Aldehydes, Ketones And Carboxylic Acids

- Notes for the chapter to serve as a quick revision companion!
1. Functional Groups
Aldehydes, ketones and carboxylic acids are organic compounds that contain a carbonyl group (carbon and oxygen with a double bond, >C=O).
  • In aldehydes, the carbonyl group is attached to one H atom, and one C atom of an alkyl or aryl group.
  • In ketones, the carbonyl group is attached to two C atoms belonging to two different alkyl or aryl groups.
  • In carboxylic acids, the carbonyl group is attached to one C atom of an alkyl or aryl group, and an O atom of the hydroxyl moiety (-OH). The word 'carboxyl' is a combination of carbonyl (>C=O) and hydroxyl (-OH).
2. Structure and Nature of Carbonyl Group (>C=O)
Let's look at the structural features and natureof the carbonyl group of aldehydes and ketones:
2.1. Structure of Carbonyl Group of Aldehdyes and Ketones
  • The carbonyl C atom is hybridised.
  • It is bonded to 3 other atoms through 3 bonds.
  • The 4th valence electron of the carbonyl carbon remains in p-orbital. This unhybridised p-orbital overlaps with a p-orbital of O to form a bond.
  • The bond angles of the carbonyl carbon and the 3 other atoms to which it is attached are approximately .
  • The structure formed is trigonal coplanar structure.
  • Apart from this, the O atom has 2 non-bonding electron pairs.
  • The C atom of the carbonyl group, and the three atoms it is attached to lie in one plane, while the electron cloud exists above and below the plane.
2.2. Nature of Carbonyl Group of Aldehdyes and Ketones
  • The O atom in >C=O is more electronegative than carbon.
  • It pulls the shared electrons closer, and acquires partial negative charge, while the C atom acquires partial positive charge.
  • Overall, the >C=O bond is polarised.
  • As a result of this, the C atom of >C=O is electrophilic, while the carbonyl oxygen is nucleophilic.
  • Because of the polar nature, the molecules of aldehydes and ketones have high dipole moments and are more polar than alkyl halides and ethers.
3. Structure and Nature of Carboxyl Group (-COOH) 3.1. Structure of Carboxyl Group of Carboxylic Acids
  • The carboxyl group of carboxylic acids comprises of a carbonyl group (>C=O) and a hydroxyl group (-OH).
  • In the carbonyl group, both C and O are hybridised.
  • The O atom of hydroxyl group is also hybridised.
  • The carboxyl group has a trigonal planar structure.
  • One of the lone pair of electrons of oxygen conjugates with the system of the >C=O group.
  • The bond angles of the carboxyl carbon are approximately .
3.2. Nature of Carboxyl Group of Carboxylic Acids
  • As stated earlier, the carboxyl group of carboxylic acids is made of a carbonyl group (>C=O) and a hydroxyl group (-OH).
  • The carbonyl group of carboxylic acids behaves differently from that of aldehydes and ketones.
  • The carbon in >C=O of carboxylic acids is less electrophilic than that of aldehydes and ketones.
  • This is because of the presence of resonance in the molecule, which makes the C atom of the carbonyl group less positive, and hence, less electrophilic.
1. Nomenclature of Aldehydes
1.1. Common Names of Aldehydes
  • Aldehydes are often called by their common name.
  • The common name is derived from the common name of the corresponding carboxylic acids.
  • The 'ic' of the carboxylic acid common name is repaced with 'aldehyde'.
  • The names reflect the Greek or Latin terms for the source of the acid.
  • Location of substituent in the parent carbon chain is indicated by Greek letters , , , , and so on.
  • -carbon is the one directly linked to the aldehyde group, -carbon the next, and so on.
  • Example: : Acetaldehyde (Derived from the name of the corresponding carboxylic acid - acetic acid). Other examples of common names of aldehydes include benzaldehyde, -Bromobutyraldehyde.
1.2. IUPAC Names of Aldehydes
a) For aliphatic aldehydes
  • IUPAC names of aliphatic aldehydes are drived from the names of corresponding alkanes.
  • The '-e' from the name of alkane is replaced with '-al'.
  • The longest carbon chain is numbered starting from the carbon of the carbonyl group if no other high priority group is present.
  • Substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain.
b) For aromatic aldehydes
  • For aldehyde group attached to a cycloalkane, the suffix 'carbaldehyde' is added after the full name of the cycloalkane.
  • Numbering of carbon atoms of cycloalkane starts from the carbon atom attached to the aldehyde group.
  • Name of simplest aromatic aldehyde carrying the aldehyde group on a benzene ring is benzenecarbaldehyde. But, in this case, the common name benzaldehyde is also accepted by IUPAC.
  • Other aromatic aldehydes are named as substituted benzaldehydes.
2. Nomenclature of Ketones
2.1. Common Names of Ketones
  • Common names of ketones are derived by naming two alkyl or aryl groups bonded to the carbonyl group.
  • Locations of substituents are indicated by Greek letters, , and so on.
  • This assigning of letters begins with the carbon atoms next to the carbonyl group, indicated as .
  • A few ketones have historical common names.
  • For example, acetone (which is the the simplest dimethyl ketone).
2.2. IUPAC Names of Ketones
  • For aliphatic ketones, the IUPAC names are derived from the names of corresponding alkanes.
  • The ending '-e' from the name of the alkane is replaced with '-one'.
  • The numbering of the parent carbon chain begins from the end nearer to the carbonyl group.
  • Substituents are prefixed in alphabetical order along with numerals indicating their positions in the carbon chain.
  • For cyclic ketones, the carbonyl carbon is numbered one.
3. Nomenclature of Carboxylic Acids
3.1. Common Names of Carboxylic acids
  • Common names of carboxylic acids end with the suffix, '-ic'.
  • They are derived from the Greek or Latin names of their natural sources.
  • For example, the name acetic acid is derived from the Latin term, acetum, which means vinegar. Similarly, formic acid is derived from the Latin term formic, which means red ants since the acid was first obtained from them.
3.2. Common Names of Carboxylic acids
  • The IUPAC names of aliphatic carboxylic acids are given by replacing the ending '-e' in the name of corresponding alkane with '-oic' acid.
  • When numbering the parent carbon chain, carboxylic carbon is numbered one.
  • For naming carboxylic acids with multiple carboxyl group, the alkyl chain excluding the carboxyl groups is numbered.
  • In this case, the number of carboxyl groups is indicated by adding the relevant prefix like di, tri, and so on (dicarboxylic acid, tricarboxylic acid, etc.)
  • Position of -COOH groups are indicated by the arabic numeral before the multiplicative prefix.
1. Physical Properties of Aldehydes and Ketones
  • At room temperature, methanal is a gas and ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid.
  • Boiling points of aldehydes and ketones are higher than amines, esters, ethers and hydrocarbons of comparable molecular masses.
  • The high boiling points are due to dipole-dipole interactions in aldehydes and ketones.
  • Moreover, boiling points of aldehydes and ketones are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding.
Ketone > Aldehyde > Amine > Ester > Ether > Alkane
  • Lower members of aldehydes and ketones like methanal, ethanal and propanone are miscible with water because of hydrogen bonding with water.
  • Solubility of aldehydes and ketones decreases on increasing the length of alkyl chain.
  • All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, and so on.
  • Lower aldehydes have sharp pungent odours.
  • With increasing size of the molecule, its odour becomes less pungent and more fragrant.
  • This is why most naturally occurring aldehydes and ketones are used in perfumery and food industry.

2. Physical Properties of Carboxylic Acids

  • Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours.
  • Higher acids are wax like solids and odourless as they are less volatile.
  • The boiling points of carboxylic acids are higher than aldehydes, ketones and even alcohols of comparable molecular masses because of intermolecular hydrogen bonding.
  • These hydrogen bonds are not broken completely even in vapour phase. So, they exist as dimers in vapour phase or in aprotic solvents.
  • Simple aliphatic carboxylic acids upto four carbon atoms are miscible in water because of hydrogen bonding with water.
  • As the number of carbon atoms increases, solubility of the carboxylic acid decreases.
  • Higher members of carboxylic acids are insoluble in water due to the increased hydrophobic interaction of hydrocarbon part.
  • Benzoic acid (simplest aromatic carboxylic acid) is nearly insoluble in cold water.
  • Carboxylic acids are soluble in less polar organic solvents like benzene, ether, alcohol, chloroform, and so on.
3. Acidity of -Hydrogens of Aldehydes and Ketones
  • -hydrogen of aldehydes and ketones is acidic in nature.
  • This is because of the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.
4. Acidic Strength of Carboxylic acids
  • When mixed with water, carboxylic acids dissociate and give resonance stabilised carboxylate anions and hydronium ion.
  • Carboxylic acids are weaker than mineral acids, but stronger than alcohols and many simple phenols.
  • They are among the most acidic organic compounds.
  • Higher acidity of carboxylic acids as compared to phenols is because the conjugate base of carboxylic acid (a carboxylate ion), is stabilised by two equivalent resonance structures.
  • In carboxylic acids, the negative charge is delocalised over the more electronegative oxygen atom.
  • On the other hand, the conjugate base of phenol (phenoxide ion) has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom.
  • So, resonance in phenoxide ion is less effective than that in carboxylate ion, making carboxylic acids more acidic than phenols.
  • Electron withdrawing groups increase acidic strength of carboxylic acids by stabilising the conjugate base by delocalisation of negative charge by inductive or resonance effects.
  • Electron donating groups decrease the acidity by destabilising the conjugate base.
  • Direct attachment of groups like phenyl or vinyl increases acidity of corresponding carboxylic acid, because of resonance as illustrated below:
  • Electron withdrawing group on phenyl of aromatic carboxylic acid increases acidity.
  • Electron donating group on phenyl of aromatic carboxylic acid decrease their acidity.

5. Uses of Aldehydes

  • Aldehydes have prominent sweet-smelling odours. So, they are used to make perfumes and colognes. An example of this is the use of benzaldehyde.
  • They also have special tastes because of which they are used in flavoring food. For example, vanillin and cinnamaldehyde.
  • In industries, aldehydes have a number of uses.
  • They are used to manufacture products like synthetic resins, dyes, and so on.
  • For example, formaldehyde is used to produce polymers like bakelite, melamine, and adhesives.
  • Also, it is used as formalin (40%) solution to preserve biological specimens.
  • It is also used as a fungicide and germicide.
  • Another important use of formaldehyde is to prepare bakelite (a phenol-formaldehyde resin).
  • Acetaldehyde is used as a starting material in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.
  • Benzaldehyde is used to prepare dyes.
  • Aldehydes are also used as solvents.

6. Uses of Ketones

  • Ketones also have prominent odours and so are used in perfumery.
  • Some ketones like acetophenone and carvone are found in essential oils. They have therapeutic properties like sedative, analgesic, mucolytic and healing properties.
  • They are also used as solvents. For example, acetone and ethyl methyl ketone.
  • They also serve as starting materials for manufacture of other chemicals, adhesives, paints, and rubber.
  • They even find applications in printing and electroplating.

7. Uses of Carboxylic Acids

  • Carboxylic acids are used in a number of industries.
  • For example, methanoic acid is used to manufacture textiles, rubber, dyes and leather. It is also used in electroplating industries.
  • Hexanedioic acid is used to manufacture nylon-6, 6.
  • Esters of carboxylic acids are used in making perfumes. For example, esters of benzoic acid.
  • They are also used in the food industry.
  • For instance, ethanoic acid is used as vinegar, sodium benzoate is used as preservative, tartaric acid is used as an emulsifier, citric acid as an acidulant, and ascorbic acid as an anti-oxidant.
  • Carboxylic acids are also used to manufacture soaps and detergents. For instance, higher fatty acids like palmitic acid and stearic acid.
  • Carboxylic acids are also used to manufacture drugs. For instance, Aspirin prepared from acetic and salicylic acid.
1. Preparation of Aldehydes
1.1. From Acyl Chlorides (Acid Chlorides) - Rosenmund reduction
  • Aldehydes can be prepared from acyl acyl chlorides by hydrogenating them over palladium on barium sulphate catalyst.
  • This is Rosenmund reduction.
1.2. From Nitriles
a) Through Stephen reaction
  • First, nitriles are reduced to corresponding imine using stannous chloride in the presence of hydrochloric acid.
  • Next, the imine is hydrolysed to give corresponding aldehyde.
  • This is Stephen reaction.
b) Using DIBAL-H
  • First, nitriles are reduced by diisobutylaluminium hydride (DIBAL-H) to imines.
  • Next, the imine is hydrolysed to give corresponding aldehyde.
1.3. From Esters
Esters can be reduced to aldehydes using DIBAL-H, similar to the reaction with nitriles.
1.4. From Hydrocarbons
By converting the methyl group of methylbenzene (Toluene) to an intermediate which is difficult to oxidise further, we can prepare aromatic aldehydes. Here are some methods for the same:
a) Oxidation of methylbenzene using chromyl chloride - Etard Reaction
  • First, chromyl chloride oxidises methyl group of methylbenzene to a chromium complex.
  • Hydrolysis of this chromium complex gives corresponding benzaldehyde.
  • This is Etard reaction.
b) Oxidation of methylbenzene using chromic oxide
  • Methylbenzene is converted to benzylidene diacetate using chromic oxide in acetic anhydride.
  • The benzylidene diacetate is then hydrolysed to corresponding benzaldehyde using aqueous acid.
c) Side chain chlorination of methylbenzene followed by hydrolysis
  • The side chlorination of toluene gives benzal chloride.
  • Benzal chloride on hydrolysis gives benzaldehyde.
  • This is a commercial method for manufacturing benzaldehyde.
d) Gatterman-Koch reaction
  • Treating benzene or its derivatives with CO and HCl in the presence of anhydrous or CuCl gives benzaldehyde or substituted benzaldehyde.
  • This is Gatterman-Koch reaction.
2. Preparation of Ketones
2.1. From Acyl Chlorides
  • Reaction of acyl chlorides with dialkylcadmium gives ketones.
  • Dialkylcadmium is prepared by the reaction of cadmium chloride with Grignard reagent.a
2.2. From Nitriles
  • Nitriles are treated with Grignard reagent to form a complex.
  • This on hydrolysis forms ketone.
2.3. From Benzene or Substituted Benzenes - Friedel-Crafts Acylation Reaction
  • Benzene or substituted benzene is treated with acid chloride in the presence of anhydrous aluminium chloride to give ketone.
  • This is Friedel-Crafts acylation reaction.
Chemical Reactions of Aldehydes and Ketones 1. Nucleophilic Addition Reactions
Aldehydes and ketones undergo nucleophilic addition and nucleophilic addition-elimination reactions because of the presence of the carbonyl functional group.
1.1. Addition of Hydrogen Cyanide (HCN)
  • Aldehydes and ketones give cyanohydrins upon reaction with hydrogen cyanide (HCN).
  • The reaction is very slow with pure HCN.
  • To catalyse the reaction, a base is used.
  • The cyanide ion generated as a result is a stronger nucleophile.
  • So, it readily adds to aldehydes and ketones to give cyanohydrin, which is very useful to manufacture other chemicals.
1.2. Addition of Sodium Hydrogensulphite
  • Aldehydes and ketones form addition products with sodium hydrogensulphite.
  • The addition compound produced in the reaction is water soluble and can be converted back to the original carbonyl compound when it is treated with dilute mineral acid or alkali.
  • This reaction is useful for the separation and purification of aldehydes.
1.3. Addition of Alcohols
  • Aldehydes react with monohydric alcohols in presence of dry HCl to produce alkoxyalcohol intermediate, known as hemiacetal.
  • The hemiacetal further reacts with one more molecule of alcohol to give acetal, a gem-dialkoxy compound.
  • Ketones react with ethylene glycol to form cyclic products known as ethylene glycol ketals.
  • Dry HCl protonates the oxygen of aldehydes and ketones.
  • This way, it increases the electrophilicity of the carbonyl carbon. Thus enabling the nucleophilic attack of ethylene glycol.
  • Acetals and ketals can be hydrolysed using aqueous mineral acids to corresponding aldehydes and ketones.
1.4. Addition of Ammonia and its Derivatives
  • Nucleophiles like ammonia and its derivatives can add to the carbonyl group of aldehydes and ketones.
  • The reaction is reversible and catalysed by acid.
  • Product formation is favoured in equilibrium because of rapid dehydration of the intermediate to form .
1.5. N-Substituted Derivatives of Aldehydes and Ketones (>C=N-Z)
1.6. Mechanism of Nucleophilic Addition Reactions
  1. Nucleophile attacks the electrophilic carbon atom of polar carbonyl group.
  2. Its direction of attack is perpendicular to the plane of hybridised orbitals of carbonyl carbon.
  3. As a result, hybridisation of carbon changes from to .
  4. A tetrahedral alkoxide intermediate is produced which captures a proton from the reaction medium, thus forming an electrically neutral product.
  5. As a result, nucleophile () and () are added across the carbon-oxygen double bond.
1.7. Reactivity of Aldehydes and Ketones to Nucleophilic Addition
  • Aldehydes are more reactive than ketones in nucleophilic addition reactions
  • This is due to steric and electronic reasons.
  • Steric reason is that in ketones, two relatively large substituents (alkyl groups) hinders the approach of nucleophile to carbonyl carbon.
  • On the other hand, aldehydes have only one alkyl group.
  • Electronic reason is that the presence of two alkyl groups in ketones reduces the electrophilicity of the carbonyl carbon more effectively.
  • This makes them less reactive than aldehydes to nucleophilic attack.
2. Reduction
(i) Reduction to Alcohols
  • Aldehydes are reduced to primary alcohols by sodium borohydride or lithium aluminium hydride and by catalytic hydrogenation.
  • Similarly, ketones are reduced to secondary alcohols.
(ii) Reduction to Hydrocarbons
  • When aldehydes and ketones are treated with zinc amalgam and concentrated HCl, the carbonyl group is reduced to a hydrocarbon.
  • This reaction is called Clemmensen reduction.
  • Reduction to hydrocarbon can also be carried out by Wolff-Kishner reduction.
  • In this method, aldehydes and ketones are treated with hydrazine.
  • The resulting product is heated with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol.
3. Oxidation
  • Aldehydes get oxidised easily to carboxylic acids by oxidising agents like nitric acid, potassium permanganate, and potassium dichromate.
  • Mild oxidising agents like Tollens' reagent and Fehling's reagent also oxidise aldehydes.
  • On the other hand, ketones need vigorous conditions to get oxidised.
  • They need strong oxidising agents and high temperatures.
  • The oxidation of ketones involves carbon-carbon bond cleavage, as a result of which a mixture of carboxylic acids is formed with ketones having lesser number of carbon atoms than the parent ketone.
3.1. Tollens' Test
  • When aldehyde is warmed with freshly prepared Tollens' reagent (ammoniacal silver nitrate solution), silver is formed which produces a bright silver mirror on the test tube walls.
  • In this test, aldehydes are oxidised to corresponding carboxylate anion in alkaline medium.
3.2. Fehling's Test
  • For this test, Fehling's reagent is used which is made of two solutions, Fehling solution A (aqueous copper sulphate) and Fehling solution B (alkaline sodium potassium tartarate; Rochelle salt).
  • The two solutions are mixed in equal amounts before carrying out the test.
  • When aldehyde is heated with Fehling's reagent, a reddish brown precipitate is formed.
  • In the reaction, aldehydes are oxidised to corresponding carboxylate anion.
  • However, aromatic aldehydes do not give a positive test.
3.3. Oxidation of Methyl Ketones by Haloform Reaction
  • Carbonyl compounds with at least one methyl group linked to the carbonyl carbon atom (methyl ketones) are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids.
  • These sodium salts have one carbon atom less than that of carbonyl compound and the methyl group is converted to haloform.
  • If a carbon-carbon double bond is present in the molecule, it is not affected in the reaction.
  • An example of haloform reaction is Iodoform reaction with sodium hypoiodite.
  • It is used to detect group or group that gives group when oxidised.
4. Reactions Due to -Hydrogen
4.1. Aldol Condensation
  • Carbonyl compounds with at least one -hydrogen undergo aldol reaction in the presence of dilute alkali as catalyst.
  • In this reaction, -hydroxy aldehyde (aldol) or -hydroxy ketone (ketol) is formed in the case of aldehydes and ketones, respectively.
  • Since both aldehyde and alcohol groups are present in the product, this reaction is called 'aldol' reaction, derived from the names of the two functional groups (aldehyde and alcohol).
  • Although ketones give ketols (ketone and alcohol functional group compounds), the general name aldol condensation still applies to their reactions.
  • Aldol and ketol lose water to give ,-unsaturated carbonyl compounds.
4.2. Cross Aldol Condensation
  • Aldol condensation that is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation.
  • When both these contain -hydrogen atoms, a mixture of four products is obtained.
  • Even ketones can be a component in the cross aldol reactions.
Cannizzaro Reaction
  • Aldehydes without an -hydrogen atom undergo self oxidation and reduction (disproportionation) reaction when they are heated with concentrated alkali.
  • As a result, onee molecule of the aldehyde is reduced to alcohol, and another one is oxidised to carboxylic acid salt.
6. Electrophilic Substitution Reaction
  • Aromatic aldehydes and ketones undergo electrophilic substitution.
  • The carbonyl group acts as a deactivating and meta-directing group.
Preparation of Carboxylic Acids 1. From Primary Alcohols and Aldehydes
  • Primary alcohols can be oxidised to give carboxylic acids using oxidising agents.
  • For this purpose, oxidising agents like potassium permanganate in neutral, acidic or alkaline media, potassium dichromate and chromium trioxide in acidic media (Jones reagent) are used.
2. From Alkylbenzenes
  • To prepare aromatic carboxylic acids, vigorous oxidation of alkyl benzenes can be done using chromic acid or acidic or alkaline potassium permanganate.
  • In this process, the entire side chain (irrespective of its length) is oxidised to give carboxyl group irrespective of length of the side chain.
  • But, this process is useful only to oxidise primary and secondary alkyl groups. It's ineffective for tertiary alkyl group.
3. From Nitriles and Amides
  • First step is to hydrolyse nitriles to amides, while maintaining mild reaction conditions to stop the reaction at amide stage.
  • Next, the amides are converted to carboxylic acids in the presence of catalyst (H+ or OH-).
4. From Grignard Reagents
  • Aldehydes can be prepared from Grignard reagents by reacting them with carbon dioxide (dry ice).
  • As a result, they form salts of carboxylic acids.
  • The carboxylic acids in turn give the corresponding carboxylic acids after acidification with mineral acid.
5. From Acyl Halides and Anhydrides
  • Acid chlorides on hydrolysis give carboxylic acids.
  • When hydrolysed with aqueous base, they give carboxylate ions.
  • On acidification, these carboxylate ions give the corresponding carboxylic acids.
  • Anhydrides can also be hydrolysed to the corresponding carboxylic acid using water.
6. From Esters
  • Carboxylic acids can be obtained through acidic hydrolysis of esters.
  • Basic hydrolysis results in the formation of carboxylates.
  • Carboxylates give the corresponding carboxylic acids on acidification.
Chemical Reactions of Carboxylic Acids 1. Reactions Involving Cleavage of O-H Bond (Reactions with Metals and Alkalies)
  • Carboxylic acids, like alcohols, react with electropositive metals to evolve hydrogen.
  • Like phenols, carboxylic acids also react with alkalies to form salts.
  • But, unlike phenols, they react with weaker bases like carbonates and hydrogencarbonates to evolve carbon dioxide.
  • Using this reaction, the presence of carboxyl group in an organic compound can be detected.
2. Reactions Involving Cleavage of C-OH Bond
1. Formation of Anhydride
Carboxylic acids when heated with mineral acids like or with give corresponding anhydride.
2. Formation of Esters
Carboxylic acids form esters with alcohols or phenols in the presence of a mineral acid like concentrated or HCl gas, that act as a catalyst.
Mechanism of esterification of carboxylic acids:
  • Esterification reaction of carboxylic acids with alcohols is a nucleophilic acyl substitution.
  • The protonation of the carbonyl oxygen of the carboxylic acid activates the carbonyl group towards nucleophilic addition of the alcohol.
  • Next, troton transfer in the tetrahedral intermediate converts the hydroxyl group into group.
  • The group is a better leaving group.
  • So, it is eliminated as neutral water molecule.
  • The protonated ester formed as a result loses a proton and forms the ester.
3. Reactions with , and
  • Hydroxyl group of carboxylic acids, like alcohols, can be easily replaced by Cl when treated with , or .
  • The reaction gives acyl chloride.
4. Reaction with Ammonia
  • Carboxylic acids react with ammonia to give ammonium salt.
  • On further heating at high temperature, the ammonium salt gives amides.
3. Reactions Involving -COOH Group
1. Reduction
Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or diborane.
2. Decarboxylation
  • When the sodium salts of carboxylic acids are heated with sodalime (NaOH and CaO in the ratio of 3 : 1), they lose carbon dioxide to form hydrocarbons.
  • This reaction is called decarboxylation.
  • Kolbe electrolysis is also an example of decarboxylation reaction.
4. Substitution Reactions in the Hydrocarbon Part
1. Halogenation
  • Carboxylic acids having an -hydrogen are halogenated to give -halocarboxylic acids when reacted with chlorine or bromine.
  • This reaction happens in the presence of red phosphorus.
  • The halogenation occurs at the -position.
  • This reaction is called Hell-Volhard-Zelinsky reaction.
2. Ring Substitution
  • Aromatic carboxylic acids undergo electrophilic substitution reactions.
  • In these reactions, the carboxyl group acts as a deactivating and meta-directing group.
  • But, aromatic carboxylic acids do not undergo Friedel-Crafts reaction.
  • That's because the carboxyl group is deactivating.
  • So, the catalyst aluminium chloride, which is a Lewis acid, gets bonded to the carboxyl group.