18.

1: CARBOXYLIC ACIDS AND DERIVATIVES

Objectives
By the end of this sub-topic learners should be able to:
 Interpret the nomenclature and formulae of carboxylic acids and their derivatives.
 Describe the formation of carboxylic acids from alcohols, aldehydes and nitriles.
 Recall the reactions of carboxylic acids in the formation of salts, esters and acyl
chlorides.
 Explain the acidity of carboxylic acids and of chlorine substituted ethanoic acids in
terms of their structures.
 Describe the reactions of acyl chlorides with water, alcohols, phenols and primary
amines.
 Explain the relative ease of hydrolysis of acyl chlorides, alkyl chlorides and aryl
chlorides.
 Illustrate the formation of esters from carboxylic acids or acyl chlorides using ethyl
ethanoate and phenyl benzoate as examples.
 Describe the acid and base hydrolysis of esters.
 State the uses of carboxylic acids and esters.

Introduction

 Carboxylic acids are organic compounds widely found in nature and they are also
present in manufactured products such as soaps and oils.
 Carboxylic acids have the functional group, carboxyl;

 Their general formula is written as ROOH, where R is an alkyl,aryl group orhydrogen.

 They are named using the suffix – oic after a prefix indicating the number of carbon
atoms including the one in the carboxyl group.
 The first two members of the series, whose systematic names are methanoic acid
and ethanoic acid, are usually called by their traditional names, formic acid and
acetic acid respectively
Table 9.1.1: Examples of carboxylic acids

Formula Name Boiling point/ ℃

HCOOH Methanoic acid 101
CH 3 COOH Ethanoic acid 118
CH 3 CH 2 COOH Propanoic acid 141
CH 3 ¿ Octatonic acid 237
C 6 H 5 COOH Benzoic acid 249

Nomenclature of carboxylic acids

 Consider the following carboxylic acids;

Methanoic acid Ethanoic acid

 Where there are substituents or side chains on the carbon chain, they are numbered
using position, counting from the carbon of the carboxylic acid as carbon number 1.
 Consider the following carboxylic acids;

3 – Methylbutanoic acid 2 – chloropropanoic acid

  Aromatic carboxylic acids are named by adding the suffix – carboxylic acid to the
name of parent hydrocarbon. However, the suffix –oic acid can be used as well.

Benzenecarboxylic acid 4 – Iodobenzoic acid

 So Benzenecarboxylic acid can be named benzoic acid.

Carboxylic acid derivatives

 Carboxylic acid derivatives are compounds that contain a carbonyl group, but now
there is an electronegative atom (oxygen, nitrogen, or a halogen) attached to the
carbonyl carbon.
 This difference in structure leads to a major change in reactivity.
 The acid derivatives have the general formula

 Where R is an alkyl or aryl group and R’ is the derivative.

 In acid derivatives the OH group of the acid is replaced by different groups as shown
below;

Ester amide acid chloride anhydride

Esters

 Esters are named after the aryl or alkyl derivatives of the parent acid.
 An ester name has two parts - the part that comes from the acid (-oate) and the part
that shows the alkyl group (methyl or ethyl etc.)
 Consider the ester below;

The ester is named methyl propanoate.

Amides

 On amides the −OH group of the parent carboxylic acid is replaced by an−NH 2
group. They are named using the suffix – amide.
 Consider the amide below;

The amide is named ethanamide.

Acyl chlorides
 Acid chlorides or acyl chlorides are named using the suffix – oyl chlorides. Consider
the acyl chloride below;

The acid chloride is named ethanoyl chloride.

 Anhydrides
 Anhydrides are named as the anhydride of the parent acid.
 There are two types of anhydrides namely symmetrical anhydrides and mixed
anhydrides.
 Symmetrical anhydrides are derived from two molecules of the same acid and they
are named as the anhydride of the parent acid.
 Mixed anhydrides are derived from two different acids and they are named by listing
the parent acids in alphabetical order.

Ethanoic anhydride (symmetrical) Ethanoic propanoic anhydride (mixed)

Physical properties of carboxylic acids

 Carboxylic acids are soluble in water, they do not dimerise in water, but form
hydrogen bonds with water.
 Carboxylic acids are polar since there is highly electronegative oxygen and due to the
presence of the hydroxyl in the carboxyl group, they are able to form hydrogen
bonds with water molecules.
 Smaller carboxylic acids (C1 to C5) are soluble in water, whereas larger carboxylic
acids (C6 and above) are less soluble due to the increasing hydrophobic nature of the
hydrocarbon chains.
 The boiling points of carboxylic acids increases as the molecules get bigger.
 Carboxylic acids exhibit strong hydrogen bonding between molecules. They
therefore have high boiling points compared to other substances of comparable
molar mass.
  Carboxylic acids have even higher boiling points then alkanes and alcohols.
Carboxylic acids, similar to alcohols, can form hydrogen bonds with each other as
well as van der Waals dispersion forces and dipole-dipole interactions.
 However, unique to carboxylic acids, hydrogen bonding can occur between two
molecules to produce a dimer.
 Carboxylic acids tend to have strong odours, especially those that are volatile.
Common odours can be found in vinegar, which contains ethanoic acid, and rancid
butter, which contains butanoic acid.
 Esters of carboxylic acids tend to have pleasant odours, so they are usually used to
make perfumes.
 The acids with more than 10 carbon atoms are wax like solids, and their odour
diminishes with increasing molar mass and resultant decreasing volatility.

Preparation of carboxylic acids

Oxidation of alcohols or aldehydes

 Primary alcohols and aldehydes can be oxidised to carboxylic acids in acidic solution
by potassium dichromate (VI).
 Primary alcohols are oxidised to carboxylic acids via aldehydes. Below is an example
of ethanol;
Reflux withexcess
CH 3 CH 2 OH + [O] H SO ∧Cr O2−¿ CH 3 CHO → CH 3 CO 2 H ¿
2 4 2 7
→

 In this reaction, ethanol is first oxidised to ethanal and the oxidation continues to
produce the ethanoic acid.

Hydrolysis of nitriles

 The hydrolysis of nitriles, which are organic molecules containing a cyano group,
leads to carboxylic acid formation.

 These hydrolysis reactions can take place in either;

1. Acidic solutions
2. Basic solutions
 1. Acidic hydrolysis of nitriles
 The nitrile is heated under reflux with dilute hydrochloric acid.
 For example, with ethanenitrile and hydrochloric acid you would get ethanoic
acid and ammonium chloride.
CH 3 CN + 2 H 2 O + HCl →CH 3 COOH + NH 4 Cl

 Ethanoic acid is only a weak acid and so once it gains the hydrogen ion, it does
not loose it easily.

2. Basic hydrolysis of nitriles

 The nitrile is heated under reflux with sodium hydroxide solution.
 For example, with ethanenitrile and sodium hydroxide,a sodium salt is formed
and ammonia gas is given off as well.

CH 3 CN + H 2 O + NaOH →CH 3 COONa + NH 3

 To get a free carboxylic acid in this case, the final solution is acidifyed with a strong
acid such as dilute hydrochloric acid or dilute sulphuric acid.
 The ethanoate ion in the sodium ethanoate will react with hydrogen ions to give the
carboxylic acid.

Chemical reactions of carboxylic acids

 Carboxylic acids undergo reactions to produce derivatives of the acid. The most
common derivatives formed are

1. Salts

2. Acyl chlorides

3. Esters

1. Carboxylic acids reactions to form salts

 Carboxylic acids reacts with metals, alkalis and carbonates to form salts.
  Consider the reaction of a carboxylic acid and magnesium;

2 RCOOH (aq) + Mg (s )→ ¿ ¿ + H 2 (g)

 Consider the reaction of a carboxylic acid sodium carbonate;

2 RCOOH (aq) + Na2 CO 3 (s)→ 2 RCOONa (aq) + CO 2 (g) + H 2 O(l)

 The evolution of carbon dioxide from this reaction is used to distinguish carboxylic
acids from alcohols and phenols.

 Consider the reaction of a carboxylic acid and sodium hydroxide;

RCOOH (aq) + NaOH (aq)→ RCOONa(aq) + H 2 O(l)

2. Carboxylic acids reaction to form acyl chlorides

 Carboxylic acids are converted to acyl chlorides by their reaction with phosphorous
pentachloride (PCl¿¿ 5)¿ or by sulphur dichloride oxide(SOCl¿¿ 2)¿.

 So, every one mole of HCl is produced from every one mole of acid used.
 If the carboxylic acid in the reaction above is ethanoic acid and it reacts with
phosphorous pentachloride the resulting product will be ethanoyl chloride.
 Consider the reaction of benzoic acid and sulphur dichloride oxide which gives
benzoyl chloride;

C 6 H 5 COOH (s ) + SOCl2 (l)→C 6 H 5 COCl (l) + SO 2(g) + HCl( g)

3. Carboxylic acids reaction to form esters (Esterification)

 Carboxylic acids undergo a reaction with alcohols in the presence concentrated
sulphuric acid to form an ester and water.
 This reaction is called esterification.
 Consider the reaction of ethanoic acid and ethanol and the ester group formed is
circled in red;
 If benzoic acid undergoes a chemical reaction with ethanol in the presence of
concentrated sulphuric acid, ethyl benzoate and water are produced.
 The reverse of esterification is ester hydrolysis.

Mechanism for esterification

Step 1

 In the first step, the ethanoic acid takes a proton (H+) from the concentrated
sulphuric acid.
 This proton is attached to one of the lone pairs on the oxygen which is double-
bonded to the carbon.

 The transfer of the proton to the oxygen gives it a positive charge. The positive
charge is delocalised over the whole of the right-hand end of the ion.
Step 2
 The positive charge on the carbon atom is attacked by one of the lone pairs on the
oxygen of the ethanol molecule.

 A water molecule is lost from the ion.

Step 3
 The hydrogen is removed from the oxygen by reaction with the hydrogensulphate
ion which was formed way back in the first step.

 On the last step, the ester ethyl ethanoate is formed and the catalyst sulphuric acid
is regenerated.

Acidity
 Carboxylic acids can dissociate in aqueous solution into carboxylate ions and
protons.
 Two factors which influence the ionisation of an acid are:
1. The strength of the bond being broken
2. The stability of the ions being formed
 The strengths of weak acids are measured on the pKa scale. The smaller the number
on this scale, the stronger the acid is.
 Three compounds are compared with their pKa values;
Table 18.2.1: pKa values for different compounds
Compounds PKa
Ethanoic acid 4.76
Phenol 10
Ethanol 16

Ethanoic acid Phenol Ethanol

 The acidity of carboxylic acids compared to alcohols is explained by the relative

stabilisation of the carboxylate ion by the delocalisation of electron.
 When an alcohol donates its proton, it becomes a negative ion called an alkoxide ion,
−¿¿
RO .When a carboxylic acid donates its proton, it becomes a negatively charged
ion, ROO−¿ ¿, called a carboxylate ion.
 A carboxylate ion is much more stable than the corresponding alkoxide ion because
of the existence of resonance structures for the carboxylate ion which disperse its
negative charge.
 Only one structure can be drawn for an alkoxide ion, but two structures can be
drawn for a carboxylate ion.
 When two or more structures that differ only in the positions of valence electrons
can be drawn for a molecule or ion, it means that its valence electrons are
delocalized, or spread over more than two atoms.
 Ethanoic acid has pKa = 4.74, alcohols have pKa ~ 16, so carboxylic acids are about
1011times more acidic than alcohols.
 The difference lies in the fact that the carboxylate anion has the negative charge
spread out over two oxygen atoms, whereas the alcohol has the negative charge
localized on a single oxygen atom

Substituent Effects on Acidity

 Some atoms or groups, when attached to a carbon, are electron-withdrawing, as
compared with a hydrogen atom in the same position.
 Any substituent that stabilizes a negative charge will enhance the dissociation
process, and therefore result in a stronger acid.
 For example, consider ethanoic acid and its substitution with chlorine.

Ethanoic acid Chloroethanoic acid Di-chloroethanoic acid

pKa = 4.78 pKa = 2.86 pKa = 1.26

Tri-chloroethanoic acid
pKa = 0.64

 The acidic strength as shown by the pKa values, increases from ethanoic acid to tri-
chloroethanoic acid.
 This is because chlorine has a higher electronegativity than hydrogen, the electrons
in the Cl−C bond are drawn further from the carbon than the electrons in the
corresponding H−C bond.
  Thus, chlorine is considered to be an electron-withdrawing group. This effect is
called the inductive effect, in which a substituent affects a compound’s distribution
of electrons.
 Similarly, chloroethanoic acid, in which the strongly electron-withdrawing chlorine
replaces a hydrogen atom, is about 100 times stronger as an acid than ethanoic acid
and nitroethanoic acid is even stronger.
 An even greater effect is found in trichloroethanoic acid shown in the structures
above, whose acid strength is about the same as that of hydrochloric acid.
 There are a number of such effects, and atoms or groups may be electron
withdrawing or electron-donating as compared with hydrogen.
 The presence of such groups near the COOH group of a carboxylic acid often has an
effect on the acidity.
 In general, electron-withdrawing groups increase acidity by increasing the stability of
the carboxylate ion.
 In contrast, electron-donating groups decrease acidity by destabilizing the
carboxylate ion.

 For example, the methyl group, CH 3 is generally regarded as electron-donating.

 This explains why ethanoic acid, CH 3 COOH , is about 10 times weaker as an acid
than methanoic acid, HCOOH .

Reactions of acyl chlorides

1. Hydrolysis
2. Ester formation
3. Amide formation
 Acyl chlorides are the most reactive of the carboxylic acid derivatives and therefore
can be readily converted into other carboxylic acid derivatives.
 Consider the two different acyl chlorides;

1. Hydrolysis
 Acyl chlorides reacts rapidly (hydrolysis) with water to produce the corresponding
carboxylic acid and hydrochloric acid.
 Consider the reaction of ethanoyl chloride and water to produce ethanoic acid and
hydrochloric acid.

 When silver nitrate is to the solution of the products, it gives an immediate

precipitate of silver chloride, .
 This is the test that distinguish acyl chlorides from other organic halogens
compounds.
 Only acyl chlorides react directly with silver nitrate solution to give an immediate
precipitate of silver chloride.

2. Ester formation
 Acyl chlorides react readily with alcohols and phenol to produce esters.
 For example the reaction of ethanoyl chloride and propan-1- ol to give propyl
ethanoate and hydrochloric acid.
CH 3 COCl (l) + CH 3 CH 2 CH 2 OH (l)→CH 3 CO−O−CH 2 CH 2 CH 3 (l) + HCl
 When phenol reacts with acyl chlorides, it is dissolved in alkaline solution (sodium
hydroxide or pyridine) to remove the HCl produced and an ester is formed.
 Consider the reaction of benzoyl chloride and phenol;

 For every one mole of −OH in the acyl chloride that reacts, also one mole of HCl is
produced.

3. Amide formation
 Acyl chlorides reacts with ammonia, primary and secondary amines to produce
amides.
 The chlorine atom is very easily replaced by other things. For example, it is easily
replaced by a−NH 2 group to make an amide.
 Ethanoyl chloride reacts violently with a cold concentrated solution of ethylamine.
 A white solid product is formed which is a mixture of N-ethylethanamide (an N-
substituted amide) and ethylammonium chloride.

 To make ethanamide from ethanoyl chloride, ethanoyl chloride is added to a

concentrated solution of ammonia in water.

CH 3 COCl + NH 3 → CH 3 CO NH 2 + HCl

 The hydrogen chloride produced reacts with excess ammonia to give ammonium
chloride.
Ease of hydrolysis of halogen compounds
 There are three types of halogen compounds which are alkyl chlorides, aryl chlorides
and acyl chlorides.
 Acyl chlorides - these contain a −COCl group, e.g. ethanoyl chloride,CH 3 COCl , or
benzoyl chloride, C 6 H 5 COCl.
 Alkyl chlorides: These have a chlorine attached to a carbon chain, e.g. chloroethane,
C 2 H 5 Cl
 Aryl chlorides: These have a chlorine attached directly to a benzene ring, e.g.
chlorobenzene, C 6 H 5 Cl.
 Any hydrolysis reaction which happens involving any of these chlorides can be
thought of as nucleophilic substitution.
 Consider the following compounds; ethanoyl chloride, chloro ethane and
chlorobenzene.
 The ease of hydrolysis of halogen compounds rely on the degree of electron
deficiency of the carbon atom which is bonded to the halogen, chlorine.
 In acyl chloride, the carbon atom is bonded to both chlorine and oxygen atoms
which are highly electronegative and they withdraw electron from the carbon atom.

 Therefore, the acyl chloride can readily attract nucleophiles, ¿.

 In alkyl chlorides (chloro ethane), the carbon atom to which the chlorine atom is
bonded is electron deficiency since the electrons are being withdrawn by the
chlorine atom.
  Therefore, the alkyl chloride can attract nucleophiles like OH −¿¿ but not more than
acyl chlorides.
 So ethanoyl chloride is more reactive than ethane chloride.
 In aryl chlorides (chlorobenzene), the lone pair of electrons on the chlorine atom
interacts with the benzene ring and there is resonance.
 So, because of resonance, a cloud of delocalised electrons will result and it increases
the C−Cl bond strength and decreases the polarity of the bond.
 Hence, the chlorine atom in chlorobenzene cannot be removed easily as a result the
chloro benzene is almost unreactive.

 Therefore acyl chlorides are the most reactive and easily hydrolysed followed by
alkyl chlorides and aryl chlorides are the least hydrolyse

Acid and base hydrolysis of esters

 Esters are neutral compounds, unlike the acids from which they are formed.
 They undergo hydrolysis reactions to give carboxylic acids and alcohols or phenol.
 However, they undergo two types of hydrolysis that is acid and base hydrolysis.
 An example of an ester

Acid hydrolysis
 Acidic hydrolysis is the reverse of esterification and it is a slow reaction.
 To speed up the reaction, the ester is heated with a large excess of water containing
a strong-acid catalyst like dilute hydrochloric acid or dilute sulphuric acid.
 Like esterification, the reaction is reversible and does not go to completion.
 Consider the reaction of ethyl ethanoate and water to give ethanoic acid and
ethanol;
+¿
CH 3 COO CH 2 CH 3 (l) + H 2 O(l) H CH 3 COOH (aq )¿ + C 2 H 5 OH (aq)
⇔

 Remember that in acidic hydrolysis, water splits the ester bond.

 The H from water joins to the oxygen atom in the O-R’ part of the original ester and
give an alcohol, and the OH of water joins to the carbonyl carbon atom, R - C = O,
and gives the carboxylic acid.

Base hydrolysis
 In this hydrolysis, a base such as sodium hydroxide or potassium hydroxide is used to
hydrolyse an ester.
 The carboxylic acid formed by the hydrolysis reacts with the excess base to form a
carboxylate salt and an alcohol.
 So, the formation of the carboxylate salt removes the carboxylic acid from the
equilibrium mixture.
 Therefore, the ester hydrolysis in the presence of a base can proceed to completion.
 Consider the reaction of propyl ethanoate and sodium hydroxide in aqueous solution
to give sodium ethanoate and propanol.
+ ¿¿
CH 3 CH 2 COO CH 2 CH 3 + NaOH →CH 3 CO O−¿ Na ¿
+ CH 3 CH 2 CH 2 OH

 This mixture is relatively easy to separate, provided an excess of sodium hydroxide

solution has been used, there will not be any ester left
 The alcohol formed can be distilled off, which is an easy process.
 Esters are therefore hydrolysed more effectively in basic than in acidic solution.
  However, the base is used up in the reaction hence it is not acting as a catalyst but
rather a reagent.
 Since soaps are prepared by the base hydrolysis of fats and oils, base hydrolysis of
esters is called saponification.
 Saponification is the process of making soaps.
 In a saponification reaction, the base is a reactant, not simply a catalyst and the
reaction goes to completion.

Fats and oils

 Fatty acids are carboxylic acids consisting of a long hydrocarbon chain at one end
and a carboxyl group (-COOH) at the other end.
 They are generally represented as RCOOH. Fatty acids are naturally found in plants
and animals and they are a source of energy in the diet.
 There are two groups of fatty acids: saturated fatty acids and unsaturated fatty acids.
 Saturated fatty acids – these are fatty acids which contain carbon-carbon single
bonds called saturated fatty acids, examples include stearic acid ¿) and palmitic acid
(C ¿ ¿ 15 H 31 COOH )¿. These are solids at room temperature called fats.
 Unsaturated fatty acids – these are fatty acids which contain one or more double
bonds between carbon atoms, examples include oleic acid (C ¿ ¿ 17 H 33 COOH )¿.
These are liquids at room temperature called oils.
 Long chain fatty acids always exist as triglycerides and are found in fats and oils.
 Triglycerides are esters of fatty acids and are formed by combining fatty acids with
glycerol.
 Consider a triglycerol given below;
Soap formation: saponification
 When a triglyceride is hydrolysed in a basic medium, the products of the reactionare
glycerol and the salts of the respective fatty acids.
 Soaps are sodium or potassium salts of long chain fatty acids.
 So, when triglycerides in fat or oil react with aqueous NaOH or KOH, they are
convertedinto glycerol and the salts of the respective fatty acids (soap).
 The R groups represents carboxylic acid chains.
 Below is the hydrolysis of a triglyceride;

 This is called alkaline hydrolysis of esters and since this reaction leads to the
formation of soap, it is called the Saponification process.
 This process has been done many years ago, especially in the rural areas, where
animal fats were boiled with water and the ashes of a wood fire, which contained
potassium carbonate a base.
 The soap formed remains in suspension form in the mixture, it is then precipitated as
a solid from the suspension by adding common salt to the suspension. This process is
called salting out of soap.

Soap action
 The soap molecule has two parts: a polar group ¿and a non-polar group (R-
hydrocarbon part).
 The polar group is called the head and the non-polar group is called the tail. Thus,
the soap molecule has a polar head and a non-polar hydrocarbon tail.
  The polar head is hydrophilic in nature (water loving) and the non-polar tail is
hydrophobic (water repelling) in nature.
 Soap is an excellent cleanser because of its ability to act as an emulsifying agent.
 An emulsifier is capable of dispersing one liquid into another immiscible liquid.
 This means that while oil which attracts dirt, do not naturally mix with water, soap
converts oil or grease or dirt into a suspension of tiny droplets in water and can then
be removed.
 However,though soaps are excellent dirt removers, they do have disadvantages. As
salts of weak acids, they are converted by mineral acids into free fatty acids:
 These fatty acids are less soluble than the sodium or potassium salts and form a
precipitate or soap scum.
 For this reason, soaps are ineffective in acidic water. Also, soaps form insoluble salts
in hard water, such as water containing magnesium, calcium, or iron.

Uses of carboxylic acids and esters

Uses of esters
 Esters that have fragrant odours are used as a constituent of perfumes, essential oils,
artificial food flavourings and cosmetics.
 Esters are used as an organic solvent for paints and varnishes.They are used to
manufacture sunburn lotions, nail polish removers and glues.
 Esters are also used in the production of polyester like Terylene. The long-chain
polyester molecules are unreactive and have great tensile strength. Therefore, they
are used to make polyester threads in the production of synthetic fabrics.
 Nitrate esters, such as nitro-glycerine, are known for their explosive properties
 Polyesters are used to make plasticisers in the manufacture of plastics.
 Methyl ester is a biodiesel
 As for large esters, they are used in the production of soaps and detergents. Soaps
and detergents are used to remove impurities from surfaces of human skin, clothes
and other solids when dissolving in water.

Uses of carboxylic acids

 Carboxylic acids make up a series of fatty acids which are extremely good for human
health.
 The omega-6 and omega-3 are essential fatty acids which are not produced by the
body.
 Manufacturing of soaps need higher fatty acids. Soaps are usually sodium or
potassium salts of higher fatty acids such as stearic acid.
 Food industry uses many organic acids like citric acidand lactic acid for the
production of soft drinks, food products etc. For example, acetic acid is used in
making vinegar used in food preparation.
 Carboxylic acids such as benzoic and sorbic acid are used as preservatives.
 In pharmaceutical industry organic acids are used in many drugs such as aspirin,
phenacetin. Ascorbic acid is used for the manufacture of medicines based on
vitamin C.

Figure 18.2.1: products manufactured by carboxylic acids

 Acetic acids are often used as a coagulant in the manufacturing of rubber. Organic
acids find huge application in making dye stuff, perfumes and rayon.
 They are used in the manufacture of soaps, detergents, shampoos, cosmetics and
metal cleaning products (Oleic acid).Manufacture of toothpaste (Salicylic acid).
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