In this revision web notes, we will learn about Aldehydes, Ketones and Carboxylic Acids Class 12 Chemistry Notes. We will cover all basic knowledge about Aldehydes, Ketones and Carboxylic Acids in this article. So, enjoy learning chem with Chemistry Notes Info @ https://chemistrynotesinfo.com
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.
In carboxylic acids, the carbonyl carbon atom is attached to an alkyl group (R) and – OH group.
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.
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.
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.
- By hydration of alkynes
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.
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 + SnCl2 + 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.
- 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.
- By side chain chlorination followed by hydrolysis
Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde.
PREPARATION OF KETONES
1) From acyl chloride
On treating acyl chlorides with dialkyl-cadmium chloride with Grignard reagent, gives ketones.
2) From nitriles
Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.
3) 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.
- 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.
- 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.
While 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.
- 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 hydrocarbons
Aldehyde and ketones can be reduced to hydrocarbons by using two different reducing agents.
Wolff – kishner reduction
Oxidation of aldehyde gives carboxylic acid containing the same number of carbon atoms as the original aldehyde.
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.
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) + 3O–H → 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.
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.
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.
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.
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 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-
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.
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.
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.
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.
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.
PHYSICAL PROPERTIES OF CARBOXYLIC ACIDS
1) Physical nature
Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids. The higher acids are waxy solids.
Aliphatic carboxylic acids upto nine carbon atoms have pungent smell, while higher acids are odourless.
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
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.
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 – COO–Na+ + H2 ↑
ii) Action of NaOH
R – COOH (Acid) + NaOH (Alkali) –Neutralization → R – COO–Na+ (Salt) + H2O
iii) Action of NaHCO3 or Na2CO3
R – COOH + 2NaHCO3 → R – COO–Na+ + H2O + CO2 ↑
2R – COOH + Na2CO3 → 2R – COO–Na+ + 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.
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.
iii) Reaction with ammonia
Carboxylic acids or acid chlorides react with ammonia to give ammonium salts which on heating decomposes to form amides.
iv) Reaction with PCl3, PCl5 and SOCl2
3R – COOH + PCl3 → 3R – COCl + H3PO3
R – COOH + PCl5 → R – COCl + POCl3 + HCl
R – COOH + SOCl3 → R – COCl + SO2 + 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)
ii) 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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