Chemistry

UNIT 7: CARBOXYLIC ACIDS AND ACYL CHLORIDES

Key unit competency:

The learner should be able to compare the chemical nature of the carboxylic acids

and acid halides to their reactivity.

Learning objectives

• Explain the physical properties and uses of carboxylic acids and acyl chlorides

• Describe the inductive effect on the acidity of carboxylic acid

• Explain the reactions of carboxylic acids and acyl chlorides

• Apply the IUPAC rules to name different carboxylic acids acyl chlorides

• Write the structural formula and isomers of carboxylic acids

• Distinguish between carboxylic acids from other organic compounds using

appropriate chemical test

• Prepare carboxylic acids from oxidation of aldehydes or primary alcohols

• Compare the physical properties of carboxylic acids to those of alcohols

• Outline the mechanisms of esterification and those of reaction of acyl

chlorides with ammonia , amines and alcohols

• Develop a culture of working as a team group activities and self-confidence

in presentation

• Appreciate the uses of carboxylic acids as the intermediate compounds in

industrial processes such as aspirin, vinegar and perfumes

Carboxylic acid is classified in the family of organic compounds due to the presence

of carboxyl group (-COOH) in their chemical formula. The general formula for

carboxylic acids is R-COOH where R- refers to the alkyl group of the molecule.

7.1.1. Nomenclature

Carboxylic acids are named by following the general rules of naming organic

compounds, where the suffix ‘oic acid’ is added to the stem name of the longest

carbon chain that contains the acid functional group. The side branches are also

positioned by starting from the carbon with carboxylic functional group.

The carboxylic group takes priority to other functional group when numbering

carbons in thecase of substituted chain.

• Optical isomers

Optical isomers have the same molecular formula and the same structural formula,

but they are different in the spatial arrangement of atoms and their optical properties.

An organic compound shows optical isomerism, when there is chiral carbon (a

carbon atom attached to four diverse groups) in its structure. A chiral carbon is also

known as asymmetric carbon. 

Just as the right hand and left hand are mirror images of another but not superimposable, optical isomers, also known as enantiomers, are different from each other and can have different properties. For example, muscles produce D-lactic acid when they contract, and a high amount of this compound in muscles causes muscular pain and cramps.

These molecules are optical isomers, because they have opposite optical activities.They can be distinguished by a plane-polarized light where one enantiomer rotates the light to the right while the other rotates it to the left.

Enantiomers are often identified as D- or L- prefixes because of the direction in which

they rotate the plane polarized light as shown in figures 7.2 and 7.3. Enantiomers

that rotate plane polarized light in clockwise direction are known as dextrorotatory

(right-handed) molecules and enantiomers that rotate plane polarized light in

anticlockwise direction are known as levorotatory (left-handed) molecules.

A solution containing equal amounts of enantiomers, 50% levorotatory and 50%

dextrorotatory is known as a racemic mixture that will not rotate polarized light,

because the rotations of the two enantiomers cancel each other out. 

7.2. Physical properties of carboxylic acids

a. Physical state

Many carboxylic acids are colorless liquids with disagreeable odors. Aliphatic

carboxylic acids with 5 to 10 carbon atoms are all liquids with a “goaty” odors (odor

of cheese). These acids are also produced by the action of skin bacteria on human

sebum (skin oils), which accounts for the odor of poorly ventilated storerooms.

The acids with more than 10 carbon atoms are wax-like solids, and their odor

diminishes with increasing molar mass and resultant decreasing volatility.

Anhydrous acetic acid freezes at (17^oC) slightly below ordinary room temperature,

reason why it is called glacial acetic acid (Figure 7.4). But a mixture of acetic acid

with water solidifies at much lower temperature.

b. Melting and boiling point

Carboxylic acids show a high degree of association through hydrogen bonding.

Because of this, they have high melting and boiling points compared to other

organic compounds of the same mass or number of carbon atoms.

Carboxylic acids have high melting and boiling points because their hydrogen

bonds enhance the possibility of bringing two acid molecules together by forming

a kind of dimer.

7.3. Acidity of carboxylic acids

Solutions of carboxylic acid turn blue litmus paper red; they do not change the

color of red litmus paper; therefore, they are acids as other mineral acids such as

HCl (aq).

Organic or carboxylic acids are weak acids in opposition to some mineral acids such

as hydrochloric acids which are strong acids.According to Arrhenius’ theory of acids

and bases, strong acids dissociate completely in water to give hydrogen ion, H⁺(aq) or

H₃O⁺, whereas weak acids dissociate partially. The hydrogen ion released combines with a water molecule to form H₃O⁺ a hydrate positive ion called hydronium H₃O⁺:

The carboxylate ion formed by ionization of the acid is more stable than the acid

because it has many resonance structures.

Ethanoic acid is a weaker acid than methanoic because its methyl group has a

positive inductive effect; that is to mean that it pushes electrons towards the O-H

bond hence make hydrogen ion stable and not easily leaving.

The greater the number of such groups, the greater the effected and therefore

the weaker will be the acid.For example, 2,2-dimethylpropanoic is weaker than

2-methylpropanoic acid which is in turn weaker than propanoic.

The same rule applies to the increase in the length of the alkyl group chain.Butanoic

acid is a weaker acid than propanoic acid which shows that the acidity strength

decreases as the alkyl chain increases.

On the other hand, when an electron withdrawing group (a group with a negative

inductive effect) is present, the opposite effect is observed. For example,

chloroethanoic acid is a stronger acid than ethanoic acid. This is because chlorine

being electronegative, will withdraw electron towards itself thus reducing the

electron density around the O-H bond thus weakening it. It causes O-H bond to

easily break, and the concentration of hydrogen ions will be high in the solution.

The more the number of groups with negative inductive effect, the greater is the

effect and hence the more acidic will be the solution. Trifluoroacetic acid is more

acidic than trichloroacetic, dichloroacetic, chloroacetic and acetic acid because

fluorine is more electronegative than chlorine and hydrogen. It will strongly

withdraw electron towards itself, hence makes easier for the proton to leave.It must

also be noted that the further away the electronegative element, the less the effect.

For example, 3-chlorobutanoic acid is therefore a weaker acid than 2-chlorobutanoic

acid.

7.4. Preparation of carboxylic acids

Carboxylic acids are common and vital functional group; found in amino acids,

fatty acids etc. and provide the starting raw material for acid derivatives such as

acyl chlorides, amides, esters and acid anhydrides. There are several methods

of preparation of carboxylic acids where the most common are discussed in this

section.

7.4.1. From primary alcohols and aldehydes

Different carboxylic acids can be prepared by oxidation of either primary alcohols

or aldehydes. In the process, the mixture of alcohol is heated under reflux with an

oxidizing agent such acidified potassium permanganate or potassium dichromate.

Primary alcohols are first oxidized to aldehydes then further oxidation of aldehydes

produces carboxylic acid.

7.4.2. Hydrolysis of acid nitriles and amides with acid or alkali

When nitriles are hydrolyzed by water in acidic medium and the mixture is submitted

to heat, the reaction yields carboxylic acids.

7.4.3. From dicarboxylic acid

Monocarboxylic acids can be prepared by heatingcarboxylic acids which have two

carboxylic functional groups attached to the same carbon atom.

Note that the reaction is used to reduce length of the carbon chain. The mono

carboxylic acid prepared has one carbon atom less than the starting dicarboxylic

acid.

7.4.4. From organomagnesium compounds (Carboxylation reaction)

Grignard reagents react with carbon dioxide gas, and when the intermediate compound formed is hydrolyzed it finally forms carboxylic acid.

Mechanism:

7.4.5. From alkenes (Oxidation of alkenes)

Carboxylic acids are also obtained by heating alkenes with concentrated acidified

potassium permanganate. The reaction unfortunately forms a mixture of compounds

that mustbe later separated.

Note that the hydrolysis of carboxylic acid derivatives such as amide, esters, acyl

chloride and acid anhydrides also produce the corresponding acids.

7.4.6. Laboratory preparation of acetic acid

7.5. Reactions of carboxylic acids

The reaction of acids with carbonates is the basis for the chemical test of carboxylic

acid functional group and it can be used to distinguish carboxylic acids from other

functional groups in qualitative analysis.

Carbon dioxide produced is also tested by lime water and it turns lime water milky

(Figure 7.8)

It is noted that the oxygen atom in the ester formed comes from the alcohol and the

one in water is from the acid. In the mechanism of esterification, the acid loses -OH

group while the alcohol loses H-atom.

7.5.3. Reduction of carboxylic acids

Carboxylic acids are reduced to primary alcohols on treatment with reducing agent

such as LiAlH₄ in dry ether or by use of hydrogen in the presence of Ni catalyst. The

reduction does not form aldehyde as an intermediate product, like in oxidation of

primary alcohols.

7.6. Uses of carboxylic acids

Food industry and nutrition

• Food additives: Sorbic acid, benzoic acid, etc.

• Main ingredient of common vinegar (acetic acid).

• Elaboration of cheese and other milk products (lactic acid).

Pharmaceutical industry

• Antipyretic and analgesic (acetylsalicylic acid or aspirin).

• Active in the process of synthesis of aromas, in some drugs (butyric or butanoic acid).

• Antimycotic and fungicide (Caprylic acid and benzoic acid combined with salicylic acid).

• Active for the manufacture of medicines based on vitamin C (ascorbic acid).

• Manufacture of some laxatives (Hydroxybutanedioic acid).

 Other industries

• Manufacture of varnishes, resins and transparent adhesives (acrylic acid).

• Manufacture of paints and varnishes (Linoleic acid).

• Manufacture of soaps, detergents, shampoos, cosmetics and metal cleaning products (Oleic acid).

• Manufacture of toothpaste (Salicylic acid).

• Production of dyes and tanned leather (Methanoic acid).

• Manufacture of rubber (Acetic acid).

• Preparation of paraffin candles (Stearic acid)

7.7. Acyl chlorides and nomenclature 

Acyl halides are compounds with the general formula where the–OH group of

carboxylic acid has been substituted by a halogen atom. The acyl remaining

structure is represented as:

Their isomers can be chain isomerism, positional isomerism and functional isomerism

with chloro aldehydes and ketones, alcohols with double bond C=C and chlorine as

a substituent, cyclic ethers with chlorine.

Acid chlorides have not many applications in our everyday life, but industrially they

are used in synthesis of perfumes and nylons, which are polymers of high importance

in textile industry. They can also be used in pharmaceutical industries to synthesize

drugs with aromatic ester or amide functional groups like aspirin or paracetamol.

7.7.1. Physical properties

Appearance

Acyl chlorides are colourless fuming liquids. Their characteristic strong smell is

caused by hydrogen chloride gas that is produced when they get in contact with

moisture (see figure 7.7). For example, the strong smell of ethanoyl chloride is a

mixture of vinegar odour and the acrid smell of hydrogen chloride gas.

b. Solubility

Acyl chlorides are slightly soluble in water due to their small dipole that can interact

with the polarity of water molecule. They cannot be said to be soluble in water

because they readily react with water. It is impossible to have a simple aqueous

solution of acyl chlorides, rather we have the products of their reaction with water.

c. Boiling and melting points

Acyl chloride molecules interact by Van der Waals forces whose strength increases

with the increase in molecular masses of the compounds. 

The boiling and melting points of acyl chlorides increases as their molecular masses

rise. They have lower boiling and melting points than alcohols and carboxylic acids

of the same number of carbon atoms, because they lack hydrogen bonds.

7.7.2. Reactions of acyl chlorides

The chemistry of acyl chlorides is dominated by nucleophilic substitution, where

a stronger nucleophile replaces chlorine atom of acyl chloride. They undergo

nucleophilic substitution reactions more easily than alkyl halides and carboxylic

acids because the nucleophile targets the carbon which is deficient in electrons and

-Cl is better leaving group than -OH group.

The common reactions of acyl chlorides include reactions with water, alcohols and

ammonia and amines. These reactants have a very electronegative element that

has a free lone pair of electrons to act as a nucleophile.

Reaction with alcohols

They react with alcohol to produce esters with high yields than esterification of an

alcohol and carboxylic acid, since Cl-atom in acyl chloride is a better leaving group

than O-H for the case of carboxylic acid. The difference in electronegativity is the

main reason for this observation.

Reaction with ammonia and amine

Acyl chlorides react with ammonia and amines to yield amides. Ammonia, primary

amines and secondary amines form primary amides, secondary amides and tertiary

amides respectively.