S7

//M0//QN1//CS//DL0

Aldehydes, ketones, and carboxylic acids are vital in the chemical industry and biological systems. They are found in natural substances like vanillin from vanilla beans and cinnamaldehyde from cinnamon. These compounds are extensively used in perfumes, flavourings agents, and pharmaceuticals. For instance, acetone serves as a common solvent, while benzaldehyde is used in dyes and perfumes. Chemically, these compounds exhibit significant reactivity due to the polar nature of the carbonyl group (>C=O), which makes them electrophilic and attack to nucleophilic addition reactions.(a) What are carbonyl compounds?(b) Why are aldehydes more reactive than ketones?(c) What role do aldehydes and ketones play in industry?(d) How does the carbonyl group influence reactivity?

//X

(a) Carbonyl compounds contain the >C=O group. Aldehydes and ketones are examples, where aldehydes have at least one hydrogen atom bonded to the carbonyl carbon, while ketones have two alkyl or aryl groups attached.

(b) Aldehydes are more reactive because they have less steric hindrance and fewer electron-donating groups, which makes the carbonyl carbon more electrophilic.

(d) The electronegativity difference between carbon and oxygen in the carbonyl group induces polarity, making the carbon atom electrophilic and prone to nucleophilic attack.

//M0//QN2//CS//DL0

Aldehydes and ketones undergo nucleophilic addition reactions due to the electrophilic nature of the carbonyl carbon. For example, the reaction with hydrogen cyanide (HCN) forms cyanohydrins, which are important intermediates in organic synthesis. The reaction is catalyzed by a base that generates the cyanide ion (CN–), which attacks the carbonyl carbon to form a tetrahedral intermediate.(a) What is a cyanohydrin?(b) Why are aldehydes more reactive in nucleophilic addition than ketones?(c) What is the significance of cyanohydrins?(d) Why is the reaction with HCN slow without a catalyst?

//X

(a) It is a compound where a hydroxyl group
(–OH) and a cyanide group (–CN) are attached to the same carbon.

(b) Aldehydes are less sterically hindered and have fewer electron-donating groups.

(d) Pure HCN has weak nucleophilicity, so a base is used to generate the stronger nucleophile, CN–.

//M0//QN3//CS//DL0

When an aldehyde with no α-hydrogen reacts with concentrated aqueous NaOH, half the aldehyde is converted to a carboxylic acid salt, and the other half is converted to an alcohol. In this reaction, one molecule of the aldehyde is oxidized, and the other is reduced. This process is called the Cannizzaro reaction.(a) What are the products of the reaction between benzaldehyde and formaldehyde with aqueous NaOH?(b) Which compounds undergo Cannizzaro reactions?(c) What are the products when trichloroacetaldehyde undergoes Cannizzaro reaction with NaOH?(d) What is the slowest step in the Cannizzaro reaction?

//X

(a) Benzyl alcohol and sodium format are formed.

(b) Compounds without α-hydrogen, like formaldehyde and benzaldehyde, undergo this reaction.

(d) The transfer of hydride ion (H–) from one aldehyde molecule to another is the rate-determining step.

//M0//QN4//CS//DL0//EQ

Ethanal undergoes aldol condensation in the presence of dilute alkali to form 3-hydroxybutanal, which dehydrates to form

but-2-enal (crotonaldehyde).

Reaction Steps:

(1) Formation of aldol :

2CH3CHO

CH3CH(OH) CH2CHO

(2) Dehydration :

CH3CH(OH)CH2CHO

CH3CH=CHCHO

(a) What is the product of aldol condensation of ethanal?(b) Why is dilute NaOH used as a catalyst?(c) What are the functional groups in the aldol intermediate?(d) Why is the product α,β-unsaturated?

//X

(a) But-2-enal (crotonaldehyde).

(b) It generates the enolate ion, which is necessary for the reaction.

(d) Dehydration of the aldol forms a conjugated double bond system. i.e α,β-unsaturated aldhyde.

//M0//QN5//CS//DL0

A pharmaceutical synthesis unit produces Paracetamol, Aspirin and various aromatic drugs. These reactions involve aldehydes, ketones, and carboxylic acid. During the development of new drug intermediates, chemist perform experiments on:

Nucleophilic addition reaction of carbonyl compound.

Oxidation - reduction behavior

Acidity of carboxylic acid.

Reactivity of aromatic vs aliphatic aldehydes

Derivatives such as hydrazones, oximes and 2, 4-DNP compounds.

They observe reaction:

Compound	With Tollen's reagent	With 2, 4-DNP	lodoform Test
Ethanal	+ve	+ve	+ve
Propanone	–ve	+ve	+ve
Benzaldehyde	+ve	+ve	–ve
Acetic Acid	–ve	–ve	–ve

Additional Facts:

Aromatic aldehydes oxidize easily but do not give iodoform test.

Ketones generally resist oxidation except methyl ketones.

Carboxylic acid are least reactive toward nucleophiles.

(a) Why does ethanal give a positive iodoform test but benzaldehyde does not?(b) Explain why aldehydes are more reactive than ketones toward nucleophilic addition.(c) Why does propanone fail the Tollen's test while ethanol passes it?(d) Arrange the following in increasing acidity: Ethanol, Acetic Acid, Phenol.

//X

(a) Iodoform test is positive for compounds containing CH3 – C = O or CH3 – CH(OH) –

Ethanol: CH3 – CHO contain CH3 – CO that is why positive

Benzaldehyde = C6H5– CHO → No CH3 group So, Negative

(b) Reason:

(i) Less steric hindrance around carbonyl carbon in aldehydes.

(ii) Aldehydes have only one alkyl group, so less +I effect

(iii) Carbonyl carbon is more δ+ → more easily attacked.

Ethanal → oxidized → give silver mirror propanone → ketone → cannot be oxidize under mild condition, so negative.

(d) Ethanol < Phenol < Acetic acid

Reason :

Carboxylate ion (–COO–) is stabilized by resonance strongest acid.

Phenoxide ion has resonance but weaker.

Ethoxide ion has no resonance → weaker acid.

//M0//QN6//CS//DL0//EQ

A chemical industry synthesizes perfume aldehydes, ketons for polymers and carboxylic acid for soaps. To optimize production, they study Cannizzaro reaction, Aldol condensation, decarboxylation, esterification and oxidation of 1° and 2° alcohols. They conduct experiments:(i) Benzaldehyde gives Cannizzaro Reaction in concentrated base.(ii) Acetaldehyde undergoes aldol condensation.(iii) Heating sodium acetate with soda lime gives methane (decarboxylation).(iv) Ethanol + Acetic Acid → Ethyl acetate.(v) Acidified KMnO4 oxidizes toluene → benzoic Acid. Additional insight :

Aldehydes without α - hydrogen → Cannizzaro.

Aldehydes with α - hydrogen → Aldol

Carboxylic acid lose CO2 during decarboxylation.

(a) Why does benzaldehyde undergoes Cannizzaro reaction while acetaldehyde undergoes aldol condensation?(b) Write the product and mechanism type when sodium acetate is heated with soda lime.(c) Explain why esterification requires acid catalyst.(d) Predict the product when benzaldehyde + NaOH (Conc.) react.

//X

(a) (i) Benzaldehyde has no α - hydrogen, so cannot form enolate → undergoes Cannizzaro.

(ii) Acetaldehyde had α - hydrogen, so form enolate → undergoes aldol condensation.

(b) CH3COONa + NaOH

CH4 + Na2CO3

This is decarboxylation reaction.

(c) Acid (H+) : It activates carbonyl carbon increase electrophilicity and removes water to shift equilibrium forward. Thus, ester formation increases.

(d) It's Cannizzaro reaction.

2C6H5CHO → C6H5CH2OH + C6H5COONa

+ Benzyl Sodium

Conc. NaOH alcohol benzoate

Carbonyl Group: C=O functional unit where carbon is sp2 hybridised and electrophilic (Cδ+).

Aldehyde: Carbonyl compound with at least one H on the carbonyl carbon (R–CHO).

Ketone: Carbonyl compound with two carbon substituents on the carbonyl carbon (R–CO–R′).

Carboxylic Acid: Compound containing –COOH; acidic due to resonance-stabilised carboxylate.

Acyl Chloride (Acid Chloride): R–CO–Cl, highly reactive derivative of carboxylic acids used for acylation.

Nucleophilic Addition: Mechanism where a nucleophile attacks Cδ+ of C=O to give a tetrahedral intermediate.

Cyanohydrin: Product of HCN addition to a carbonyl (R–C(OH)–CN), useful for chain extension.

Bisulfite Addition: Reversible addition of NaHSO3 to carbonyls forming water-soluble adducts used for purification.

Hemiacetal / Acetal: Hemiacetal = R–C(OH)(OR′); acetal = R–C(OR′)2 formed by alcohol addition (acetal = protected carbonyl).

Imine (Schiff Base): C=NR formed by condensation of carbonyl with primary amine (–H eliminated).

Oxime / Hydrazone / Semicarbazone: C=N derivatives (from hydroxylamine, hydrazine, semicarbazide) used for characterization.

Clemmensen Reduction: Zn(Hg)/HCl reduction of C=O → CH2 (useful for acid-sensitive substrates).

Wolff–Kishner Reduction: NH2NH2 / KOH, high-temp reduction of C=O → CH2 (strongly basic conditions).

Tollen’s Test: Ag(NH3)+2 oxidises aldehydes to carboxylates; gives silver mirror (detects aldehydes).

Fehling’s Test: Cu2+ (alkaline) reduces to Cu2O (brick-red) with aliphatic aldehydes (not aromatic).

Aldol Condensation: Base/acid-catalysed coupling of carbonyls forming β-hydroxy carbonyls → α,β-unsaturated carbonyls on dehydration.

Cross-aldol: Aldol between two different carbonyls leading to mixed products (regioselectivity matters).

Cannizzaro Reaction: Disproportionation of non-enolisable aldehydes in conc. base → one molecule reduced to alcohol, one oxidised to acid.

Rosenmund Reduction: H2 / Pd–BaSO4 hydrogenation of acyl chlorides → aldehydes (partial reduction).

Stephen Reaction: Reduction of nitriles (SnCl2/HCl) to iminium then hydrolysis to aldehyde.

DIBAL-H / LiAlH(i-Bu)2: Selective hydride reagents — DIBAL at low temp reduces esters/nitriles to aldehydes; LiAlH4 fully reduces to alcohols.

Popov’s Rule (Oxidative Cleavage): In oxidation of unsymmetrical ketones, the C–CO bond breaking pattern leaves the keto carbon with the smaller alkyl group.

Hell–Volhard–Zelinsky (HVZ) Reaction: α-halogenation of carboxylic acids via acyl halide intermediate (X2 / P).

Esterification (Fischer): Acid-catalysed reversible condensation of carboxylic acid + alcohol → ester + water.

Decarboxylation: Loss of CO2 from carboxylates on heating (e.g., with soda-lime) yielding hydrocarbon (often 1 C less).

Acid Derivatives Interconversions: Carboxylic acids → acyl chlorides (SOCl2/PCl5), anhydrides, esters, amides — key synthetic transformations.

Acidity Factors: Electron-withdrawing groups (EWG) increase carboxylic acid acidity; resonance stabilisation of anion is crucial.