Preparation for Aldehydes and Ketones

1. From alcohols

There are two commons ways of converting alcohols to aldehydes or ketones.

(a) By oxidation: Oxidation of alcohols by common oxidants such as K2Cr2O7/H+, KMnO4/H+,KMnO4/OH-, CrO3/H+ gives aldehydes or ketones.

R-CH2-OH + [O] ⟶ R-CHO + H2O

R-CH2(OH)-R + [O] ⟶ R-CO-R

(b) By catalytic dehydrogenation: This method is suitable for volatile alcohols and is of industrial application. In this method alcohol vapours are passed over heavy metal catalysts such as Ag or copper.

R-CH2(OH) ⟶ RCHO + H2

R-CH(OH)-R' ⟶ R-CO-R' + H2

2. From acid chlorides

By catalytic hydrogenation of acid chlorides with Pd and BaSO4 poisoned by traces of S or quinoline i.e., Lindlar’s catalyst produces aldehyde. It is also called Rosenmund reduction.

R-CO-Cl + H2 ⟶ R-CHO + HCl

CH3-CO-Cl + H2 ⟶ CH3-CHO + HCl

3. From Hydrocyanic acid

Treatment of hydrocyanic acid with Grignard reagent followed by hydrolysis form aldehyde.

H-C☰N + CH3MgBr ⟶ CH3-CH=N.MgBr ⟶ CH3-CHO + Br-Mg-NH2

4. Etard reaction

Chromylchloride (CrO2Cl2) Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde. This reaction is called Etard reaction.

5. By Gatterman – Koch reaction

When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.

Physical Properties

1. Physical state: Methanal is a pungent smelling gas, ethanal is volatile liquid with b.p. = 294 K. Rest of the aldehydes and ketones containing upto eleven carbon atoms are colourless liquids while higher members are solids.

2. Solubility: Aldehydes and ketones upto four carbon atoms are miscible with water due to presence of H-bonding between polar carbonyl group and water molecules.

Solubility of these compounds decreases rapidly on increasing length of alkyl chain and higher members are practically insoluble in water. All these compounds are quite soluble in organic solvents such as CH3OH, ether, CHCl3, benzene etc. Ketones, themselves are good solvents.

3. Boiling points: Boiling points of aldehydes and ketones are higher than those of non-polar compounds i.e., hydrocarbons and weakly polar compounds, such as ether, due to dipole-dipole interactions but lower than that of alcohols of comparable molar masses, due to absence of intermolecular H-bondings.

4. Smell: Lower aldehydes have exceptionally unpleasant odour whereas other members generally have pleasant odour. As the size of molecule increases smell becomes less and less pungent and more fragrant.

Chemical Reactions

(i) Addition of hydrogen cyanide

Acetaldehyde in reaction with hydrogen cyanide gives acetaldehyde cyanohydrins.

(ii) Addition of Grignard reagent

Formaldehyde when reacted with Grignard reagent produces primary alcohol.

Acetaldehyde when reacts with Grignard reagent, it produces secondary alcohol.

Acetone when reacts with Grignard reagent, it produces tertiary alcohol.

(iii) Addition of ammonia derivative

Aldehyde or ketone when reacted with ammonia derivatives forms a group containing −C=N− group.

(vii) Reaction with alkalis

Aldehydes and ketones in the presence of dilute sodium hydroxide give aldols. These reactions are given below:

Note: The reaction of aldehydes and ketones with alkalies is known as an aldol condensation reaction. Aldehydes and ketones show so many chemical reactions because they are very reactive.

Uses of Aldehydes and Ketones

Aldehydes and ketones are used in chemical industries as solvents, starting material and reagents for synthesising the other products, e.g.,

1. 40% aq. HCHO known as formalin and is used in preserving biological specimens.

2. Formaldehyde is also used as disinfectant.

3. HCHO is used in manufacturing synthetic polymers like Bakelite.

4. Acetaldehyde is used for silvering of mirrors.

5. Benzaldehyde as a flavouring agent in perfume industry.

6. Acetone is used as a solvent in industries and laboratories.

7. Acetone is a common constituent of nail polishes. Many aldehydes and ketones such as butyraldehyde, vanilin, acetophenone, camphor etc. are known for their odours and flavours.

Nomenclature of Carboxyl Group

Common names of carboxylic acids are based on their source of origin e.g., Formic acid (HCOOH) was first obtained from red ants (Latin Formica means ants) similarly acetic acid (CH3COOH) is so named because it was obtained from vinegar (Latin Acetum means vinegar) butyric acid (CH3CH2CH2COOH) from rancid butter (Latin Butyrum means butter) etc. In common system position of substituents is indicated by Greek letters α, β, γ, δ etc.

Structure of Carboxyl Group

Methods of Preparation of Carboxylic Acids

1. From primary alcohols

Primary alcohols are easily oxidised to carboxylic acids with oxidants such as KMnO4, in neutral, acidic or alkaline media or by CrO3 and K2Cr2O7 in acidic media.

CH3-CH2-OH ⟶ CH3COOH

2. From oxidation of aldehydes and ketones

Aldehydes can be oxidised to carboxylic acids even with mild oxidants while ketones are oxidised with difficulty and usually mixture of acids is obtained.

CH3CHO ⟶ CH3COOH

CH3COCH3 ⟶ CH3COOH + CO2 + H2O

3. By hydrolysis of nitriles (cyanides) and amides

Nitriles are hydrolysed to amides and then to carboxylic acids in the presence of H+ or OH- as catalyst. If conditions used are mild, then reaction can be stopped even at amide.

R-CN + 2H2O ⟶ RCONH2 ⟶ RCOOH + NH3

4. From Grignard reagent

Reaction of Grignard reagent with dry ice (i.e., solid CO2) in ethereal solution followed by acidic hydrolysis produces carboxylic acid containing one more carbon atom than Grignard reagent.

5. From hydrolysis of acid halides and anhydrides

Acid chlorides and anhydrides are readily hydrolysed with aqueous base to form carboxylate ions which form carboxylic acid on acidification.

RCOCl ⟶ RCOOH + HCl

Physical Properties

1. Physical State: First 3 members are colourless liquids with pungent smell, next 6 members are oily liquids with unpleasant odour. Higher acids are colourless waxy solids. Benzoic acid and its homologues are colourless solids.

2. Solubility: Among aliphatic acids first four members are very soluble in water and solubility decreases gradually with rise of molecular mass. All are soluble in alcohol or ether. Benzoic acid is sparingly soluble in cold water but is soluble in hot water, alcohol, ether.

3. Boiling Point: Due to the presence of H-bond, boiling points of carboxylic acids are higher than corresponding aldehydes, ketones, ethers or even alcohols of comparable molar masses.

4. Melting Point: Melting point of carboxylic acid increases irregularly with increasing molar mass, upto first ten numbers, melting point of carboxylic acids containing even number of carbon atoms is higher than the next lower and higher member containing odd number of carbon atoms.

Chemical Reactions

1. Reaction with Metals

The reaction of carboxylic acids with metals like K, Na, Mg, and Ca results in the formation of salts. A proton will be released from the carboxyl group of the carboxylic acid where the metal substation will occur during the reaction phase. H2 gas is produced as a result of the reaction.

2CH3COOH + 2Na ⟶ 2CH3COONa + H2

2. Reaction with Alkalis

The reaction of carboxylic acids with alkalis results in the formation of salts and water.

CH3COOH + NaOH ⟶ CH3COONa + H2O

3. Reaction with Carbonates and Bicarbonates

Carboxylic acids decompose with carbonates and bicarbonates to produce salts, vapor, and carbon dioxide gas.

2CH3COOH + Na2CO3 ⟶ 2CH3COONa + H2O + CO2

2CH3COOH + NaHCO3 ⟶ CH3COONa + H2O + CO2

4. Formation of Acyl Chlorides

Acid chlorides are formed when carboxylic acids react with thionyl chloride (SOCl2), phosphorus pentachloride (PCl5), or phosphorus trichloride (PCl3).

RCOOH + SOCl2 ⟶ RCOCl + SO2 + HCl

RCOOH + PCl5 ⟶ RCOCl + PCl3 + HCl

3RCOOH + PCl3 ⟶ 3RCOCl + H3PO3

5. Formation of Esters (Esterification)

When carboxylic acids and alcohols are heated in the presence of concentrated sulphuric acid or dry hydrochloric acid, fruity esters are formed.

R-COOH + R'OH ⇌ RCOOR' + H2O

6. Formation of Amide

Ammonium salts are formed by treating carboxylic acids with ammonia. Ammonium salts lose a water molecule when heated, resulting in the formation of amides.

9. Hell-Volhard Zelinsky Reaction

Carboxylic acids shape -substituted carboxylic acids when they react with a chlorine (Cl2) or bromine (Br2) molecule. In the presence of red phosphorus, the reaction takes place. The H.V.Z. reaction, or Hell-Volhard Zelinsky reaction, is the name given to this reaction.