constant), that is the equilibrium constant (Ka) for the above equilibrium, is given by the expression:

Ka =

  [H+(aq)] [RCOO(aq)]

 ––––––––––––––––––  mol dm-3

       [RCOOH(aq)]

Like pH, the range of Ka is so great, it is often quoted on the logarithmic scale where

 pKa = -log10(Ka/mol dm-3)

 

The stronger the acid, the greater the Ka and the lower the pKa values.

In the data table next in section 6.4.2 I've quoted both values.

See equilibrium calculations section 5c for calculations involving pH, Ka and pKa

 

Dissociation of dicarboxylic and tricarboxylic acids in aqueous solution

The ionization of a dicarboxylic acid requires two equations e.g. in 'shorthand' for propanedioic acid

(i)  HOOC-CH2-COOH(aq)  HOOC-CH2-COO-(aq)  +  H+(aq)

(ii)  HOOC-CH2-COO-(aq)  -OOC-CH2-COO-(aq)  +  H+(aq)

For a tricarboxylic acid like citric acid, there will be three ionisation equations.

HOOCCH2C(OH)(COOH)CH2COOH(aq)  ===> HOOCCH2C(OH)(COO-)CH2COOH(aq)  +  H+(aq)

HOOCCH2C(OH)(COO-)CH2COOH(aq)  ===> HOOCCH2C(OH)(COO-)CH2COO-(aq)  +  H+(aq)

HOOCCH2C(OH)(COO-)CH2COO-(aq)  ===> -OOCCH2C(OH)(COO-)CH2COO-(aq)  +  H+(aq)

These acids are referred to as dibasic or tribasic acids and also diprotic or triprotic acids.

 

Why are carboxylic acids stronger acids than alcohols?

CH3COOH pKa = 4.76 (weak acid)  and  CH3CH2OH pKa ~16 (extremely weak acid).

diagram of carboxylic acid what influences the strength of the acid electron shifts minus inductive -I effect plus inductive +I effects advanced organic chemistry

The are two principal reasons

(i) The carbonyl C=O group pulls the electron clouds away from the O-H group making the hydrogen atom more δ+, making the H+ ion easier to remove by a lone pair donating base e.g. water :OH2.

Look at the electron shift above towards the more electronegative oxygen δ+C=Oδ (the top part of the diagram).

(ii) The delocalisation of the negative charge between the two oxygen atoms of the carboxylate ion, lowers the potential energy of the system, giving extra stability to the anion (diagram above).

You can think of each oxygen as carrying half a single negative charge.

The negative charge on the anion from an alcohol, an alkoxide ion e.g. CH3CH2O- from CH3CH2OH, cannot be delocalised, so no extra stability.

The alkoxide ion is a stronger conjugate base than the carboxylate ion.

In fact in water, e.g. the ethoxide ion, rapidly hydrolyses yielding the original alcohol, ethanol, and the hydroxide ion, so forming an alkaline solution.

CH3CH2O-(aq)  +  H2O(l)  ===>  CH3CH2OH(aq)  +  OH-(aq)

 


6.4.2 Structure of carboxylic acids and the value of the ionisation constant Ka (dissociation constant)

Although they are generally weak acids, the range of ionisation is quite wide and greatly affected by substituent groups in the 'alky' or 'aryl' hydrocarbon part of the molecule, so, like with pH, a logarithmic (base 10) scale is used ...

... for examples see the relative strengths of carboxylic acids in the data table below.

diagram of carboxylic acid what influences the strength of the acid electron shifts minus inductive -I effect plus inductive +I effects advanced organic chemistry

If the R group contains electronegative, electron cloud withdrawing atoms, the electron shift affects the δ-O-Hδ+ bond (-I induction effect) and promotes the donation of a proton to a base (left of bottom diagram).

On the other hand, other groups like alkyl groups, can have a +I inductive electron shift effect and make the proton less available to a base (right of bottom diagram).

 

Reminders of mathematical connections:

Ka = [H+(aq)] [RCOO-(aq)] / [RCOOH(aq)]    (R = H, alkyl, aryl, the latter may be substituted with other atoms)

pKa = - log10(Ka/moldm-3)   and  Ka = 10-pKa

Like the pH scale, a base 10 logarithmic scale means each unit of pH is equal to a factor of 10 in the hydrogen ion concentration [H+(aq)] and each unit of pKa represents a factor of 10 in the value of the ionization constant (dissociation constant) in aqueous solution.

Note: For dicarboxylic acids or tricarboxylic acids, the ionisation decreases from the 1st, 2nd ionisation etc.

so that:   Ka1 > Ka2  etc.  and  pKa1 < pKa2 etc.

Linear monocarboxylic acids Abbreviated formula pKa Ka/mol dm-3 Comments
Methanoic acid HCOOH 3.75 1.78 x 10-4  
Ethanoic acid CH3COOH 4.76 1.74 x 10-5 Vinegar has a pH of ~3
Propanoic acid CH3CH2COOH 4.87 1.35 x 10-5 After ethanoic, most have a similar pKa
Butanoic acid CH3(CH2)2COOH 4.82 1.51 x 10-5  
Pentanoic acid CH3(CH2)3COOH 4.86 1.38 x 10-5  
Chloroethanoic acid ClCH2COOH 2.86 1.38 x 10-3 Three substituted ethanoic acids - these show the increasing effect of one to three electronegative atoms on the extent of dissociation into ions. You get similar effects with F, Br, I, OH and NO2 substituent groups.
Dichloroethanoic acid Cl2CHCOOH 1.29 5.13 x 10-2
Trichloroethanoic acid Cl3CCOOH 0.65 2.24 x 10-1
         
Dicarboxylic aliphatic  acids Abbreviated formula pKa Ka/mol dm-3 Comments
Ethanedioic acid HOOCCOOH pKa1 = 1.23

pKa2 = 4.28

Ka1 = 5.89 x 10-2

Ka2 = 5.25 x 10-5

Two ionisations possible to yield hydrated protons H3O+. Note the order  Ka1 > Ka2  and  pKa1 < pKa2 for dibasic acids.
Propanedioic acid HOOCCH2COOH pKa1 = 2.83

pKa2 = 5.69

Ka1 = 1.48 x 10-3

Ka2 = 2.04 x 10-6

 
Butanedioic acid HOOCCH2CH2COOH Ka1 = 4.22

Ka2 = 5.64

Ka1 = 6.03 x 10-5

Ka2 = 2.29 x 10-6

 
         
Aromatic acids Abbreviated formula pKa Ka/mol dm-3 Comments
Benzoic acid C6H5COOH 4.20 6.31 x 10-5  
2-chlorobenzoic acid ClC6H4COOH 2.94 1.48 x 10-3 The presence of the electronegative chlorine atom, increases the strength of the acid compared to benzene.
3-chlorobenzoic acid ClC6H4COOH 3.83 1.48 x 10-4
4-chlorobenzoic acid ClC6H4COOH 3.99 1.02 x 10-4
2-nitrobenzoic acid O2NC6H4COOH 2.17 6.76 x 10-3 The presence of the electronegative nitro group -NO2, increases the strength of the acid compared to benzene, more so than the a single chlorine atom.
3-nitrobenzoic acid O2NC6H4COOH 3.45 3.55 x 10-4
4-nitrobenzoic acid O2NC6H4COOH 3.43 3.72 x 10-4
benzene-1,2-dicarboxylic acid C6H4(COOH)2 pKa1 = 2.98

pKa2 = 5.41

Ka1 = 1.05 x 10-3

Ka2 = 3.89 x 10-6

Two ionisations possible to yield hydrated protons H3O+. Note the order  Ka1 > Ka2  and  pKa1 < pKa2 for dibasic acids.
benzene-1,3-dicarboxylic acid C6H4(COOH)2 pKa1 = 3.46

pKa2 = 4.60

Ka1 = 3.47 x 10-4

Ka2 = 2.51 x 10-5

 
benzene-1,4-dicarboxylic acid C6H4(COOH)2 pKa1 = 3.51

pKa2 = 4.82

Ka1 = 3.09 x 10-4

Ka2 = 1.51 x 10-5

 
         

Structure notes: 2-chlorobenzoic acid structural formula  3-chlorobenzoic acid structural formula  4-chlorobenzoic acid structural formula 2-, 3- and 4-chlorobenzoic acid  

2-nitrobenzoic acid structural formula advanced A level organic chemistry  3-nitrobenzoic acid structural formula advanced A level organic chemistry  4-nitrobenzoic acid structural formula advanced A level organic chemistry  2-, 3- and 4-nitrobenzoic acid  

(c) doc b (c) doc b (c) doc b structures of the aromatic dicarboxylic acids


6.4.3 Observations - comparing the reactivity of weak carboxylic acids and strong mineral acids

Carboxylic acids are weak acids

Typically weak acid solutions have a pH of around 2 to 6 (yellow–orange–pink with universal indicator), which is somewhat higher than strong acid solutions with a pH of 0 to 1 (always <2).

They are called weak acids because only a few % of the molecules in aqueous ionise to release protons (hydrogen ions, H+).

It is the generation of hydrogen ions that makes aqueous solutions of carboxylic acids acidic.

e.g. for ethanoic acid (vinegar) around 98% remains unionised i.e. as the original neutral molecule and only ~2% form ethanoate ions and hydrogen ions..

CH3COOH(aq) (c) doc b CH3COO(aq) + H+(aq)

Propanoic acid and butanoic acid are equally weak carboxylic acids.

CH3CH2COOH(aq) (c) doc b CH3CH2COO(aq) + H+(aq)

CH3CH2CH2COOH(aq) (c) doc b CH3CH2CH2COO(aq) + H+(aq)

This is a reversible reaction with only 2% of the weak acid ionised on the right–hand side of the equilibrium.

At similar aqueous solution concentrations, strong acids like hydrochloric acid and sulfuric acid have a low pH of 0 or 1, because they are fully ionised (for H2SO4 this only applies to the 1st ionisation).

So when you dissolve gaseous hydrogen chloride or liquid sulfuric acid in water, the ionisation is ~100% e.g.

HCl(g)  ===> H+(aq)  +  Cl-(aq)    and   H2SO4(l)  ===> H+(aq)  +  HSO4-(aq)

 

Even so, weak acids like ethanoic acid will turn blue litmus pink and universal indicator gives a red colour  in aqueous solution and usually liberate carbon dioxide from carbonates (and CO2 test) - simple tests for an acidic substance.

Carboxylic acids react with (i) metals, (ii) oxides and hydroxides, (iii) hydrogencarbonates and carbonates and (iv) ammonia.

(see sections 6.4.4, 6.4.5, 6.4.6 and 6.4.7).

 

Comparing the pH and rates of reaction of weak and strong acids of equal molarity

e.g. 1.0 mol dm-3 solutions of ethanoic acid and hydrochloric acid.

For equimolar solutions, the solution of the strong acid will have a much great concentration of hydrogen ions, so its pH will be much lower - use an accurately buffer calibrated pH meter

 

The pH of solutions of equal concentration e.g. of molarity 1.0 mol/dm3

The pH of a strong acid might be pH 0-1 (hydrochloric, sulfuric or nitric acids).

The pH of a weak acid might be typically pH 3-6 (vinegar ~pH 3, carbonic acid pH 4-5).

 

The rate of reaction with metals.  (1 molar means 1.0 mol/dm3, sometimes written as 1M for shorthand))

If you put magnesium ribbon into 1 molar solutions of hydrochloric acid (strong, high % ionisation so high H+(aq) concentration) and 1 molar solution of ethanoic acid (weak, low percentage ionization so much lower H+(aq) concentration), you can see the difference in the fast and slow 'fizzing' rates!

You can repeat the experiment using calcium carbonate (limestone granules) instead of magnesium ribbon and collect carbon dioxide gas.

You can do simple rate of reaction experiments comparing how fast the gas is evolved from the reaction mixture.

The above links takes you to a page where the experimental procedures are described, little point in repeating them here.

Factors affecting the rates of Reaction - theory and methods of measuring the speed of a reaction (c) Doc Brown

The basic experimental procedure is shown in the diagram above and a graph of typical results below.

The above graph shows the sort of results you might expect by adding the same masses of magnesium ribbon or calcium carbonate granules to the same volume of ethanoic acid, CH3COOH, or hydrochloric acid, HCl, of equal concentration e.g. both acids with a concentration of 1.0 mol/dm3.

Remember that its the hydrogen ion, the H+(aq) ion (H3O+), is the active chemical species in acid solutions NOT a 'HCl' or a 'H2SO4' or a 'CH3COOH' molecule.

 

Electrolysis observations

Since stronger/weak acid solutions (or alkalis) contain more/less hydrogen ions, they are better/poorer conductors of electricity.

e.g. If you carry out electrolysis experiments with the same molarity solutions of hydrochloric acid and ethanoic acid, you get a greater rate of hydrogen collected at the negative cathode from the hydrochloric acid compared to the ethanoic acid.

2H+(aq)  +  2e-  ===> H2(g)

The apparatus is the same as for electrolysing water acidified with sulfuric acid.

You must use solutions of the same concentration and electrolysed them for the same length time at the same voltage (potential difference, p.d.) before measuring the gas volumes of hydrogen formed. (Electrolysis methods).

From the strong acid solution, you should get a greater volume of hydrogen in the same time.

The greater concentration of ions in the strong acid solution reduces the electrical resistance and more current flows via the greater number of ions present to carry it, hence more hydrogen ions are reduced at the cathode to form hydrogen.

 

For a more detailed discussion of these points see The theory of acids and bases AND make sure you know the difference between 'strength' and concentration !!!

'Strength' is about how ionised is the acid i.e. how big is Ka (or Kb for bases)

'Concentration' is about how many particles of the solute per unit volume e.g. molarity in mol dm-3.

 

Despite being a weak acid, carboxylic acids like ethanoic acid behave like any other acid and react with metals, alkalis and carbonate to form salts and fizzing here and there!


6.4.4 The reaction of carboxylic acids with metals

The salt names depends on the name of the acid, but the end of the name is ... oate.

So aqueous solutions of methanoic acid form methanoate salts, ethanoic acid gives ethanoate salts, propanoic acid gives propanoate salts and butanoic acid gives butanoate salts on neutralisation etc.. reaction

The salts can be crystallised from the solution by evaporation.

Metals dissolve in aqueous carboxylic acid solutions to form a salts and hydrogen e.g.

(i) ethanoic acid + magnesium ==> magnesium ethanoate + hydrogen

2CH3COOH(aq)  +  Mg(s)  ===>  (CH3COO)2Mg(aq)  +  H2(g)

2CH3COOH(aq)  +  Mg(s)  ===>  2CH3COO-(aq)  +  Mg2+(aq)  +  H2(g)

(ii) butanoic acid + zinc ====> zinc butanoate + hydrogen

2CH3CH2CH2COOH(aq)  +  Zn(s)  ===>  (CH3CH2CH2COO)2Zn(aq)  +  H2(g)

2CH3CH2CH2COOH(aq)  +  Zn(s)  ===>  2CH3CH2CH2COO-(aq)  +  Zn2+(aq)  + H2 (g)

-

Not on the syllabus, but an interesting tragic story of 'old acetic acid' and lack of appreciation of chemical hazards.

Ethanoic acid very slowly reacts with lead to form lead(II) ethanoate (old name lead acetate), once called 'sugar of lead'

2CH3COOH(aq)  +  Pb(s) ===>  (CH3COO)2Pb(aq)  +  H2(g)

The salt formed was called 'sugar of lead' because it had a sweet taste, but, ironically, its a deadly nerve toxin !

Cider makers in the past had dipped rods of lead into cider to neutralise any acetic acid that had formed and sweeten the beverage.

Unfortunately, lead is one of many heavy metals and that are highly toxic and lead compounds affect the brain and nervous systems and can be fatal.

Cases of lead poisoning have occurred through millennia, including the Romans, by using lead piping, lead pots in food preparation or concentrating liquids in cooking.

So, any cider left over that goes sour, dispose of it or, even better, let it turn completely into cider vinegar for the kitchen!


6.4.5 Reaction of carboxylic acids with oxides & hydroxides (soluble or insoluble bases)

(a) Alkalis (soluble bases) are neutralised by carboxylic acids to form a carboxylic acid salt and water  e.g.

The ionic equation is:  RCOOH(aq)  +  OH-(aq)  ===> RCOO-(aq)  +  H2O(l)

R = H, alkyl (e.g. CH3CH2) or aryl (e.g. C6H5)

(i) ethanoic acid + sodium hydroxide ===> sodium ethanoate + water

CH3COOH(aq)  +  NaOH(aq)  ===>  CH3COONa(aq)  +  H2O(l)

CH3COOH(aq) + OH-(aq) ===> CH3COO-(aq) + H2O(l)

The pH of the neutralised solution is ~9.

(ii) propanoic acid + potassium hydroxide ===> potassium propanoate + water

CH3CH2COOH(aq) + KOH(aq) ===> CH3CH2COOK(aq) + H2O(l)

(iii) butanoic acid + sodium hydroxide ===> sodium butanoate + water

CH3CH2CH2COOH(aq) + NaOH(aq) ===> CH3CH2CH2COONa(aq) + H2O(l)

(iv) benzoic acid  +  sodium hydroxide  ===>  sodium benzoate  +  water

(c) doc b (aq)  +  NaOH(aq)  ===>  (aq)  +  H2O(l)

(v) Making soluble aspirin

from: (aq)  +   NaOH(aq)  ===> (aq)  +  H2O(l)

 

If you gradually add alkali to the carboxylic acid, you get a characteristic curve of pH changes.

pH curve for adding strong base sodium hydroxide to weak carboxylic acid ethanoic acid advanced A level organic chemistry

Curve (2) + (4): Adding a strong base to weak acid, end point (i3), at pH ~9.

pH change at end–point reasonable sharp e.g. you can titrate weak organic acids like ethanoic acid with standard sodium hydroxide solution

NaOH is a strong soluble base.

Suitable indicators: 

phenolphthalein (pKind 9.3, range 8.3–10.0), end-point is the first permanent pink.

Thymol blue (base, pKind 8.9, range 8.0–9.6)

This neutralisation reaction can be used for the quantitative analysis of carboxylic acids.

You can even analyse sparingly soluble carboxylic acids by dissolving it in aqueous ethanol solvent.

 

For more details see:

Volumetric titration procedures and calculations for acid-alkali titrations

Advanced level acid-base titration calculation questions

Advanced level theory of pH curves and indicator choice for acid - alkali titrations

For a dibasic acid, there are two stages of neutralisation and two possible salts can be crystallised from solution, depending on the ratio of alkali added to the carboxylic acid e.g. for ...

(iv) Ethanedioic acid

HOOC-COOH(aq)  +  NaOH(aq)  ===> HOOC-COO-Na+(aq)  +  H2O(l)

HCOO-COO-Na+(aq)  +  NaOH(aq)  ===>  Na+-OOC-COO-Na+(aq)  +  H2O(l)

Overall

HOOC-COOH(aq)  +  2NaOH(aq)  ===> Na+-OOC-COO-Na+(aq)  +  2H2O(l)

(v) Butanedioic acid

HOOCCH2CH2COOH(aq)  +  NaOH(aq)  ===> HOOCCH2CH2COO-Na+(aq)  +  H2O(l)

HCOOCH2CH2COO-Na+(aq)  +  NaOH(aq)  ===>  Na+-OOCCH2CH2COO-Na+(aq)  +  H2O(l)

Overall

HOOCCH2CH2COOH(aq)  +  2NaOH(aq)  ===> Na+-OOCCH2CH2COO-Na+(aq)  +  2H2O(l)

 

Dibasic carboxylic acids (dicarboxylic acids) can also be titrated with standard sodium hydroxide solution.

pH curve for adding strong base sodium hydroxide to weak dicarboxylic acid ethanedioic acid propanedioic acid butanedioic acid advanced A level organic chemistry The more complicated pH curve, adding alkali to a dicarboxylic acid

There are two inflexion points on the pH curve corresponding to the half and full neutralisation of the dibasic/diprotic acid.

The equations for the two step neutralisation of ethanedioic acid are given below, including the ionic equations.

HOOC–COOH(aq) + NaOH(aq) ==> HCOO–COONa+(aq) + H2O(l)

ionically: HOOC–COOH(aq) + OH(aq) ==> HCOO–COO(aq) + H2O(l)

HCOO–COONa+(aq) + NaOH(aq) ==> Na+–OOC–COONa+(aq) + H2O(l)

ionically: HCOO–COO(aq) + OH(aq) ==> OOC–COO(aq) + H2O(l)

To detect the 2nd end–point, and hence the acid quantitatively, phenolphthalein indicator (pKind 9.3, range 8.3–10.0) is used, since it is essentially a weak acid–strong base titration.

Other acids like propanedioic acid (malonic acid) and butanedioic acid (succinic acid) behave, and be titrated, in the same way.

 

You can titrate citric acid in fruit juice with phenolphthalein indicator.

The end-point is the first permanent pink.

The overall neutralisation equation is:

HOOCCH2C(OH)(COOH)CH2COOH(aq)  +  3OH-(aq)  ===>  -OOCCH2C(OH)(COO-)CH2COO-(aq)  + 3H2O(l)

 

For more details see:

Volumetric titration procedures and calculations for acid-alkali titrations

Advanced level acid-base titration calculation questions

Advanced level theory of pH curves and indicator choice for acid - alkali titrations

 

(b) Insoluble bases dissolve in carboxylic acids to give a salt and water e.g.

(i) zinc oxide + ethanoic acid ====> zinc ethanoate + water

2CH3COOH(aq)  +  ZnO(s)  ===>  (CH3COO)2Zn(aq)  +  H2O(l)

2CH3COOH(aq)  +  ZnO(s)  ===>  2CH3COO-(aq)  +  Zn2+(aq)  +  H2O(l)

(ii) ethanoic acid + calcium hydroxide ====> calcium ethanoate + water

2CH3COOH(aq)  +  Ca(OH)2(s)  ===>  (CH3COO)2Ca(aq)  +  2H2O(l)

2CH3COOH(aq)  +  Ca(OH)2(s)  ===>  2CH3COO-(aq)  +  Ca2+(aq)  +  2H2O(l)

(iii) propanoic acid + magnesium hydroxide ====> magnesium propanoate + water

2CH3CH2COOH(aq)  +  Mg(OH)2(s)  ===>  (CH3CH2COO)2Mg(aq)  +  2H2O(l)

2CH3CH2COOH(aq)  + Mg(OH)2(s)  ===> 2CH3CH2COO-(aq) +  Mg2+(aq) + 2H2O(l)

(iv) butanoic acid + magnesium hydroxide ====> magnesium butanoate + water

2CH3CH2CH2COOH(aq)  +  Mg(OH)2(s)  ===> (CH3CH2CH2COO)2Mg(aq)  +  2H2O(l)

2CH3CH2CH2COOH(aq)  + Mg(OH)2(s)  ===> 2CH3CH2CH2COO-(aq) +  Mg2+(aq) + 2H2O(l)

 

The colourless salts can be crystallised out by carefully evaporating most of the water off.


6.4.6 The reaction of carboxylic acids with carbonates and hydrogencarbonates

Carbonate and hydrogencarbonate bases produce a carboxylic acid salt, water and carbon dioxide gas e.g.

(i) ethanoic acid + sodium hydrogen carbonate ==> sodium ethanoate + water + carbon dioxide

CH3COOH(aq) + NaHCO3(s) ====> CH3COONa(aq) + H2O(l) + CO2(g)

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

(ii) ethanoic acid + sodium carbonate ====> sodium ethanoate + water + carbon dioxide

2CH3COOH(aq) + Na2CO3(s) ====> 2CH3COONa(aq) + H2O(l) + CO2(g)

2CH3COOH(aq) + Na2CO3(s) ====> 2CH3COO-(aq)  +  2Na+(aq) + H2O(l) + CO2(g)

Sodium carbonate is quite soluble in water, but I've just assumed your adding the acid solution to the solid.

(iii) propanoic acid + potassium carbonate ====> potassium propanoate + water + carbon dioxide

2CH3CH2COOH(aq) + K2CO3(s) ====> 2CH3CH2COOK(aq) + H2O(l) + CO2(g)

2CH3CH2COOH(aq) + K2CO3(s) ====> 2CH3CH2COO-(aq)  +  2K+(aq) + H2O(l) + CO2(g)

(iv) ethanoic acid + magnesium carbonate ==> magnesium ethanoate + water + carbon dioxide

2CH3COOH(aq) + MgCO3(s)  ====> (CH3COO)2Mg(aq) + H2O(l) + CO2(g)

2CH3COOH(aq) + MgCO3(s)  ===> 2CH3COO-(aq)  + Mg2+(aq) + H2O(l) + CO2(g)

(v) butanoic acid + calcium carbonate ==> calcium butanoate + water + carbon dioxide

2CH3CH2CH2COOH(aq) + CaCO3(s)  ====> (CH3CH2CH2COO)2Ca(aq) + H2O(l) + CO2(g)

2CH3CH2CH2COOH(aq) + CaCO3(s)  ===> 2CH3CH2CH2COO-(aq)  + Ca2+(aq) + H2O(l) + CO2(g)

 

The colourless salts can be crystallised out by carefully evaporating most of the water off.


6.4.7 The reaction of carboxylic acids with aqueous ammonia

Aqueous ammonia solution forms ammonium salts e.g.

methanoic acid + ammonia ====> ammonium methanoate

HCOOH(aq)  +  NH3(aq)  ===> HCOONH4(aq)

ethanoic acid + ammonia ==> ammonium ethanoate

CH3COOH(aq)  +  NH3(aq)  ===> CH3COONH4(aq)

pH of the neutralised salt solution is  ~7

Here we are adding a weak base to a weak acid (or vice versa!)

 

The pH curve for adding a weak acid to a weak base

pH curve for adding weak carboxylic acid ethanoic acid to weak base ammonia advanced A level organic chemistry Curves (2) + (3) adding weak acid to weak base

 

The pH curve for adding a weak base to a weak acid

pH curve for adding weak base ammonia to weak carboxylic acid ethanoic acid advanced A level organic chemistry Curves (2) + (3) adding weak base to weak acid

 

In both cases, the point of inflexion in the pH curve is not sufficient to give a sharp end-point - not a viable quantitative titration.

 

For details see

Advanced level theory of pH curves and indicator choice for acid - alkali titrations

 


Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of Carboxylic Acids and Derivatives NOTES

 All Advanced Organic Chemistry Notes

 

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