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Reaction of Aldehydes and Ketones - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:35 AM | #JEE Main

Quick Facts

  • Nucelophilic Addition Reaction, Intermolecular Cannizaro Reaction is considered one of the most asked concept.

  • 102 Questions around this concept.

Solve by difficulty

The increasing order of the rate of HCN addition to compounds A-D is

A.\; \; HCHO

B.\; \; CH_3COCH_3

C.\; \; PhCOCH_3

D.\; \; PhCOPh

Which one is most reactive towards Nucleophilic addition reaction?

Reaction of a carbonyl compound with one of the following reagents involves nucleophilic addition followed by elimination of water. The reagent is :

Which of the following reactions will yield 2,2­-dibromopropane?

Given    Fe^{3+}(aq)+e^{-}\rightarrow Fe^{2+}(aq);E^{\circ}=+0.77\, V

Al^{3+}(aq)+3e^{-}\rightarrow Al(s);E^{\circ}=-1.66\, V

Br_{2}(aq)+2e^{-}\rightarrow 2Br^{-};E^{\circ}=+1.09V

Considering the electrode potentials, which of the following represents the correct order of reducing power?

Which of the following series correctly represents relations between the elements from X to Y?

X\rightarrow Y

Silver mirror test is given by which one of the following compounds ?

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Major product formed in the following reaction is a mixture of :


This reduction reaction is known as :

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The correct order of bond dissociation energy among , N_{2},O_{2},O_{2}^{-} is shown in which of the following arrangements ?

Concepts Covered - 6

Nucelophilic Addition Reaction

(i) Mechanism of nucleophilic addition reactions:
A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon. The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu and H+ across the carbon oxygen double bond as shown in the figure below.

(ii) Reactivity
Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.

Reduction and Oxidation Reaction

Reduction to hydrocarbons:
The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and concentrated hydrochloric acid (Clemmensen reduction) hydrazine hydrazone or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).

Oxidation
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.

Intermolecular Aldol Condensation

It is the condensation taking place when two different aldehydes or two different ketones or one aldehyde and one molecule of ketone both containing \alpha-H atoms. A number of products due to self-condensation and cross condensation is obtained. The reaction occurs as follows.

For example,


 

Intramolecular Aldol Condensation

The aldehydes and ketones undergo a number of reactions due to the acidic nature of \alpha-H, which in turn is due to the strong electron-withdrawing effect of the (C=O) group and resonance stabilisation of the conjugate base. When two molecules of the same aldehyde or ketone containing \alpha-H atom condense together in the presence of dilute alkali, such as NaOH, KOH, K2CO3, Na2CO3, or at least 2 \alpha-H-atoms to give a molecule of aldol or ketol, it is called aldol condensation. On heating, it loses a molecule of H2O to give a molecule of α, β-unsaturated aldehyde or ketone.

For example,

Intermolecular Cannizaro Reaction

When two different aldehydes lacking α-H atom are reacted in the presence of a strong base, they undergo disproportionation or redox reaction to give a molecule of alcohol and salt of an acid. Alcohol is obtained from the less reactive aldehyde and acid salt is obtained from the more reactive aldehyde. In this reaction, OH- attacks at the C of (C=O) group of more reactive aldehyde and gives adduct anion from which H- ion is transferred to the less reactive aldehyde. It gives acid ion from more reactive aldehyde and alcohol from less reactive aldehyde.

For example,

Intramolecular Cannizaro Reaction

Two molecules of the same aldehyde lacking α-H atom undergo disproportionation or redox reaction in the presence of a strong base to give a molecule of alcohol and a molecule of the salt of an acid.

For example,

Study it with Videos

Nucelophilic Addition Reaction
Reduction and Oxidation Reaction
Intermolecular Aldol Condensation

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Books

Reference Books

Nucelophilic Addition Reaction

Chemistry Part II Textbook for Class XII

Page No. : 366

Line : 8

Intermolecular Aldol Condensation

Chemistry Part II Textbook for Class XII

Page No. : 371

Line : 22

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