Properties Of Aldehydes And Ketones: Notes and Important Questions by PW

Aldehydes and ketones contain the carbonyl group (C=O), but predicting their reactions can be challenging when several reagents, reaction conditions and exceptions are involved. Understanding the electrophilic nature of the carbonyl carbon, the effect of steric hindrance and the presence or absence of an alpha-hydrogen helps you distinguish reaction products and identify the correct tests.

The Properties of Aldehydes and Ketones One-Shot Video by PW covers key concepts, tricks and previous year questions to help you build conceptual clarity and practise applying the reactions. Use the following questions to revise important reactions and test your understanding.

Nucleophilic Addition Reaction Of Carbonyl Compounds

Oxygen is more electronegative than carbon, so the carbonyl bond is polarised:

• Carbon carries a partial positive charge.
• Oxygen carries a partial negative charge.
• The carbonyl carbon acts as the electrophilic centre.

General Mechanism

1. A nucleophile attacks the carbonyl carbon.
2. The weaker pi bond breaks.
3. The electron pair moves onto oxygen, forming an alkoxide intermediate.
4. The negatively charged oxygen captures a proton from the reaction medium.

Thus, the nucleophile adds to carbon, while H+ adds to oxygen.

During this reaction, the carbonyl carbon changes hybridisation from sp2 to sp3. The originally planar carbonyl carbon becomes tetrahedral.

The carbonyl group is planar, allowing nucleophilic attack from both faces. If two different groups are attached to the carbonyl carbon, attack from either side can form two enantiomers in nearly equal amounts. The product is then a racemic mixture.

Factors Affecting Nucleophilic Addition

Electronic Effects

Electron-withdrawing groups increase the electron deficiency of the carbonyl carbon. This increases its electrophilicity and speeds up nucleophilic attack.

Electron-donating groups reduce the electron deficiency of the carbonyl carbon and slow the reaction.

Steric Effects

Bulky groups near the carbonyl carbon hinder the approach of the nucleophile.

The general reactivity order is:

Formaldehyde > Other Aldehydes > Ketones

Reasons include:

• Formaldehyde has minimum steric hindrance.
• Aldehydes have one carbon group and one hydrogen.
• Ketones have two carbon groups, causing greater steric hindrance and electron donation.

Formaldehyde reacts fastest among carbonyl compounds in nucleophilic addition reactions.

Important Nucleophilic Addition Reactions

Cyanohydrin Formation

Carbonyl compounds react with hydrogen cyanide to form cyanohydrins. The cyanide ion attacks the carbonyl carbon, and the alkoxide intermediate is protonated.

A cyanohydrin contains:

• A hydroxyl group
• A cyano group

HCN is a weak acid and gives a low concentration of CN−. Adding NaOH or KOH increases CN− concentration through formation of NaCN or KCN.

Therefore, NaCN generally makes the reaction faster than HCN.

Cyanohydrins can undergo hydrolysis. Complete hydrolysis of the cyano group produces a carboxylic acid.

Sodium Bisulfite Addition

Sodium bisulfite, NaHSO3, reacts through the sulfur atom of the bisulfite ion.

• Compound: Aldehydes: Reaction With Sodium Bisulfite: Give the reaction.
• Compound: Methyl Ketones: Reaction With Sodium Bisulfite: Give the reaction.
• Compound: Non-Methyl Ketones: Reaction With Sodium Bisulfite: Generally do not react.

A white crystalline addition product is formed. This reaction helps identify, separate, and purify aldehydes and methyl ketones.

The adduct can be filtered from unreactive impurities. Treatment with a base regenerates the original carbonyl compound. The sodium bisulfite adduct is stable enough for separation but decomposes in basic medium, allowing recovery of the carbonyl compound.

Grignard Reaction

A Grignard reagent has the formula RMgX. Its carbon atom has nucleophilic character and attacks the carbonyl carbon.

After protonation of the alkoxide intermediate:

• Carbonyl Compound: Formaldehyde: Alcohol Formed: Primary Alcohol.
• Carbonyl Compound: Aldehyde: Alcohol Formed: Secondary Alcohol.
• Carbonyl Compound: Ketone: Alcohol Formed: Tertiary Alcohol.

The reactivity order remains:

Formaldehyde > Aldehyde > Ketone

Reaction With Alcohols

In the presence of dry HCl, a carbonyl compound reacts with alcohol. Acid protonates the carbonyl oxygen and increases the electrophilicity of the carbonyl carbon.

• Starting Compound: Aldehyde: One Mole Of Alcohol: Hemiacetal; Two Moles Of Alcohol: Acetal.
• Starting Compound: Ketone: One Mole Of Alcohol: Hemiketal; Two Moles Of Alcohol: Ketal.

A hemiacetal or hemiketal contains one OH and one OR group. An acetal or ketal contains two OR groups on the same carbon.

One mole of a diol can provide two hydroxyl groups and form a cyclic acetal or ketal.

Acetals and ketals are stable in basic medium but hydrolyse in acidic medium. Hemiacetals and hemiketals can hydrolyse in both acidic and basic media.

Reaction With Ammonia Derivatives

Ammonia derivatives have the general form NH2Z. Nitrogen attacks the electrophilic carbonyl carbon. After proton transfer and loss of water, a carbon–nitrogen double bond is formed.

• Reagent: Primary Amine: Product: Imine.
• Reagent: Hydroxylamine: Product: Oxime.
• Reagent: Hydrazine: Product: Hydrazone.
• Reagent: Phenylhydrazine: Product: Phenylhydrazone.
• Reagent: Semicarbazide: Product: Semicarbazone.
• Reagent: 2,4-Dinitrophenylhydrazine: Product: 2,4-DNP Derivative.

Brady’s reagent is 2,4-dinitrophenylhydrazine. It gives red, orange, or yellow precipitates with aldehydes and ketones. Memory Tip: Remember “ROY” for Red–Orange–Yellow.

The common pattern is:

Nucleophilic attack, intermediate formation, proton transfer, and final product formation.

Oxidation And Reduction Of Carbonyl Compounds

Aldehydes are easily oxidised to carboxylic acids:

RCHO → RCOOH

In basic medium, the product is a carboxylate salt.

Ketones require strong oxidising conditions and may undergo carbon–carbon bond cleavage. According to Popoff’s rule, the carbonyl group remains with the smaller alkyl group during oxidative cleavage.

Carbonyl compounds can be reduced to alcohols using LiAlH4 or NaBH4. Hydride attacks the carbonyl carbon, forming an alkoxide that is protonated during work-up.

Clemmensen reduction uses Zn(Hg) and concentrated HCl:

C=O → CH2

Wolff–Kishner reduction uses hydrazine, KOH, ethylene glycol, and heat. It also converts C=O into CH2, but occurs in a strongly basic medium.

Tollens’ Test

Tollens’ reagent is ammoniacal silver nitrate. Aldehydes are oxidised to carboxylate ions, while Ag+ is reduced to metallic silver.

A bright silver mirror forms on the test tube.

Memory Tip: For Tollens’ test, remember: Aldehyde oxidises; Ag+ reduces to Ag.

• Compound: Aldehyde: Tollens’ Test: Positive.
• Compound: Ordinary Ketone: Tollens’ Test: Negative.
• Compound: Alpha-Hydroxy Ketone: Tollens’ Test: Positive.
• Compound: Formic Acid: Tollens’ Test: Positive.
• Compound: Hemiacetal: Tollens’ Test: Positive.
• Compound: Acetal: Tollens’ Test: Negative.
• Compound: Terminal Alkyne: Tollens’ Test: Silver Precipitate.

Hemiacetals give the test because they can form aldehydes in a basic medium. Formic acid is the important carboxylic acid that gives a positive Tollens’ test.

Fehling’s Test

Fehling’s reagent contains:

• Fehling’s solution A: aqueous copper sulfate
• Fehling’s solution B: alkaline sodium potassium tartrate

Aliphatic aldehydes reduce Cu2+ to Cu+, producing a red precipitate.

• Compound: Aliphatic Aldehyde: Fehling’s Test: Positive.
• Compound: Aromatic Aldehyde: Fehling’s Test: Generally Negative.
• Compound: Ordinary Ketone: Fehling’s Test: Negative.
• Compound: Alpha-Hydroxy Ketone: Fehling’s Test: Positive.
• Compound: Hemiacetal: Fehling’s Test: Positive.

Benzaldehyde generally does not give Fehling’s test, although aliphatic aldehydes do.

Haloform And Iodoform Reactions

The haloform test is given by:

• Methyl ketones, RCOCH3
• Ethanol
• Acetaldehyde
• Secondary alcohols containing CH3CH(OH)R

Memory Tip: Look for CH3CO− or a group that can be oxidised to CH3CO−.

With iodine and base, methyl ketones form yellow iodoform, CHI3:

RCOCH3 → RCOO− + CHI3

The reaction involves repeated halogenation of the methyl group, followed by cleavage.

Positive Examples

• Acetone
• Pentan-2-one
• Ethanol
• Isopropyl alcohol
• 2-Butanol
• Acetaldehyde

Negative Examples

• Methanol
• Pentan-3-one
• Ordinary carboxylic acids
• Cyclohexanone
• Pentan-1-ol
• Pentan-3-ol

Aldol Reaction And Condensation

Aldehydes and ketones containing an alpha-hydrogen undergo aldol reaction in a basic medium.

The first product is a beta-hydroxy carbonyl compound. On heating, it loses water and forms an alpha,beta-unsaturated carbonyl compound.

• Condition: Dilute Base Without Heating: Product: Beta-Hydroxy Carbonyl Compound.
• Condition: Base Followed By Heating: Product: Alpha,Beta-Unsaturated Carbonyl Compound.

In a cross-aldol reaction, different carbonyl compounds may produce self and cross-products. Formaldehyde, which has no alpha-hydrogen, acts only as the carbonyl component.

Intramolecular aldol condensation forms rings. When possible, six-membered ring formation is generally preferred over five-membered ring formation.

Cannizzaro Reaction

Aldehydes without alpha-hydrogen undergo the Cannizzaro reaction in strong base.

Two molecules of the same aldehyde undergo simultaneous oxidation and reduction:

• One molecule forms a carboxylate salt.
• The other forms a primary alcohol.

2RCHO + OH− → RCOO− + RCH2OH

This is a disproportionation reaction because the same compound is oxidised and reduced.

In a cross-Cannizzaro reaction, two different aldehydes without alpha-hydrogen react. The more electrophilic aldehyde is oxidised, while the less reactive aldehyde is reduced.

Aldehyde And Ketone Reaction Summary

• Reaction: Nucleophilic Addition: Aldehyde: Positive; Ketone: Positive, Slower.
• Reaction: Tollens’ Test: Aldehyde: Positive; Ketone: Generally Negative.
• Reaction: Fehling’s Test: Aldehyde: Aliphatic Aldehydes Positive; Ketone: Negative.
• Reaction: Iodoform Test: Aldehyde: Acetaldehyde Positive; Ketone: Methyl Ketones Positive.
• Reaction: Aldol Reaction: Aldehyde: If Alpha-H Is Present; Ketone: If Alpha-H Is Present.
• Reaction: Cannizzaro Reaction: Aldehyde: If Alpha-H Is Absent; Ketone: Not Applicable.

Important Questions: Properties of Aldehydes and Ketones

1. Arrange formaldehyde, acetaldehyde and acetone in decreasing order of reactivity towards nucleophilic addition. Give a reason.

Solution: The order is:

Formaldehyde > Acetaldehyde > Acetone

Formaldehyde has the least steric hindrance and no electron-donating alkyl group. Acetaldehyde has one alkyl group, while acetone has two, increasing steric hindrance and reducing the electrophilicity of the carbonyl carbon.

2. What are the products when formaldehyde and acetone react separately with a Grignard reagent, followed by acidic hydrolysis?

Solution:

• Formaldehyde + RMgX, followed by hydrolysis, gives a primary alcohol.
• Acetone + RMgX, followed by hydrolysis, gives a tertiary alcohol.

The Grignard reagent adds an alkyl group to the carbonyl carbon, and protonation of the intermediate produces the alcohol.

3. How can Tollens’ and Fehling’s tests be used to distinguish benzaldehyde from acetaldehyde?

Solution:

• Tollens’ test: Both benzaldehyde and acetaldehyde generally give a positive test, producing a silver mirror.
• Fehling’s test: Acetaldehyde gives a positive test with a red precipitate of cuprous oxide, while benzaldehyde generally gives a negative test.

4. Which compounds undergo aldol condensation and Cannizzaro reaction? Explain the role of alpha-hydrogen.

Solution:

• Aldol condensation: Aldehydes and ketones containing at least one alpha-hydrogen can undergo aldol reaction in a suitable basic medium. The initial product is a beta-hydroxy carbonyl compound, which can lose water on heating.
• Cannizzaro reaction: Aldehydes without an alpha-hydrogen undergo disproportionation in concentrated base. One molecule is oxidised to a carboxylate salt, while another is reduced to a primary alcohol.

5. Which of the following gives a positive iodoform test: acetone, pentan-3-one, ethanol and cyclohexanone? Explain.

Solution: Acetone and ethanol give a positive iodoform test.

• Acetone contains the required CH3CO- group.
• Ethanol is oxidised under the reaction conditions to acetaldehyde, which gives the test.
• Pentan-3-one lacks the required methyl ketone group.
• Cyclohexanone does not contain the required CH3CO- group.

A positive iodoform test produces a yellow precipitate of CHI3.

Watch PROPERTIES OF ALDEHYDES & KETONES in ONE SHOT || All Concepts, Tricks & PYQ || NEET by PW to practise more varied questions and strengthen your confidence in identifying reaction products, applying named reactions and distinguishing aldehydes from ketones through qualitative tests.

Understanding the properties of aldehydes and ketones becomes easier when you connect reaction mechanisms with reagents, conditions and product formation. Revise nucleophilic addition, oxidation and reduction, qualitative tests, aldol condensation and the Cannizzaro reaction, then practise questions with PW One- Shot Video to improve your accuracy and confidence in Organic Chemistry.

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