Chemistry

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Amines – Classification

Amines – classification

Nomenclature

(a) Common System:

In common system, an aliphatic amine is named by prefixing alkyl group to amine. The prefixes di-,tri-, and tetra-, are used to describe two, three (or) four same substituent’s.

(b) IUPAC System:

Structure of Amines

Like, ammonia, nitrogen atom of amines is trivalent and carries a lone pair of electron and sp³ hybridised, out of the four sp³ hybridised orbitals of nitrogen, three sp³ orbitals overlap with orbitals of hydrogen (or) alkyl groups of carbon, the fourth sp³ orbital contains a lone pair of electron.

Hence, amines posses pyramidal geometry. Due to presence of lone pair of electron C-N-H (or) C-N-C bond angle is less than the normal tetrahedral bond angle 109.50. For example, the C-N-C bond angle of trimethylamine is 1080 which is lower than tetrahedral angle and higher than the H-N-H bond angle of 107˚. This increase is due to the repulsion between the bulky methyl groups.

General Methods of Preparation Amines

Aliphatic and aromatic amines are prepared by the following methods.

1. From Nitro Compounds

Reduction of Nitro compounds using H₂/Ni (or) Sn/HCl or Pd/H₂ gives primary amines.

2. From Nitriles

(a) Reduction of alkyl or aryl cyanides with H₂/Ni (or) LiAlH₄ (or) Na/C₂H₅OH gives primary amines. The reduction reaction in which Na/C₂H₅OH is used as a reducing agent is called mendius reaction.

(b) Reduction of isocyanides with sodium amalgam / C₂H₂OH gives secondary amines

3. From Amides

(a) Reduction of amides with LiAlH₄ gives amines

(b) Hoffann’s Degradation Reaction

When Amides are treated with bromine in the presence of aqueous or ethanolic solution of KOH, primary amines with one carbon atom less than the parent amides are obtained.

Example:

4. From Alkyl Halides

(a) Gabriel Phthalimide Synthesis

Gabriel synthesis is used for the preparation of Aliphatic primary amines. Phthalimide on treatment with ethanolic KOH forms potassium salt of phthalimide which on heating with alkyl halide followed by alkaline hydrolysis gives primary amine. Aniline cannot be prepared by this method because the arylhalides do not undergo nucleophilic substitution with the anion formed by phthalimide.

(b) Hoffann’s Ammonolysis

When Alkyl halides (or) benzylhalides are heated with alcoholic ammonia in a sealed tube, mixtures of 1°, 2° and 3° amines and quaternary ammonium salts are obtained.

This is a nucleophilic substitution, the halide ion of alkyl halide is substituted by the – NH₂ group. The product primary amine so formed can also has a tendency to act as a nucleophile and hence if excess alkyl halide is taken, further nucleophilic substitution takes place leading to the formation of quarternary ammonium salt. However, if the process is carried out with excess ammonia, primary amine is obtained as the major product.

The order of reactivity of alkylhalides with amines.

RI > RBr > RCl

(c) Alkyl halide can also be converted to primary amine by treating it with sodium azide (NaN₃) followed by the reduction using lithium aluminium hydride.

(d) Preparation of Aniline from Chlorobenzene

When chlorobenzene is heated with alcoholic ammonia, aniline is obtained.

5. Ammonolysis of Hydroxyl Compounds

(a) when vapour of an alcohol and ammonia are passed over alumina, W₂O₅ (or) silica at 400°C, all types of
amines are formed. This method is called Sabatier – Mailhe method.

(b) Phenol reacts with ammonia at 300°C in the presence of anhydrous ZnCl₂ to give aniline

Properties of Amines

1. Physical State and Smell

The lower aliphatic amines (C₁ – C₂) are colourless gases and have ammonia like smell and those with four or
more carbons are volatile liquids with fish like smell. Aniline and other arylamines are usually colourless but when exposed to air they become coloured due to oxidation.

2. Boiling Point

Due to the polar nature of primary and secondary amines, can form intermolecular hydrogen bonds using their lone pair of electorn on nitrogen atom. There is no such H-bonding in tertiary amines.

The boiling point of various amines follows the order,

Amines have lower boiling point than alcohols because nitrogen has lower electronegative value than oxygen and hence the N-H bond is less polar than -OH bond.

Table Boiling points of amines, alcohols and alkanes of comparable molecular weight.

Solubility

Lower aliphatic amines are soluble in water, because they can form hydrogen bonds with water molecules. However, solubility decreases with increase in molecular mass of amines due to increase in size of the hydrophobic alkyl group. Amines are insoluble in water but readily soluble in organic solvents like benzene, ether etc.

Chemical Properties

The lone pair of electrons on nitrogen atom in amines makes them basic as well as nucleophilic. They react with acids to form salts and also react with electrophiles. They form salts with mineral acids

Example:

Expression for Basic Strength of Amines

In the aqueous solutions, the following equilibrium exists and it lies far to the left hence amines are weak bases compared to NaOH.

The basicity constant Kb gives a measure of the extent to which the amine accepts the hydrogen ion (H⁺) from water, we know that,

Larger the value of Kb or smaller the value of pKb, stronger is the base.

Table: pKb values of Amines in Aqueous solution. (pK_b for NH₃ is 4.74)

Influence of Structure on Basic Character of Amines

The factors which increase the availability of electron pair on nitrogen for sharing with an acid will increase the basic character of an amine. When a +I group like an alkyl group is attached to the nitrogen increase the electron density on nitrogen which makes the electron pair readily available for protonation.

(a) Hence alkyl amines are stronger bases than ammonia.

Consider the reaction of an alkyl amine image 19 with a proton

The electron – releasing alkyl group R pushes electron towards nitrogen in the amine and provide unshared electron pair more available for sharing with proton. Therefore, the expected order of basicity of aliphatic amines (in gas phase) is

The above order is not regular in their aqueous solution as evident by their pKb values given in the table.

To compare the basicity of amines, the inductive effect, solvation effect, steric hindrance, etc., should be taken into consideration.

Solvation Effect

In the aqueous solution, the substituted ammonium cations get stabilized not only by electron releasing (+I) effect of the alkyl group but also by solvation with water molecules. The greater the size of the ion, lesser will be the solvation.

The order of stability of the protonated amines is greater the size of the ion, lesser is the solvation and lesser is the stability. In case of secondary and tertiary amines, due to steric hindrance, the alkyl groups decrease the number of water molecules that can approach the protonated amine. Therefore the order of basicity is,

Based on these effects we can conclude that the order of basic strength in case of alkyl substituted amines in aqueous solution is

The resultant of +I effect, steric effect and hydration effect cause the 2° amine, more basic.

Basic Strength of Aniline

In aniline, the NH₂ group is directly attached to the benzene ring. The lone pair of electron on nitrogen atom in aniline gets delocalised over the benzene ring and hence it is less available for protonation makes the, aromatic amines (aniline) less basic than NH₃.

In case of substituted aniline, electron releasing groups like -CH₃, -OCH₃, -NH₂ increase the basic strength and electron withdrawing group like -NO₂,-X,-COOH decrease the basic strength.

Table pK_b’s of substituted anilines (pK_b value of aniline is 9.376)

The relative basicity of amines follows the below mentioned order

Alkyl amines > Aralkyl amines > Ammonia > N – Aralkyl amines > Aryl amines

Chemical Properties of Amines

1. Alkylation

Amines reacts with alkyl halides to give successively 2° and 3° amines and quaternary ammonium salts.

2. Acylation

Aliphatic/aromatic primary and secondary amines react with acetyl chloride (or) acetic anhydride in presence of pyridine to form N – alkyl acetamide.

Example

3. Schotten – Baumann Reaction

Aniline reacts with benzoylchloride (C₆H₅COCl) in the presence of NaOH to give N – phenyl benzamide. This reaction is known as Schotten – Baumann reaction. The acylation and benzoylation are nucleophilic substitutions.

4. Reaction With Nitrous Acid

Three classes of amines react differently with nitrous acid which is prepared in situ from a mixture of NaNO₂ and HCl.

(a) Primary Amines

(i) Ethylamine reacts with nitrous acid to give ethyl diazonium chloride, which is unstable and it is converted to ethanol by liberating N₂.

(ii) Aniline reacts with nitrous acid at low temperature (273 – 278 K) to give benzene diazonium chloride which is stable for a short time and slowly decomposes even at low temperatures. This reaction is known as diazotization.

(b) Secondary Amines

Alkyl and aryl secondary amines react with nitrous acid to give N – nitroso amine as yellow oily liquid which is insoluble in water.

This reaction is known as Libermann’s nitroso test,

(c) Teritiary Amine

(i) Aliphatic tertiary amine reacts with nitrous acid to form trialkyl ammonium nitrite salt, which is soluble in water.

(ii) Aromatic tertiary amine reacts with nitrous acid at 273K to give p – nitroso compound.

5. Carbylamine Reaction

Aliphatic (or) aromatic primary amines react with chloroform and alcoholic KOH to give isocyanides (carbylamines), which has an unpleasant smell. This reaction is known as carbylamine test. This test used to identify the primary amines.

6. Mustard Oil Reaction

(i) When primary amines are treated with carbon disulphide (CS₂), N – alkyldithio carbomic acid is formed which on subsequent treatment with HgCl₂, give an alkyl isothiocyanate.

(ii) When aniline is treated with carbon disulphide, or heated together, S-diphenylthio urea is formed, which on boiling with strong HCl, phenyl isothiocyanate (phenyl mustard oil), is formed.

These reactions are known as Hofmann – Mustard oil reaction. This test is used to identify the primary amines.

7. Electrophilic Substitution Reactions in Aniline

The image 36 group is a strong activating group. In aniline the NH₂ is directly attached to the benzene ring, the lone pair of electrons on the nitrogen is in conjugation with benzene ring which increases the electron density at ortho and para position, thereby facilitating the electrophilic attack at ortho and para positions.

(i) Bromination

Aniline reacts with Br₂/H₂O to give 2, 4, 6 – tribromo aniline a white precipitate.

To get mono bromo compounds, – NH₂ is first acylated to reduce its activity.

When aniline is acylated, the lone pair of electron on nitrogen is delocalised by the neighbouring carbonyl group by resonance. Hence it is not easily available for conjugation with benzene ring.

The acetylamino group is thus less activating than the amino group in electrophilic substitution reaction.

(ii) Nitration

Direct nitration of aniline gives o and p – nitro aniline along with dark coloured ‘tars’ due to oxidation. Moreover in a strong acid medium aniline is protonated to form anilinium ion which is m – directing and hence m – nitro aniline is also formed.

To get para product, the – NH₂ group is protected by acetylation with acetic anhydride. Then, the nitrated product is hydrolysed to form the product.

(iii) Sulphonation

Aniline reacts with Conc. H₂SO₄ to form anilinium hydrogen sulphate which on heating with H₂SO₄
at 453 – 473K gives p – aminobenzene sulphonic acid, commonly known as sulphanilic acid, as the major product.

(iv) Aniline

It does not under go Friedel – Crafts reaction (alkylation and acetylation) we know aniline is basic in nature and it donates its lone pair to the lewis acid AlCl₃ to form an adduct which inhibits further the electrophilic substitution reaction.

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Nitro Compounds

Nitro compounds are considered as the derivaties of hydrocarbons. If one of the hydrogen atom of hydrocarbon is replaced by the – NO₂ group, the resultant organic compound is called a nitrocompound.

Classification of Nitrocompounds

Nitroalkanes are represented by the formula, R-NO₂ where R is an alkyl group (C_nH_2n+1-). Nitroalkanes are further classified into primary, secondary, tertiary nitroalkanes on the basis of type of carbon atom to which the nitro (-NO₂) group is attached.

Nomenclature of Nitroalkanes

In the IUPAC nomenclature, the nitroalkanes are named by adding prefi nitro before the name of alkane, the position of the nitro group is indicated by number.

ISOMERISM

Nitroalkanes exhibit chain and position isomerism among their own class and functional isomerism with alkyl nitrites and special type tautomerism can also exist in nitro alkanes having an α – H atom. For example, nitro compounds having the molecular formula C₄H₉NO₂ exhibit the following isomerisms.

Tautomerism:

Primary and secondary nitroalkanes, having α-H, also show an equilibrium mixture of two tautomers namely nitro – and aci – form.

Tertiary nitro alkanes do not exhibit tautomerism due to absence of α – H atom.

Acidic Nature of Nitro Alkanes

Th α – H atom of 1° & 2° nitroalkanes show acidic character because of the electron with drawing effect of NO₂ group. These are more acidic than aldehydes, ketones, ester and cyanides. Nitroalkanes dissolve in NaOH solution to form a salt. Aci – nitro derivatives are more acidic than nitro form. When the number of alkyl group attached to α carbon increases, acidity decreases. due to +I effect of alkyl groups.

Preparation of Nitroalkanes

1. From Alkyl Halides: (Laboratory Method)

(a) Alkyl bromides (or) iodides on heating with ethanolic solution of potassium nitrite gives nitroethane.

The reaction follows SN₂ mechanism.

This method is not suitable for preparing nitrobenzene because the bromine directly attached to the benzene ring cannot be cleaved easily.

2. Vapour Phase Nitration of Alkanes: (Industrial Method)

Gaseous mixture of methane and nitric acid passed through a red hot metal tube to give nitromethane.

Except methane, other alkanes (upto n – hexane) give a mixture of nitroalkanes due to C-C cleavage. The individual nitro alkanes can be separated by fractional distillation.

3. From α – Halocarboxylic Acid

α – choloroacetic acid when boiled with aqueous solution of sodium nitrite gives nitromethane.

4. Oxidation of Tert – Alkyl Amines

tert – butyl amine is oxidised with aqueous KMnO₄ to give tert – nitro alkanes.

5. Oxidation of Oximes

Oxidation of acetaldoxime and acetoneoxime with trifloroperoxy acetic acid gives nitroethane (1°) and 2 – nitropropane (2°) respectively.

Preparation of Nitroarenes

1. By Direct Nitration

When benzene is heated at 330K with a nitrating mixture (Con.HNO₃ + Con.H₂SO₄), electrophilic
substitution takes place to form nitro benzene. (Oil of mirbane)

On direct nitration of nitrobenzene m – dinitrobenzene is obtained

2. Indirect Method

Nitration of nitro benzene gives m-dinitrobenzene. The following method is adopted for the preparation of p-dinitrobenzene.

For example

Amino group can be directly converted into nitro group, using caro’s acid (H₂SO₅) (or) persulphuric acid
(H₂S₂O₈) (or) peroxytrifluro acetic acid (F₃C.CO₃H) as oxidising agent.

Physical Properties of Nitro Alkane

The lower nitroalkanes are colourless pleasant smelling liquids, sparingly soluble in water, but readily soluble in organic solvents like benzene, acetone etc… They have high boiling points because of their highly polar nature. Alkylnitrites have lower boiling points than nitro alkanes.

Chemical Properties of Nitroalkanes

Nitroalkanes undergo the following common reactions.

1. Reduction
2. Hydrolysis
3. Halogenations

1. Reduction of Nitroalkanes

Reduction of nitroalkanes has important synthetic applications. The various reduction stages of nitro group are given below.

The final product depends upon the nature of reducing agent as well as the pH of the medium.

Reduction of Alkyl Nitrites

Ethylnitrite on reduction with Sn / HCl gives ethanol

2. Hydrolysis of Nitroalkanes

Hydrolysis can be effected using conc. HCl or conc. H₂SO₄. Primary nitroalkanes on hydrolysis gives carboxylic acid, and the secondary nitroalkanes give ketones. The tertiary nitroalkanes have no reaction.

On the other hand, the acid or base hydrolysis of ethyl nitrite gives ethanol.

3. Halogenation of Nitroalkanes

Primary and secondary nitroalkanes on treatement with Cl₂ or Br₂ in the presence of NaOH give halonitroalkanes. The α – H atom of nitroalkanes are successively replaced by halogen atoms.

Toxicity

Nitroethane is suspected to cause genetic damage and be harmful to the nervous system.

Nef Carbonyl Synthesis:

Chemical Properties of Nitrobenzene

Electrolytic Reduction:

Reduction of Catalytic and Metal Hydrides

Nitrobenzene reduction with Ni (or) Pt, (or) LiAlH₄ to give aniline

Selective Reduction of Polynitro Compounds

Electrophilic Substitution Reaction

The electrophilic substitution reactions of nitrobenzene are usually very slow and vigorous reaction condition have to be employed (-NO₂ group is strongly deactivating and m – directing).

Nitrobenzene does not undergo Friedel – Craft reactions due to the strong deactivating nature of -NO₂ group.

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Uses of Carboxylic Acids and its Derivatives

Formic Acid

It is Used

1. For the dehydration of hides.
2. As a coagulating agent for rubber latex
3. In medicine for treatment of gout
4. As an antiseptic in the preservation of fruit juice.

Acetic Acid

It is Used

1. As table vinegar
2. For coagulating rubber latex
3. For manufacture of cellulose acetate and poly vinylacetate

Benzoic Acid

It is Used

1. As food preservative either in the pure form or in the form of sodium benzoate
2. In medicine as an urinary antiseptic
3. For manufacture of dyes

Acetyl Chloride

It is Used

1. As acetylating agent in organic synthesis
2. In detection and estimation of – OH, – NH₂ groups in organic compounds

Acetic Anhydride

It is Used

1. Acetylating Agent
2. In the preparation of medicine like asprin and phenacetin
3. For the manufacture plastics like cellulose acetate and poly vinyl acetate.

Ethyl Acetate is Used

1. In the preparation of artificial fruit essences.
2. As a solvent for lacquers.
3. In the preparation of organic synthetic reagent like ethyl acetoacetate.

Carboxylic acids and their derivatives are used in the production of polymers, biopolymers, coatings, adhesives, and pharmaceutical drugs. They also can be used as solvents, food additives, antimicrobials, and flavourings.

The functional groups at the heart of this chapter are called carboxylic acid derivatives: they include carboxylic acids themselves, carboxylates (deprotonated carboxylic acids), amides, esters, thioesters, and
acyl phosphates.

Carboxylic acids have a hydroxyl group bonded to an acyl group, and their functional derivatives are prepared by replacement of the hydroxyl group with substituents, such as halo, alkoxyl, amino and acyloxy. Some examples of these functional derivatives were displayed earlier.

Carboxylic acids are also important in the manufacture of greases, crayons, and plastics. Compounds with carboxyl groups are relatively easily converted to compounds called esters, which have the hydrogen atom of the carboxyl group replaced with a group containing carbon and hydrogen atoms.

In general, it finds use primary as an acylating agent (source of an acetyl group) for alcohols and amines. This liquid is also used to make pharmaceuticals such as aspirin and salicylic acid, as well as a preservative for wood.

Carboxylic acids are soluble in water. Carboxylic acids do not dimerise in water, but forms hydrogen bonds with water. Carboxylic acids are polar and due to the presence of the hydroxyl in the carboxyl group, they are able to form hydrogen bonds with water molecules.

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Functional Derivatives of Carboxylic Acids

Compounds such as acid chlorides, amides, esters etc., are called carboxylic acid derivatives because they differ from a carboxylic acid only in the nature of the group or atom that has replaced the – OH group of carboxylic acid.

Relative Reactivity of Acid Derivatives

The reactivity of the acid derivatives follows the order

The above order of reactivity can be explained in terms of

1. Basicity of the leaving group
2. Resonance effect

1. Basicity of the Leaving Group

Weaker bases are good leaving groups. Hence acyl derivatives with weaker bases as leaving groups (L) can easily rupture the bond and are more reactive. The correct order of the basicity of the leaving group is . Hence the reverse is the order of reactivity.

2. Resonance Effect

Lesser the electronegativity of the group, greater would be the resonance stabilization as shown below. This effect makes the molecule more stable and reduces the reactivity of the acyl compound. The order of electronegativity of the leaving groups follows the order – Cl > – OCOR > – OR > – NH₂

Hence the order of reactivity of the acid derivatives with nucleophilic reagent follows the order

acid halide > acid anhydride > esters > acid amides

Nomenclature

Acid Halides:

Methods of Preparation of Acid Chloride:

Acid chlorides are prepared from carboxylic acid by treating it with anyone of the chlorinating agent such as SOCl₂, PCl₅, or PCl₃

1. By Reaction with Thionyl Chloride (SOCl₂)

This method is superior to others as the by products being gases escape leaving the acid chloride in the pure state.

Physical Properties:

• They emit pale fumes of hydrogen chloride when exposed to air on account of their reaction with water vapour.
• They are insoluble in water but slowly begins to dissolve due to hydrolysis.

Chemical Properties:

They react with weak nucleophiles such as water, alcohols, ammonia and amines to produce the corresponding acid, ester, amide or substituted amides.

1. Hydrolysis:

Acyl halides undergo hydrolysis to form corresponding carboxylic acids

2. Reaction with Alcohols (Alcoholysis) gives esters.

3. Reaction with Ammonia (Ammonolysis) gives acid amides.

4. Reaction with 1° and 2° Amines gives N-alkyl amides.

5. Reduction

(a) When reduced with hydrogen in the presence of ‘poisoned’ palladium catalyst, they form aldehydes. This reaction is called Rosenmund reduction. We have already learnt this reaction under the preparation of aldehydes.

(b) When reduced with LiAlH₄ gives primary alcohols.

Acid Anhydride

Methods of Preparation

1. Heating carboxylic acid with P₂O₅

We have already learnt that when carboxylic acids are heated with P₂O₅ dehydration takes place to form
acid anhydride.

2. By Reaction of Acid Halide With a Salt of Carboxylic Acids

Acid chlorides on heating with sodium salt of carboxylic acids gives corresponding anhydride.

Chemical Properties

1. Hydrolysis

Acid anhydride are slowly hydrolysed, by water to form corresponding carboxylic acids.

2. Reaction With Alcohol

Acid anhydride reacts with alcohols to form esters.

3. Reaction With Ammonia

Acid anhydride reacts with ammonia to form amides.

4. Reaction with PCl₅

Acid anhydride reacts with PCl₅ to form acyl chlorides.

Esters

Methods of Preparation

1. Esterification

We have already learnt that treatment of alcohols with carboxylic acids in presence of mineral acid gives esters. The reaction is carried to completion by using an excess of reactant or by removing the water from the reaction mixture.

2. Alcoholysis of Acid Chloride or Acid Anhydrides

(ii) Treatment of acid chloride or acid anhydride with alcohol also gives esters.

Physical Properties

Esters are colour less liquids or solids with characteristic fruity smell. Flavours of some of the esters are given below.

Chemical Properties

They react with weak nucleophiles such as water, alcohols, ammonia and amines to produce the corresponding acid, ester, amide or substituted amides.

1. Hydrolysis

We have already learnt that hydrolysis of esters gives alcohol and carboxylic acid.

2. Reaction With Alcohol (Transesterification)

Esters of an alcohol can react with another alcohol in the presence of a mineral acid to give the ester of second alcohol. The interchange of alcohol portions of the esters is termed transesterification.

 Transesterification

3. Reaction With Ammonia (Ammonolysis)

Esters react slowly with ammonia to form amides and alcohol.

4. Claisen Condensation

Esters containing at least one ∝ – hydrogen atom undergo self condensation in the presence of a strong base such as sodium ethoxide to form β – keto ester.

5. Reaction with PCl₅

Esters react with PCl₅ to give a mixture of acyl and alkyl chloride

Acid Amides

Acid amides are derivatives of carboxylic acid in which the – OH part of carboxyl group has been replaced by – NH₂ group. The general formula of amides are given as follows image 21 Now, we shall focus our attention mainly on the study of chemistry of acetamide.

Methods of Preparation

1. Ammonolysis of Acid Derivatives

Acid amides are prepared by the action of ammonia with acid chlorides or acid anhydrides.

2. Heating Ammonium Carboxylates

Ammonium salts of carboxylic acids (ammonium carboxylates) on heating, lose a molecule of water to form amides.

3. Partial Hydrolysis of Alkyl Cyanides (Nitriles)

Partial hydrolysis of alkyl cyanides with cold con HCl gives amides

Chemical Properties

1. Amphoteric Character

Amides behave both as weak acid as well as weak base and thus show amphoteric character. This can be proved by the following reactions.

Acetamide (as acid) reacts with sodium to form sodium salt and hydrogen gas is liberated. 

3. Dehydration

Amides on heating with strong dehydrating agents like P₂O₅ get dehydrated to form cyanides.

4. Hof Mann’s Degradation

Amides reacts with bromine in the presence of caustic alkali to form a primary amine carrying one carbon less than the parent amide.

5. Reduction

Amides on reduction with LiAlH₄ or Sodium and ethyl alcohol to form corresponding amines.

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Acidity of Carboxylic Acids

Carboxylic acids undergo ionisation to produce H⁺ and carboxylate ions in aqueous solution. The carboxylate anion is stabilised by resonance which make the Carboxylic acid to donate the proton easily.

The resonance structure of carboxylate ion are given below.

The strength of carboxylic acid can be expressed in terms of the dissociation constant(Ka):

The dissociation constant is generally called acidity constant because it measures the relative strength of an acid. The stronger the acid, the higher will be its Ka value.

The dissociation constant of an acid can also be expressed in terms of pKa value.

pKa = – log Ka

A stronger acid will have higher Ka value but smaller pKa value.

Ka and pKa values of some Carboxylic acids of 298 K

Effect of substituents on the acidity of carboxylic acid.

(i) Electron Releasing Alkyl Group Decreases the Acidity

Th electron releasing groups (+I groups) increase the negative charge on the carboxylate ion and destabilise it and hence the loss of proton becomes difficult. For example, formic acid is more stronger than acetic acid.

(ii) Electron with Drawing Substituents Increases the Acidity

The electron – withdrawing substituents decrease the negative charge on the carboxylate ion and stabilize it. In such cases, the loss of proton becomes relatively easy. Acidity increases with increasing electronegativity of the substituents. For example, the acidity of various halo acetic acids follows the order

F – CH₂ – COOH > Cl – CH₂ COOH > Br – CH₂ – COOH > I – CH₂ – COOH

Acidity increases with increasing number of electron – withdrawing substituents on the α – carbon. For example

Cl₃C – COOH > Cl₂CH – COOH > ClCH₂COOH > CH₃COOH

The effect of various, electron withdrawing groups on the acidity of a carboxylic acid follows the order,

– NO₂ > – CN > – F > – Cl > – Br > – I > Ph

The relative acidities of various organic compounds are

RCOOH > ArOH > H₂O > ROH > RC ≡ CH