+2 UNIT 14 PAGE- 2
Relative reactivities of aldehydes and ketones
For maximum reactivity towards nucleophiles, the carbonyl carbon atom should be positive as possible and not sterically hindered by adjacent groups. The ketones have two alkyl groups on carbonyl carbon while aldehydes have one alkyl group only. Each alkyl group being electron-releasing decreases the positivity of carbon atom and on account of its bulk offers steric hindrance to approaching reagents. For both these reasons, the nucleophilic attack on the carbonyl carbon retarded. Thus the carbonyl group in ketones being influenced by two alkyl groups is less reactive than in aldehydes where the carbonyl group is under the influence of one alkyl group only. Formaldehyde having no alkyl group on carbonyl carbon is more reactive than all other aldehydes which are again more reactive than the ketones.
As we go higher in the series of carbonyl compounds, the electron releasing power of the alkyl group increases and their shielding effect enhanced. Thus the reactivity of the carbonyl group in them is progressively decreased. For example,
Problem
01. Arrange the following carbonyl compounds in increasing order of their reactivity in nucleophilic addition reactions.
(a) Ethanal, propanal, propanone, butanone.
(b) Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, acetophenone
Acidity of a-hydrogen atom
Another type of reactions of the carbonyl compounds are due to the acidity of the hydrogen atoms on carbon adjacent to carbonyl, or a-carbon atom. The acid character of the a-hydrogen atoms (attached to a-carbon) is explained as follows :
The carbonyl carbon is positive and therefore attracts the electrons in the single bond shared with a-carbon. The a-carbon in turn pulls towards it the electrons in the next single bond joining hydrogen. The drift of the electrons away from the hydrogen atom weakens the carbon-hydrogen bond. Thus there is a tendency for a-hydrogen atom to split off as proton in the presence of strong basic reagents.
The removal of proton leaving an electron pair gives a carbanion or enalolate ion which is resonance stabilised. The two contributing forms to the hybrid are :
The carbanion thus produced is a good nucleophile and can attack carbonyl group of another molecule. The formation of the carbanion followed by its addition to a carbonyl group, is the process involved in all the condensation reactions of aldehydes and ketones.
Benzaldehyde having the aldehyde group(-CHO) attached to the aryl ring undergoes most of the general reactions of the aliphatic aldehydes in which a-hydrogen is not involved. However, conjugation of the carbonyl carbon with the aryl ring reduces the electrophilic reactivity of the carbonyl-carbon atom due to delocalisation of p-electrons. Hence in general aromatic aldehydes react less readily at the CHO group than their aliphatic counter parts.
Also, the absence of a-hydrogen atom in benzaldehyde makes it possible to employ stronger basic or acidic catalysts in condensation reactions. Thus it undergoes a number of condensations which are not possible with aliphatic aldehydes.
The reactions of aldehydes and ketones can be discussed under the following heads :
1. Addition across >C=O bond.
2. Replacement of carbonyl oxygen by other groups.
3. Oxidation
4. Reduction
5. Miscellaneous reactions.
1. ADDITION ACROSS >C=O BOND
Some of the important addition reactions are discussed below.
(a) Addition of hydrogen cyanide : Both the aldehydes and ketones react with hydrogen cyanide to form addition products known as cyanohydrins. The reaction is carried out in presence of a base which also catalyses the reaction.
The reaction is believed to proceed as follows. The positively polarised carbon of carbonyl group is attached by strongly nucleophile CN- with simultaneous transfer of p-electrons to oxygen. The oxygen then combines with H+ to give the cyanohydrin.
The hydrogen of cyanohydrins either in acid or basic solution will convert the nitrile group to the carboxylic acid. Since cyanohydrins are a-hydroxynitriles, this provides a useful route for the preparation of a-hydroxy acids. For example,
Cyanohydrins are used to prepare a-hydroxy acids and a,b-unsaturated acids.
Lactic acid on subsequent heating in presence of sulphuric acid yields a,b-unsaturated acid.
( c) Addition of sodium bisulphite
Saturated solution of sodium bisulphite(NaHSO3) in water , when mixed with aldehydes and some ketones forms crystalline ‘bisulphite addition compounds’. Thus,
Almost all aldehydes form bisulphite addition compounds , but only a few ketones of the type CH3COR, where R is a primary alkyl group up to C3 in size, undergo this addition. Ketones with bulky alkyl groups fail to react with sodium bisulphite presumably because of steric hindrance and shielding the electrophilic carbon atom of the the carbonyl group.
The bisulphite addition compounds get decomposed back to the original carbonyl compounds in presence of acids or alkalies. Thus,
Hence the formation and decomposition of the bisulphite compounds serves as a powerful means of purification and separation of carbonyl compounds from non-carbonyl substances.
( c) Addition of Grignard reagent
Almost all aldehydes and ketones react with Grignard reagent to form complex adducts,
These upon hydrolysis with acid yield alcohols.
Thus the addition of Grignard reagents to carbonyl compounds provides an excellent route for the synthesis of alcohols.
In the above reaction :
· Formaldehyde forms primary alcohol.
· Aldehydes other than formaldehyde form secondary alcohols.
· Ketones form tertiary alcohols.
* Formaldehyde and methyl magnesium bromide yield ethyl alcohol.
* Acetaldehyde and methyl magnesium bromide yield iso-propyl alcohol.
* Acetone and methyl magnesium bromide yield tert-Butyl alcohol.
d) Addition of alcohols
Aldehydes react with alcohols in the presence of dry HCl gas to form acetals. In this reaction, the addition of one molecule of alcohol to one molecule of aldehyde results in the formation of hemiacetal. A hemiacetal contains both an ether as well as alcohol functional groups. It is an unstable compound and cannot be isolated. It further reacts with alcohol to form stable acetal.
Acetals are dialkoxy compounds and have properties similar to ethers. These are quite stable to alkali, but can be cleaved by acids to regenerate aldehydes. Hence acetal formation can be used to protect the aldehyde group against alkaline oxidising agents
Ketones do not ordinarily react with monohydric alcohols, however, they combine with diols to give cyclic ketals.
2. Replacement of carbonyl oxygen atom with other groups
In these reactions oxygen atom of carbonyl group is replaced by either one divalent group or two monovalent groups.
(a) Reaction with ammonia derivatives GNH2
Certain derivatives of ammonia which contain the primary amine group -NH2, add to the carbonyl group of aldehydes and ketones to form unstable intermediates. These intermediates lose a molecule of water to yield the respective ‘condensation products.
G stands for the general group bonded to -NH2 group in the ammonia detivative. The common ammonia derivatives used for the reaction and the products obtained are listed below.
The reaction product in each case is named as oxime (aldoxime or ketoxime), phenyl hydrazone or semicarbazone of the carbonyl compound from which it was obtained. Thus the various ammonia derivatives react with acetaldehyde and acetone as follows.
The product of reaction of aldehydes and ketones with the various ammonia derivatives are mostly crystalline solids having sharp melting points. For this reason, they are frequently employed for the characterization and identification of carbonyl compounds. The semicarbazones are usually high melting solids than are the oximes or phenyl hydrazones. If the oxime or phenyl hydrazone of an unknown carbonyl compound is a low melting solid or oil, the semicarbazone will usually serve to identify it. If not, 2,4-dinitrophenyl hydrazine and the product 2,4-dinitrophenyl hydrazone has a high melting point.
(b) Reaction with ammonia
Like ammonia derivatives, ammonia also reacts with aldehydes (except formaldehyde) and ketones to form the products called imines.
However, formaldehyde reacts with ammonia to form hexamethylene tetramine (CH2)6N4 also known as urotropine as shown below:
Urotropine is used as medicine to treat urinary infections.
Acetone reacts with ammonia to form diacetonamine.
Unlike aliphatic aldehydes and ketones, benzaldehyde on reaction with ammonia does not form a simple aldehyde-ammonia, but a more complex condensation product called hydrobenzamide.
( c) Reaction with primary amines
Aldehydes and ketones react with primary amines to form Schiff’s base.
(d) Reaction with PCl5 or SOCl2 (thionyl chloride) Aldehydes and ketones react with PCl5 or thionyl chloride(SOCl2) to form gem-dihalide.
3. OXIDATION
(a) Oxidation of aldehydes
Aldehydes are easily oxidised to carboxylic acids containing the same number of carbon atoms as in parent aldehyde.
The reason for this easy oxidation is the presence of a hydrogen atom on the carbonyl carbon which can be converted into -OH group without involving the cleavage of any other bond. Hence, aldehydes are oxidised not only by strong oxidising agents like KMnO4 and K2Cr2O7 but also by weak oxidising agents like bromine water, Ag+, Cu2+ etc. As a result, aldehydes act as strong reducing agents. They reduce :
(i) Tollen’s reagent (ammoniacal silver nitrate) to metallic silver.
(ii) Fehling solution (alkaline CuSO4 solution containing rochelle salt) to red precipitate of Cu2O.
(i) Reduction of Tollen’s reagent
Tollen’s reagent is ammoniacal solution of silver nitrate. On warming with this reagent, aldehydes form silver mirror on the walls of the container. The reaction is known as mirror test for aldehydes. The chemical reactions involved are :
(ii) Reduction of Fehling solution
Fehling’s solution is an alkaline solution of copper sulphate containing sodium potassium tartrate(Rochelle’s salt) as the complexing agent. Aldehydes on warming with this solution, give a red precipitate of cuprous oxide.
Since ketones are not oxidised by weak oxidising agents, such as Tollen’s reagent or Fehing solution, therefore such reagents may be used to distinguish between aldehydes and ketones.
(b) Oxidation of ketones : Ketones are oxidised only under vigorous conditions using powerful oxidising agents such as Con HNO3. KMnO4/H2SO4 , K2Cr2O7/H2SO4 etc. Oxidation of ketones involves cleavage of bond between carbonyl carbon and a-carbon on either side of keto group giving a mixture of carboxylic acids.
In the case of unsymmetrical ketones, the point of cleavage is such that keto group stays with smaller alkyl group preferentially. This is illustrated by the example given below:
(c ) Oxidation with Sodium hypohalite(NaOX) or (X2 + NaOH)
This reaction is given by acetaldehyde or methyl ketones and is known as Haloform reaction. For example, when acetaldehyde or methyl ketone is treated with sodium hpoiodite (I2 + aqueous NaOH) a yellow precipitate of iodoform is formed.
Acetaldehyde is the only aldehyde which gives haloform reaction.
The haloform reaction is used as a diagnostic test for the presence of -COCH3 group. For this purpose, the reaction is carried with iodine and alkali because, the iodoform produced being a yellow crystalline solid is then precipitated. This is known as Iodoform Test and is given by methyl ketones, acetaldehyde and compounds such as CH3CHOHR and CH3CH2OH which are oxidised to appropriate carbonyl compounds under conditions used for the reaction.
The haloform reaction is useful for distinguishing methyl ketones from other ketones. It is also used for the preparation of carboxylic acid with one carbon atom less than the original methyl ketone.
4.REDUCTION OF ALDEHYDES AND KETONES Aldehydes and ketones can be reduced to a variety of compounds under different conditions and different reducing agents.
(a) Reduction to alcohols
Aldehydes and ketones on mild reduction give primary and secondary alcohols respectively. This type of reduction is carried out either catalytically with H2 in presence of Ni, Pt or Pd or chemically by Lithium aluminium hydride(LiAlH4) or Sodium borohydride(NaBH4).
Some examples are :
It may be noted that LiAlH4 is more powerful reducing agent than NaBH4. It not only reduces aldehydes and ketones but also reduces esters, acids and nitriles as well. It is also specific that it reduces only C=O bond but not C=C bond which can otherwise be reduced by the use of H2 /Ni.
Ketones can also be reduced to secondary alcohols with aluminium isopropoxide in 2-propanol solution.
The reaction is called Meerwein-Ponndorf Reduction.
(b) Reduction to Hydrocarbons : The carbonyl group( >C=O ) can be reduced to methylene(>CH2) group resulting in the formation of alkanes by any one of the following reagents:
i) Zn(Hg)/HCl (Clemensen's reduction).
ii) NH2NH2 / KOH ( At 455-475 K)
(Wolff-Kishner reduction).
iii) HI / P at 424 K
Aldehydes and ketones are reduced to corresponding hydrocarbons by treatment with zinc amalgam and hydrochloric acid. This reaction is known as Clemensen Reduction.
For example,
When the carbonyl compound is heated with hydrazine and potassium hydroxide in high boiling polar solvent(ethylene glycol), the carbonyl group is reduced to methylene group(>CH2) giving hydrocarbons. This reaction is called Wolf-Kishner Reduction.
(b) Reduction to Pinacols
Ketones when reduced in neutral or alkaline medium, form 1,2-diols or pinacols. Thus with magnesium amalgam and water acetone is reduced as follows:
Pinacol-pinacolone rearrangement
Upon treatment with hot dil H2SO4, pinacol undergoes a rearrangement and dehydration to form a monoketone called pinacolone.
Miscellaneous Reactions
9. Reaction with Alkalies
a) Aldol Condensation
Two molecules of an aldehyde or a ketone having at least one a-hydrogen atom, condense in the presence of a dilute alkali to give b-hydroxyaldehyde or b-hydroxy ketone. The reaction is called Aldol Condensation. It involves the formation of bond between carbonyl carbon of one molecule and a-carbon atom of the other molecule.
The products of aldol condensation when heated with dilute acids undergo dehydration to form a-b-unsaturated aldehydes or ketones.
In general, all aldehydes and ketones which contain a-hydrogen atom can undergo this reaction. Those which do not contain a-hydrogen like HCHO, C6H5CHO etc. do not undergo this reaction.
(c) Crossed Aldol condensation
Aldol condensation is not confined to similar molecules of aldehydes and ketones. It can also take place between different aldehydes and ketones. This type of condensation is called Crossed Aldol condensation or mixed aldol condensation. For example, acetaldehyde undergoes this type of condensation with acetone in the presence of KCN.
Crossed Aldol Condensation can also occur between carbonyl compound having no a-hydrogen atom with aldehydes and ketones possessing a-hydrogen atom. In this case, the carbon atom having a-hydrogen atom attaches itself to the carbonyl group of the molecule which does not have a-hydrogen atom as described in the reaction between benzaldehyde and acetaldehyde.
Similarly , acetone reacts with benzaldehyde in alkaline medium to form dibenzalacetone.
(d) Cannizaro's reaction
Aldehydes which do not have a-hydrogen atom, such as formaldehyde and benzaldehyde, when warmed with concentrated (50%) alkali solution, give a mixture of alcohol and salt of a carboxylic acid.
In this reaction , the aldehyde undergoes disproportionation. It implies that one molecule of the aldehyde is oxidised to carboxylic acid and the other is redced to alcohol. Ketones do not give this reaction.
The reaction is also possible in all compounds which lack a-hydrogen atom. For example,
In crossed Cannizaro reaction, when an aldehyde is treated with concentrated alkali give brown resinous mass.
10. Perkin reaction
Perkin observed that when aromatic aldehydes heated with anhydride of an aliphatic acid in presence of the sodium salt of the same acid, an a-b-unsaturated acid is formed. This reaction is known as Perkin reaction and is shown mainly by aromatic aldehydes. For example, benzaldehyde , when heated with acetic anhydride in the presence of sodium acetate, forms cinnamic acid.
11. Reaction with acids
(a) Formation of Phorone
When acetone is saturated with hydrogen chloride gas is kept at 273 K for about a fortnight, it forms mesityl oxide and Phorone.
Two molecules of acetone condense to form mesityl oxide.
Three molecules of acetone condenses in presence of dry HCl to form phorone(2,6-Dimethyl-hepta-2,5-dien-4-one).
(b) Formation of Mesitylene
Three molecules of acetone on refluxing with concentrated sulphuric acid produces mesitylene as one of the products.
12. Substitution Reactions in Benzene nucleus in Aldehydes and Ketones : Aldehydic and keto groups are meta directing groups and therefore substitution reactions occur at meta positions. For example,
(i) Halogenation
(ii) Nitration
(iii) Sulphonation
COMMERCIALLY IMPORTANT CARBONYL COMPOUNDS
The commercially important carbonyl compounds are methanal, ethanal, benzaldehyde and propanone.
1. Formaldehyde (methanal)
It is manufactured by passing methanol vapours mixed with air over heated copper at 825-875 K.
Formaldehyde is a gas and is extremely reactive. Its boiling point is 252 K. Formaldehyde readily undergoes polymerisation yielding different products under different conditions.
Paraformaldehyde
When an aqueous solution of formaldehyde is evaporated to dryness, a white crystalline solid with fishy odour and m.p 394-396 K is obtained. It has a long chain polymer with formula (-CH2-O-CH2-O-) or (CH2O)n . H2O, where the value of n may vary from 6 to 50.
Metaformaldehyde
When allowed to stand at room temperature or its 60% aqueous solution is treated with 2% sulphuric acid, a white solid with melting point 334-335 K is formed. This is known as metaformaldehyde, trioxy methylene or trioxane.
Methanal can be readily obtained from these polymers by gentle warming.
USES
i) Methanal is used in the manufacture of bakelite, resins and other polymers.
ii) Its 40% aqueous solution known as formalin (40% formaldehyde, 8% methanol and 52 % water ) is used for preserving biological specimens.
iii) Formaldehyde is also used as an antiseptic.
2. Acetaldehyde ( Ethanal)
(i) Acetaldehyde can be prepared on a large scale by the dehydrogenation of ethyl alcohol in presence of copper catalyst at 573 K.
(ii) Acetaldehyde can be manufactured by the hydration of acetylene in the presence of 42% sulphuric acid containing mercuric sulphate at 330 K. On heating the mixture, acetaldehyde passes over and can be collected.
Acetaldehyde polymerises in presence of few drops of Con. H2SO4 and forms a trimer(paraldehyde).
Uses of acetaldehyde
i) Ethanal is used in the preparation of a number of organic compounds like acetic acid, ethyl acetate and n-butyl alcohol.
ii) Paraldehyde is used in medicine as a hypnotic.
iii) It is used in silvering of mirrors.
3. Acetone (propanone)
i) Acetone is commercially prepared by the dehydrogenation of 2-Propanol (isopropyl alcohol) in the presence of heated copper at 573 K.
ii) By the dry distillation of calcium acetate.
Uses of acetone
1. It is used as a solvent in the manufacture of smokeless powdwers (cordite) , celluloid etc.
2. It is one of the ingredients of liquid nail polish.
4. Benzaldehyde
It is manufactured by heating benzilidene chloride (obtained by chlorination of toluene) with water at 370 K in presence of iron powder as catalyst.
Benzaldehyde is a colourless oily liquid having almond odour.
Uses of benzaldehyde
i) In perfume industry.
ii) In the manufacture of dyes like malachite green.
iii) As a starting material for the synthesis of other organic compounds like benzoyl chloride, cinnamic acid etc
Distinguishing Tests of Aldehydes and ketones.
Test | Aldehydes | Ketones |
1. Tollen’s reagent test 2. Fehling solution test 3. Schiff’s test 4. with NaOH 5. With alcohols in presence of dry HCl | gives silver mirror give red ppt give pink colour give brown resinous mass(HCHOexceptin) forms acetals easily. | No silver mirror no ppt no colour no change do not form ketals easily. |
Schiff's Reagent : Schiff's reagent is an aqueous solution of magenta or pink coloured rosaniline hydrochloride which has been decolourised by passing sulphur dioxide. When aldehydes are treated with decolourised solution of Schiff's reagent, its pink or magenta colour is restored. This reaction is used as a test for aldehydes because ketones do not restore the pink colour of Schiff's reagent.
Problem
02. An organic compound (A) with molecular formula C9H10O forms orange-red precipitate with 2,4-DNP reagent gives yellow precipitate on heating with iodine in presence of sodium hydroxide. It does not reduce Tollen’s reagent or Fehling solution, nor it decolourises bromine water or Baeyer’s reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2. Identify the compounds (A) and (B) and explain the reactions involved.
CARBOXYLIC ACIDS
Carboxylic acids are compounds containing carboxyl group -COOH in their molecules. Since the group contains a carbonyl and hydroxyl group, it is called a carboxyl group. These acids are known as mono-, di-, tri-, or poly- carboxylic acids according to the number of carboxyl groups present in the molecule. They are saturated as well as unsaturated.
The monocarboxylic acids are called fatty acids as many higher members (stearic acid C17H35COOH, palmitic C15H31COOH, Oleic acid C17H33COOH etc) occur as glycerides in oils and fats and also resemle fats in some physical properties. The fatty acids are monobasic acids.
Nomenclature
1. Trivial name : Lower members are commonly known by Trivial names derived from the source of individual acids. Eg.,
Formula | Source | Trivial name |
HCOOH | red ant (formica) | formic acid |
CH3COOH | vinegar(acetum) | acetic acid |
C3H7COOH | butter(butyrum) | butyric acid |
C4H9COOH | root of valerian plant | valeric acid |
C11H23COOH | laurel oil | lauric acid |
In the case of 
-substituted acids, the positions of the substituents are indicated by Greek letters -a, b, g, d, etc. For example,
-substituted acids, the positions of the substituents are indicated by Greek letters -a, b, g, d, etc. For example,
2. Derived names : Sometimes fatty acids are named as alkyl derivatives of acetic acid. For example,
3. IUPAC Names : According to this system, the acids are named after the corresponding alkane by replacing the ending 'e' by 'oic acid'.The monocarboxylic acids are named as alkanoic acids. Carboxyl carbon is always given number 1 , while numbering the carbon atoms of the parent chain. The position of the substitutents is indicated by numbers. For example,
The Common and IUPAC names of acids are given in the following Table.
The names of aromatic acids are based on benzoic acid.
Preparation of carboxylic acids
1. From Alcohols, Aldehydes or Ketones
By the oxidation of alcohols, aldehydes or ketones with dichromate solution, fatty acids are obtained.
2. From Cyanides
Fatty acids can be prepared in good yield by the hydrolysis of alkyl cyanides with acids or alkali.
3. From Grignard Reagent
By the action of carbon dioxide on Grignard reagent , the fatty acids can be prepared
4. From Trihalogen Derivatives
By the hydrolysis of trihaloalkanes containing three halogen atoms attached to the same carbon atom, fatty acids can be formed in small amounts
5. From sodium alkoxide
By the action of carbon monoxide on sodium alkoxide under pressure, sodium salt of fatty acid is obtained, which on hydrolysis with dilute mineral acid forms fatty acids.
Preparation of aromatic acids
1. From Alkyl benzenes : By the oxidation of alkyl benzenes with alkaline KMnO4 or CrO3 in acetic acid. Example,
2. From Benzyl alcohol or Benzaldehyde : By the oxidation of benzyl alcohol with alkaline KMnO4 or dil. Nitric acid or chromic acid.
3. From Phenyl cyanide : By hydrolysis with dil. mineral acid. Thus
4. From Grignard Reagent : By treating phenyl magnesium bromide with CO2 followed by hydrolysis.
Properties of carboxylic acids
Physical Properties
i) The first three members of the carboxylic acids are colourless and have pungent smell. The next six members are oily liquids with faint unpleasant odour. Thereafter, they are colourless waxy solids. Benzoic acid and its homologues are colourless solids.
ii) The first four members are very soluble in water and the solubility decreases gradually with rise in molecular mass. However, all are soluble in alcohol or ether. Benzoic acid is sparingly soluble in cold water but is soluble in hot water, alcohol ether etc.
iii) Lower members of carboxylic acids (up to C-10) are volatile in steam while higher members are non-volatile.
iv) They have higher boiling points than the corresponding alcohols of comparable molecular masses. This is due to the fact that the pair of carboxylic acid molecules (known as dimers) are held together by hydrogen bonds. Carboxylic acids have higher boiling points than alcohols of comparable molecular masses. For example, the boiling point of ethanoic acid is 391 K, whereas that of propanol is 370 K(both have the molecular mass = 60). The higher boiling point of carboxylic acid as compared with alcohol is due to greater hydrogen bonding in acids than in alcohols. As a result, the molecules of carboxylic acids are held together by hydrogen bonds and have more attractive forces and therefore have higher boiling points
Chemical Properties of Carboxylic Acids
The important chemical properties of carboxylic acids are discussed below :
1. Acidic Character
Carboxylic acids are distinctly acidic. They ionise in water to give hydronium ion as :
However, the acidic strength of carboxylic acids is much less than that of mineral acids. The strength of acids depend upon the extent of ionisation which in turn depends upon the stability of the anion formed. Carboxylic acids are acidic because the carboxylate ions are stable and hence carboxylic acids have greater tendency to ionise to form stable carboxylate ions. This may be illustrated as follows :
The carboxyllic acid molecule has the resonance stabilised structiure. It is a resonance hybrid of the following structures.
It is clear that in resonance structure II , the oxygen atom of the hydroxyl group carries some positive charge. Consequently, the electron pair of the O-H bond is displaced towards oxygen atom. The displacement of electrons causes the release of a proton and carboxylate ion RCOO- is formed.
The carboxylate ion thus formed is also a resonance hybrid of structures III and IV as shown below.
Thus we observe that carboxylic acid as well as its anion are resonance stabilised. However, if we compare these structures, we observe that carboxylic acid structures (I & II) are not equivalent while the carboxyl ion structures (III and IV) are equivalent. Thus carboxylic acids readily gives proton to form stable carboxylate ion.
It may be noted that phenols are also acidic because phenoxide ion can be stabilised by delocalisation of negative charge into the ring. However, phenols are less acidic than carboxylic acids.
The difference in the relative strengths can be understood if we compare the resonance hybrids of carboxylate ion and phenoxide ion.
The resonance hybrid may be represented as :
The electron charge in the carboxylate ion is more dispersed in comparison to the phenoxide ion, since there are two electronegative oxygen atoms in the carboxylate ion as compared to only one oxygen atom in the phenate ion. In other words, the carboxylate ion is relatively more stable as compared to phenate ion. Thus the release of H+ ion from the carboxylic acid is comparatively easier or it behaves as a strong acid than phenol.
Effect of substituents on acidic strength of carboxylic acids
The substituents have a marked effect on the acidity of carboxylic acids.
a) Electron releasing substituents
Alkyl group is an electron releasing group. If the H atom of formic acid is replaced by -CH3 group to form acetic acid(CH3COOH), the alkyl group will tend to increase the electron density on the oxygen atom of the O-H bond. Consequently, the release of H+ ion in acetic acid will be more difficult as compared with formic acid. Apart from this, the methyl group will also destabilise the acetate ion relative to the formate ion. Thus the release of H+ ion from acetic acid will be difficult as compared to formic acid or the former is a weak acid. In general, greater the + I effect of the alkyl group attached to the carboxyl group, lesser will be the acidic strength of the carboxylic acid. The +I effect of the alkyl groups increases in the order :
-CH3 < -C2H5 < (CH3)2CH- < (CH3)3C-.
Therefore acetic acid is stronger than propionic acid CH3CH2COOH, which is still stronger than isobutyric acid (CH3)2CHCOOH and so on.
b) Electron withdrawing substituents
The electron withdrawing substituents such as halogen atom will tend to withdaw the electron charge when attached to the carboxylic acid at a specific position. Consider the example of chloroacetic acid. Chlorine atom is an electron withdrawing atom (-I Inductive effect). It withdraws the electrons from the carbon to which it is attached and this effect is transmitted throughout the chain. As a result, the electrons are withdrawn more strongly towards oxygen of O-H bond and promotes the release of proton. Consequently, acidic strength increases. Therefore, chloroacetic acid is stronger than acetic acid.
Now , the Inductive effect increases with increase in the number of chlorine atoms and therefore acid strength also increases.
Thus the acidic strength decreases in the order:
CCl3COOH > CHCl2COOH >CH2ClCOOH > CH3COOH
The electron withdrawing effect of halogens decreases very rapidly with distances from the carboxyl group. Thus, with increasing distance between halogen and carboxyl group, the acid strength decreases as it is evident from the following data.
Acid | Ka x 10-5 |
CH3CH2CH(Cl)COOH | 139 |
CH3CH(Cl)CH2COOH | 8.9 |
CH2ClCH2CH2COOH | 2.96 |
Also stronger the electron withdrawing group, greater is the strength of the acid. Thus fluoroacetic acid is stronger than chloroacetic acid and the latter is stronger than bromo and iodoacetic acid. Aromatic acids like benzoic acid is stronger than simple aliphatic acids because phenyl group is more electron withdrawing than alkyl group.
2. Salt Formation : Carboxylic acids are attacked by electropositive metals such as sodium, potassium and zinc to form their salts with the liberation of hydrogen.
3. Action with alkalies and carbonates : Carboxylic acids neutralise alkalies forming salts and decompose carbonates(or bicarbonates) evolving carbon dioxide with effervescence.
4. Formation of Functional Derivatives : The -OH group of carboxylic acids can be replaced by a number of groups such as -Cl, -OR, -NH2 and -OOCR to form acid chlorides, esters, amides and anhydrides. These compounds are collectively called Functional derivatives of acids.(a) Esters : (RCOOR') : These are obtained by treating acids with alcohols in the presence of Con. Sulphuric acid or dry hydrogen chloride.
(b) Acid Chloride : (RCOCl ) : These are obtained when acids are treated with phosphorus penta chloride or thionyl chloride.
(c) Acid Amides : (RCONH2) : These are obtained from the acids by treating them with ammonia followed by heating the ammonium salts.
(d) Acid anhydrides : (RCO)2O : An acid anhydride is obtained by treating a carboxylic acid with the corresponding acid chloride in the presence of pyridine.
5. Decarboxylation : Carboxylic acids get decarboxylated i.e., lose carbondioxide when their sodium salts are heated with soda lime (NaOH + CaO ). The condition for decarboxylation depends on their structures.
When two carboxyl groups are attached to the same carbon atom, decarboxylation takes place simply on heating.
Alkali metal salts of carboxylic acids undergo decarboxylation by electrolysis. This method is known as Kolbe’s electrolysis.
6. Reduction : Carboxylic acids on reduction with LiAlH4 in ether are reduced to alcohols. LiAlH4 is a mild reducing agent.
However, carboxylic acids on drastic reduction with concentrated hydroiodic acid and red phosphorus under high pressure yield hydrocarbon as :
6. Ring substitution in aromatic acids
Carboxyl group in benzoic acid is electron with drawing group and therefore, it is meta-directing group. Some common electrophilic substitution reactions of benzoic acid are :
(a) Bromination :
(b) Sulphonation
(c) Nitration
8. Reactions of alkyl group of carboxylic acids
a. Oxidation : When acids are treated with mild oxidising agents such as hydrogen peroxide, the alkyl group is oxidised at the b-position. For example:
Carboxylic acids can be oxidised at the a-position by oxidising agents like selenium oxide, SeO2. Ketoacids are obtained as product in this reaction.
(b) Hell Volhard Zelinsky (HVZ) reaction
(a-Halogenation of aliphatic acids)
When carboxylic acids are treated with Cl2 or Br2 in the presence of red phosphorus . the a-hydrogen atoms of carboxylic acids are replaced by chlorine or bromine. The reaction does not stop at monosubstitution but continues till all the a-hydrogens are replaced. For example:
Function of red phosphorus
Its function is to convert carboxylic acid to an acid halide, which being more reactive than the parent acid and undergoes a-halogenation easily.
This reaction has a great synthetic importance as the halogen atom can be replaced by a number of other groups giving useful products. Some examples are :
