Aldehydes, Ketones and Carboxylic Acids Class 12 Notes

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Aldehydes, Ketones and Carboxylic Acids Class 12 Chemistry Notes

A functional group in which a carbon atom is attached to an oxygen atom by a double bond and the remaining two valences of carbon atom are free is called carbonyl group (> C = O). Organic compounds containing a carbonyl group (> C = O) are called carbonyl compounds. Aldehyde and ketones are carbonyl compounds.

In aldehydes, carbonyl carbon is bonded to a hydrogen atom and an alkyl group (R), while in ketones it is attached to two same or different alkyl groups.

Aldehydes and Ketones

In carboxylic acids, the carbonyl carbon atom is attached to an alkyl group (R) and – OH group.

Carboxylic acid

Structure of carbonyl functional group

In aldehydes and ketones, the carbon of carbonyl group is SP2 hybridised and bonded to three other atoms. Carbon atom forms three sigma (σ) bonds with the bond angle of 120ᵒ which lie in one plane.

Structure of carbonyl functional group

ALDEHYDE AND KETONES

PREPARATION OF ALDEHYDE AND KETONES

1. From alcohols (by oxidation)

Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively in the presence of oxidizing agents like KMnO4, acidified K2Cr2O7 and CrO3.

ALDEHYDE AND KETONES preparation by oxidation

2. From Hydrocarbons

  • By ozonolysis of alkenes

Ozonolysis of alkenes followed by reduction with zinc dust and water gives aldehyde, ketones or a mixture of both.

Ozonolysis of alkenes followed by reduction with zinc dust and water gives aldehyde, ketones or a mixture of both.
  • By hydration of alkynes

Addition of water to acetylene in presence of H2SO4 and HgSO4 gives acetaldehyde while all other alkynes give ketones.

Addition of water to acetylene in presence of H2SO4 and HgSO4 gives acetaldehyde while all other alkynes give ketones

PREPARATION OF ALDEHYDE

1. From acylchloride (Rosenmund reduction)

Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate.

Rosenmund reduction

2. From nitriles and esters (Stephen reaction)

Nitriles are reduced to corresponding imine with tin chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.

RCN   +   SnCl +   HCl    →    RCH = NH    ___H3O+→   R – CHO

3. From aromatic hydrocarbons

  • By oxidation of methylbenzene

Chromyl chloride (CrO2Cl2) oxidizes methyl group to chromium complex which on hydrolysis give corresponding benzaldehyde. This reaction is known as Etard reaction.

Etard Reaction
  • By Gatterman – Koch reaction

When benzene or its derivative is treated with CO and HCl in presence of anhydrous AlCl3 or CuCl, it gives benzaldehyde.

Gatterman – Koch reaction
  • By side chain chlorination followed by hydrolysis

Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde.

preparation of benzaldehyde.

PREPARATION OF KETONES

1) From acyl chloride

On treating acyl chlorides with dialkyl-cadmium chloride with Grignard reagent, gives ketones.

PREPARATION OF KETONES

2) From nitriles

Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.

PREPARATION OF KETONES from nitriles

3) Friedel Crafts Acylation reaction (substituted benzenes)

Friedel Crafts Acylation reaction (substituted benzenes)

PHYSICAL PROPERTIES OF ALDEHYDE AND KETONES

1) Physical nature : Formaldehyde is a gas while Acetaldehyde and acetone are colourless and exist in liquid state.

2) Odour :  Aldehydes and ketones have generally pleasant smell.

3) Solubility : Aldehydes and ketones upto four carbons atoms are miscible with water and are soluble in organic solvents such as ether, alcohol etc.

4) Boiling Point: Boiling points of aldehydes and ketones are higher than those of hydrocarbons of comparable molecular masses but lower than those of corresponding alcohols and carboxylic acid.

CHEMICAL REACTIONS OF ALDEHYDE AND KETONES

1. Nucleophilic addition reactions

  • Addition of hydrogen cyanide (HCN)

When hydrogen cyanide is added to an aldehyde or a ketone, cyanohydrins compounds are formed. In this reaction, nucleophile CN attacks carbonyl group. Addition of hydrogen cyanide to carbonyl group is a reversible reaction.

When hydrogen cyanide is added to an aldehyde or a keton, cyanohydrins compounds are formed
  • Addition of Sodium bisulphate (NaHSO3)

Aldehydes and ketones when treated with saturated aqueous solution of sodium bisulphate give addition products. This reaction is used for separation and purification of aldehyde and ketones from other organic compounds.

Sodium bisulphate reaction with aldehyde and ketone
  • Addition of Grignard Reagents (R – MgX)

The addition of Grignard reagent, (R – MgX) on carbonyl compounds lead to the formation of alcohols.

Formaldehyde → primary alcohols (10)

Higher aldehyde → secondary alcohols (20)

Ketones → tertiary alcohols (30)

  • Addition of alcohols

Aldehydes react with alcohols in presence of an acid to form acetal.

Aldehydes react with alcohols in presence of an acid to form acetal.

While ketones react with alcohols in presence of  an acid to form ketal.

ketones react with alcohols in presence of  an acid to form ketal
  • Addition of ammonia and its derivatives

Nucleophiles, such as ammonia and its derivatives H2N – Z add to the carbonyl group of aldehydes and ketones.

Z = Alkyl, aryl, OH, NH2 etc.

Addition of ammonia and its derivatives

2. Reduction

  • Reduction of alcohols

Aldehydes and Ketones can be easily reduced to primary and secondary alcohols respectively by using NaBH4, LiAH4, Sodium – amalgam and water or by hydrogenation.

reduction of alcohol
  • Reduction of hydrocarbons

Aldehyde and ketones can be reduced to hydrocarbons by using two different reducing agents.

Clemmensen Reduction

Clemmensen’s reduction - Clemmensen Reduction  - Chemistry Notes Info - ChemNotesInfo - ChemistryNotesInfo

Wolff – kishner reduction

Wolff – kishner reduction

3. Oxidation

Oxidation of aldehyde gives carboxylic acid containing the same number of carbon atoms as the original aldehyde.

Oxidation of aldehyde gives carboxylic acid

Oxidation of ketone gives carboxylic acid containing less number of carbon atoms than the original ketone.

e.g. Acetone on oxidation using CrO3 gives acetic acid.

Oxidation of ketone gives carboxylic acid

Ketones and aldehyde can be distinguished by using following mild oxidizing reagents.

  • Tollen’s test

R – CHO (Aldehyde)    +    2[Ag(NH3)2]+ (Tollen’s reagent)  +   3OH    →    R – COO   +   2Ag   +   2H2O   +   NH3  

  • Fehling’s test

R – CHO (Aldehyde)  +   2Cu2+  +   5OH-   (Fehling’s solution) →    R – COO   +   Cu2O   +   3H2O

4. Haloform reaction

A Ketone having at least one methyl group attached to carbonyl carbon and acetaldehyde is oxidized by sodium hypo-halite to give sodium salt of carboxylic acid and haloform.

Haloform reaction

5. Adol Condensation

Two molecules of an aldehyde or ketone (containing α – H – atom) in the presence of dilute basic solution of NaOH, KOH, Ba(OH)2, K2CO3, or Na2CO3 or dil. HCl undergo addition reaction to give corresponding β – hydroxy aldehyde (adol) or β – hydroxy Ketones (Ketol). This reaction is called adol condensation or self oxidation.

e.g.

1) Acetaldehyde and 2) Acetone

1) Acetaldehyde and 2) Acetone

6. Cannizzaro reaction

Self Oxidation and reduction of aldehydes (which do not posses on α – hydrogen atom) in the presence of concentrated alkali (aqueous or alcoholic) is known as Cannizzaro reaction. The reaction products are alcohol and salt of carboxylic acid. e.g.     

Cannizzaro reaction

7. Electrophilic Substitution reaction

Carbonyl group in the aromatic aldehydes and ketones is deactivating and metadirecting. Thus, aromatic aldehydes and ketone undergo electrophilic substitution to give m – nitrobenzaldehyde.

e.g. Benzaldehyde undergoes electrophilic substitution to give m – nitrobenzaldehyde.

Benzaldehyde undergoes electrophilic substitution to give m – nitrobenzaldehyde

USES OF ALDEHYDES

1) Formalin is used as preservative for biological specimens. Formaldehyde is used for silvering mirror and for production of several plastic and resins, Bakelite etc.

2) Acetaldehyde is used in the manufacture of acetic acid, ethyl acetate, vinyl acetate, polymers and drugs.

3) Benzaldehyde is used in perfumery and in dye industries.

USES OF KETONES

1) Acetone and ethyl methyl ketone are common industrial solvents.

2) Many ketones are well known for their odors and flavors e.g. acetophenone.

3) Acetone is also use as one of the constituents of liquid nail polish. It is used in manufacture of explosive, lacquers, paint removers, plastics, drugs, adhesives and disinfectants.

We are learning chemistry notes of XII class NCERT CBSE Books chapter “Aldehydes, Ketones and Carboxylic Acids Class 12 Notes”. Firstly we learn about Aldehydes and Ketones, Finally we learn about Carboxylic Acids.

CARBOXYLIC ACIDS

Carboxylic acids are the organic compounds containing one or more carboxyl group in their molecule. The functional group of the carboxylic acids is carboxyl group or – COOH. The carboxylic acids may be aliphatic (R – COOH) or aromatic (Ar – COOH) depending upon whether – COOH group is attached to aliphatic alkyl chain or aryl group respectively.

The general formula is-

Aliphatic Carboxylic Acid      and         Aromatic Carboxylic Acid - 
Aldehydes, Ketones and Carboxylic Acids

STRUCTURE OF CARBOXYLIC GROUP

The carbon atom in carboxylic group is sp2 hybridized. The C – C = O and O = C – O bond angles are approximately 120ᵒ.

STRUCTURE OF CARBOXYLIC GROUP
- Aldehydes, Ketones and Carboxylic Acids Class 12 Notes

METHODS OF PREPARATION OF CARBOXYLIC ACID

1) From primary alcohols and aldehydes (by oxidation)

Primary alcohols and aldehydes are easily oxidized to carboxylic acid by oxidizing agents like acidic or alkaline KMnO4, acidified K2Cr2O7 or CrO3 in glacial acetic acid.

R – CH2OH (1ᵒ Alcohol)   —K2Cr2O7 or dil. H2SO4 —> R – CHO (Aldehyde)      —K2Cr2O7 or dil. H2SO4 —-> R – COOH (Carboxylic Acid)  

2) From alkyl benzenes (by oxidation)

Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzene with chromic acids or acidic or alkaline potassium permanganate or acidified K2Cr2O7.

PREPARATION OF CARBOXYLIC ACID From alkyl benzenes (by oxidation) - XII Class NCERT CBSE Chemistry Revision Short notes in hindi english by ChemistryNotesInfo www.ChemistryNotesInfo.com

3) From nitriles and amines (by hydrolysis)

When a nitrile (alkyl cyanide) is boiled with dilute mineral acid, it gives corresponding carboxylic acids. Amide is the intermediate product.

PREPARATION OF CARBOXYLIC ACID From nitriles and amines (by hydrolysis) - Aldehydes, Ketones and Carboxylic Acid

4) From Grignard reagent

When solution of Grignard reagent in dry ether is added to solid carbon dioxide (dry ice), it gives complex (magnesium salt of carboxylic acid), which on acid hydrolysis gives the corresponding carboxylic acid.

PREPARATION OF CARBOXYLIC ACID From Grignard reagent

5) From acyl halide and anhydride

Acid chloride when hydrolyzed with water gives carboxylic acid.

R – COCl (Acid Chloride)     H2O  →    R – COOH (Carboxylic acid)   +    HCl

6) From esters

When an ester is heated with dilute H2SO4 undergo hydrolysis to give carboxylic acid and alcohol.

PREPARATION OF CARBOXYLIC ACID From esters - Chem Notes by "Chemistry Notes Info"

PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS

1) Physical nature

Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids. The higher acids are waxy solids.

2) Odour

 Aliphatic carboxylic acids upto nine carbon atoms have pungent smell, while higher acids are odourless.

3) Solubility

All are soluble in organic solvents like alcohol and ether. Benzoic acid is nearly soluble in cold water but is soluble in hot water, alcohol, ether.

4) Boiling points

Carboxylic acids have higher boiling points than aldehydes, ketones, ether and even alcohol of comparable molecular masses.

REACTIONS OF CARBOXYLIC ACIDS

1) Reaction involving cleavage of O – H bond

Acidic Nature

Among the organic compounds, the carboxylic acids have maximum acidity. However, their acidity is lower than that of mineral acids.

Carboxylic acid dissociates in water to give resonance stabilized carboxylate anions and hydronium ion.

Carboxylic acid dissociates in water to give resonance stabilized carboxylate anions and hydronium ion

The equilibrium constant is Keq is given by

  Keq   =    [RCOO] [H3O+] / [RCOOH] [H2O]

Since concentration of water remains constant.

 Ka  =   Keq X[H2O]   =   [RCOO][H3O+] / [RCOOH]

The strength of carboxylic acid is also expressed in terms of their pKa values. The stronger carboxylic acids have lower pKa values,

  pKa   =   – log Ka

Following reactions explain the acidic properties of carboxylic acids

i) Action of active metals

2R – COOH  +  2Na  →  2R – COONa+  +  H2  ↑

ii) Action of NaOH

R – COOH (Acid)  +  NaOH (Alkali)   –Neutralization  →   R – COONa+ (Salt)  +    H2

iii) Action of NaHCO3 or Na2CO3

R – COOH  +  2NaHCO3  →  R – COONa+  +  H2O   +   CO2  ↑

2R – COOH  +  Na2CO3  →  2R – COONa+  +  H2O   +   CO2  ↑

2) Reaction involving cleavage of C – OH bond

i) Formation of anhydride

Carboxylic acids on heating with minerals acids such as H2SO4 or with P2O5 give corresponding anhydride.

Formation of anhydride from carboxylic acid

ii) Esterification (Action of alcohols)

Carboxylic acids are esterified with alcohols or phenols in the presence of mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.

Esterification (Action of alcohols)

iii) Reaction with ammonia

Carboxylic acids or acid chlorides react with ammonia to give ammonium salts which on heating decomposes to form amides.

Reaction with ammonia of carboxylic acids

iv) Reaction with PCl3, PCl5 and SOCl2

3R – COOH   +   PCl3    →   3R – COCl    +    H3PO3

R – COOH   +   PCl5    →    R – COCl   +   POCl +   HCl

R – COOH   +   SOCl  →    R – COCl   +   SO +   HCl

3) Reactions involving – COOH group

i) Formation of primary alcohols (Reduction)

Strong reducing agents like lithium aluminium hydride or diborane reduces carboxylic acids to primary alcohols.

R – COOH (Carboxylic Acid) —  i) LiAlH4/ ether ii) H3O+   →    R – CH2OH (Primary Alcohol)

ii) Formation of hydrocarbons (decarboxylation)

Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime.

R – COONa (Sodium Carboxylate) + NaOH    Δ CaO  → R – H (Alkane) + Na2CO3

4) Substitution reaction in the hydrocarbon part

i) Halogenation (Hell – volhard – zelinsky reaction)

Halogenation (Hell – volhard – zelinsky reaction)

ii) Ring Substitution (Friedel Crafts reaction)

Ring Substitution (Friedel Crafts reaction)

USES OF CARBOXYLIC ACIDS

1. Methanoic (formic) acid is used in rubber, textile, dyeing, leather and electroplating industries.

2. Ethanoic (Acetic) acid is a solvent and used as vinegar in food industry.

3. Esters of benzoic acid are used in perfume industries.

4. Sodium benzoate is used as a food preparation of soaps and detergents.

5. Higher fatty acids are used in preparation of soaps and detergents.

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