Aldehydes, Ketones and Carboxylic Acids Notes
Study Notes
Topics
5Chapter Overview
Overview
This chapter studies organic compounds containing the carbonyl group, especially aldehydes, ketones and carboxylic acids. Aldehydes have the group -CHO, ketones have >C=O within the carbon chain, and carboxylic acids have -COOH. Their chemistry is governed by the polarity of the carbonyl bond: carbon becomes electrophilic and oxygen becomes electron rich. NEET questions commonly test nomenclature, preparation methods, nucleophilic addition, oxidation, reduction, aldol condensation, Cannizzaro reaction, acidity of carboxylic acids, named reactions and laboratory tests. Mastery comes from connecting structure with reactivity: aldehydes are generally more reactive than ketones, and carboxylic acids are acidic due to resonance-stabilised carboxylate ions.
- 1The chapter is built around electronic effects, steric effects and resonance.
- 2Nucleophilic addition is the characteristic reaction of aldehydes and ketones.
- 3Carboxylic acids undergo reactions involving cleavage of O-H, C-OH or C=O bonds.
- 4Electron-withdrawing groups increase acidity of carboxylic acids; electron-donating groups decrease acidity.
- 5Named reactions such as Rosenmund reduction, Stephen reaction, Clemmensen reduction, Wolff-Kishner reduction, HVZ reaction, aldol and Cannizzaro are high-yield NEET areas.
- 6Laboratory tests are frequently asked as assertion-reason and match-the-column questions.
AKC Order
Remember the chapter as A-K-C: Aldehydes and Ketones first, then Carboxylic acids. First learn carbonyl polarity, then addition, then oxidation-reduction, then acidity.
Aldehyde Tests
Aldehydes give a 'silver mirror' in Tollens' test: A for Aldehyde, A for Ag mirror.
Everyday Occurrence
Formaldehyde is used as formalin, acetaldehyde occurs in ethanol metabolism, acetone is a common solvent, and acetic acid is present in vinegar.
NEET Pattern
A typical NEET question may ask which compound gives Tollens' test, iodoform test, aldol condensation or Cannizzaro reaction.
Treating Aldehydes and Ketones as Equally Reactive
Aldehydes are generally more reactive toward nucleophilic addition due to less steric hindrance and weaker +I effect than ketones.
Confusing Aldol and Cannizzaro
Aldol needs alpha hydrogen; Cannizzaro needs no alpha hydrogen and concentrated alkali.
Open-chain saturated aldehydes and ketones with one carbonyl group have the same molecular formula but different structures, making them functional isomers.
Variables
n=Number of carbon atoms in the molecule
This formula represents open-chain saturated monocarboxylic acids such as formic acid, acetic acid and propionic acid.
Variables
n=Number of carbon atoms in the acid
Aldehydes & Ketones
Overview
Aldehydes and ketones are carbonyl compounds containing a C=O group. In aldehydes, the carbonyl carbon is bonded to at least one hydrogen, so the functional group is -CHO. In ketones, the carbonyl carbon is bonded to two carbon groups, giving R-CO-R'. They are classified as aliphatic or aromatic and named using suffixes -al for aldehydes and -one for ketones. The carbonyl carbon is sp2 hybridised, trigonal planar and electrophilic because oxygen pulls electron density. Their physical properties depend on polarity, molecular mass and absence of intermolecular hydrogen bonding. Aldehydes are generally more reactive than ketones because ketones have two electron-donating alkyl groups and greater steric hindrance.
- 1Methanal is formaldehyde and ethanal is acetaldehyde; propanone is acetone.
- 2Aromatic aldehydes such as benzaldehyde have -CHO directly attached to an aryl group.
- 3In naming, the aldehyde carbon is always carbon 1 in an open chain.
- 4Ketone carbonyl position is indicated by the lowest possible locant.
- 5The carbonyl group has both sigma and pi bonds; pi electrons are pulled toward oxygen.
- 6Dipole-dipole attractions explain the moderate boiling points of aldehydes and ketones.
Suffix Trick
Aldehyde has A, so use -al. Ketone has O, so use -one.
Reactivity Trick
Aldehyde has one bulky group; ketone has two. Less crowding means faster attack, so aldehyde is more reactive.
Classification Example
CH3CHO is an aliphatic aldehyde, C6H5CHO is an aromatic aldehyde, CH3COCH3 is a symmetrical ketone, and CH3COC2H5 is an unsymmetrical ketone.
Naming Example
CH3CH2CHO is propanal. CH3COCH2CH3 is butan-2-one because the carbonyl carbon gets the lowest possible number.
Wrong Numbering in Aldehydes
Do not assign a locant to the aldehyde carbon in simple chains; it is automatically carbon 1.
Assuming Carbonyl Compounds Self Hydrogen Bond
Aldehydes and ketones can accept hydrogen bonds from water but cannot donate hydrogen bonds to themselves.
The carbonyl carbon is attached to one hydrogen and one alkyl, aryl or hydrogen group.
Variables
R=Alkyl, aryl or hydrogen group
CHO=Aldehydic functional group
The carbonyl carbon is bonded to two carbon groups, which may be same or different.
Variables
R, R'=Alkyl or aryl groups
CO=Carbonyl group
Preparation Methods
Overview
Preparation methods connect functional group interconversion across alcohols, hydrocarbons, acid derivatives, nitriles and Grignard reagents. Aldehydes are prepared by controlled oxidation of primary alcohols, dehydrogenation, Rosenmund reduction of acid chlorides, Stephen reduction of nitriles and ozonolysis of alkenes. Ketones are prepared by oxidation of secondary alcohols, hydration of alkynes, Friedel-Crafts acylation, reaction of acid chlorides with dialkyl cadmium or organocuprates, and ozonolysis. Carboxylic acids are prepared by oxidation of primary alcohols and aldehydes, hydrolysis of nitriles, hydrolysis of acid derivatives, Grignard carboxylation and side-chain oxidation of alkyl benzenes. NEET often tests reagent selectivity and product prediction.
- 1Use PCC or mild oxidants to stop primary alcohol oxidation at aldehyde stage.
- 2KMnO4 or K2Cr2O7/H+ oxidises primary alcohols and aldehydes to acids.
- 3Acid chlorides are more reactive than aldehydes, so Rosenmund catalyst is poisoned to prevent over-reduction.
- 4Aromatic methyl groups are oxidised to benzoic acid by strong oxidising agents.
- 5Friedel-Crafts acylation is useful for preparing aromatic ketones.
- 6Dialkyl cadmium reacts with acid chlorides to give ketones without further addition.
Primary Alcohol Path
Primary alcohol has two oxidation stops: first aldehyde, final acid. PCC parks at aldehyde; KMnO4 keeps moving to acid.
Grignard Plus CO2
CO2 adds one carbon. Count carbons before and after to avoid product mistakes.
Solved Conversion
Convert ethanol to ethanoic acid: CH3CH2OH is oxidised using acidified KMnO4 or K2Cr2O7 to CH3COOH.
PYQ Concept
If an alkene on ozonolysis gives only acetone, the alkene is 2,3-dimethylbut-2-ene because both double-bond carbons have two methyl groups.
Using Strong Oxidant for Aldehyde Preparation
Strong oxidants usually over-oxidise primary alcohols to carboxylic acids. Use PCC or suitable controlled methods for aldehydes.
Forgetting Hydrolysis Step
Stephen reaction, nitrile hydrolysis and Grignard carboxylation require hydrolysis/acid work-up to give final products.
Controlled oxidation or catalytic dehydrogenation of a primary alcohol gives an aldehyde.
Variables
R=Alkyl or aryl group
[O]=Oxidising agent such as PCC
Secondary alcohols are readily oxidised to ketones.
Variables
R, R'=Carbon groups attached to the carbinol carbon
[O]=Oxidising agent
Physical & Chemical Properties
Overview
The properties of aldehydes and ketones arise from the polar carbonyl group. Physically, they show dipole-dipole interactions, moderate boiling points and water solubility for lower members due to hydrogen bonding with water. Chemically, the electrophilic carbonyl carbon undergoes nucleophilic addition with HCN, NaHSO3, alcohols, ammonia derivatives and Grignard reagents. Aldehydes oxidise readily to acids, while ketones need strong conditions. Reduction gives alcohols or hydrocarbons depending on reagent. Aldol condensation occurs when alpha hydrogen is present, forming beta-hydroxy aldehydes/ketones followed by dehydration. Cannizzaro reaction occurs in aldehydes without alpha hydrogen, giving one alcohol and one carboxylate salt by disproportionation.
- 1Nucleophilic addition generally has two steps: attack of nucleophile, then protonation.
- 2Electron-withdrawing groups increase carbonyl reactivity; electron-donating groups decrease it.
- 3Steric hindrance decreases rate of nucleophilic addition.
- 4Ammonia derivatives form imines, oximes, hydrazones and semicarbazones.
- 5Aldol products are beta-hydroxy carbonyl compounds.
- 6Cross aldol reactions can give mixtures unless one reactant lacks alpha hydrogen.
- 7Cannizzaro is a redox reaction in which the same aldehyde is both oxidised and reduced.
Aldol Condition
Aldol = Alpha hydrogen Does Organic Linkage. If no alpha hydrogen, aldol cannot start.
Cannizzaro Condition
Cannizzaro says 'Cannot alpha'. Aldehydes without alpha H go to Cannizzaro in concentrated base.
Solved Example: Product of HCN Addition
Acetaldehyde plus HCN gives CH3CH(OH)CN, a cyanohydrin. The new C-C bond forms between carbonyl carbon and cyanide carbon.
Solved Example: Reaction Choice
Benzaldehyde has no alpha hydrogen, so it undergoes Cannizzaro reaction, not aldol condensation.
Applying Fehling's Test to All Aldehydes
Aliphatic aldehydes generally give Fehling's test, but aromatic aldehydes like benzaldehyde usually do not.
Forgetting Protonation in Addition
After nucleophilic attack, the alkoxide must be protonated to give the final alcohol-type product.
Confusing Reduction Products
NaBH4/LiAlH4 gives alcohols, while Clemmensen/Wolff-Kishner removes oxygen and gives hydrocarbons.
CN− attacks the carbonyl carbon and protonation gives cyanohydrin.
Variables
R=Hydrogen, alkyl or aryl group
CN=Cyano group introduced by cyanide ion
Aldehydes form primary alcohols and ketones form secondary alcohols on reduction.
Variables
[H]=Reducing equivalent from NaBH4, LiAlH4 or catalytic hydrogenation
R=Organic group
Carboxylic Acids
Overview
Carboxylic acids contain the carboxyl group, -COOH, which combines a carbonyl group and a hydroxyl group on the same carbon. They are named using the suffix -oic acid, while aromatic acids such as benzoic acid retain common names. Their high boiling points arise from strong intermolecular hydrogen bonding and dimer formation. Their acidic character is due to resonance stabilisation of the carboxylate ion after loss of H+. Electron-withdrawing groups increase acidity, especially when closer to -COOH, while electron-donating groups decrease acidity. Carboxylic acids are prepared by oxidation, hydrolysis of nitriles and derivatives, and Grignard carboxylation. They form salts, esters, acid chlorides, amides, anhydrides and undergo HVZ and decarboxylation reactions.
- 1Carboxyl carbon is sp2 hybridised and approximately planar.
- 2Lower carboxylic acids are miscible with water; solubility decreases with increasing alkyl chain length.
- 3Carboxylic acids react with NaHCO3 to release CO2, unlike most phenols.
- 4Esterification with alcohols in acid gives esters and water.
- 5Decarboxylation of sodium carboxylates with soda lime gives alkanes with one fewer carbon.
- 6Acid derivatives generally follow reactivity order: acid chloride > anhydride > ester > amide.
Acidity Rule
Withdraw electrons, acid strengthens. Donate electrons, acid weakens.
Derivative Reactivity
Remember acid derivative reactivity as CAME: Chloride, Anhydride, ester, amide.
Acidity Order Example
CCl3COOH > CHCl2COOH > ClCH2COOH > CH3COOH because increasing chlorine atoms increase the -I effect.
Derivative Example
Ethanoic acid reacts with SOCl2 to form ethanoyl chloride, which can then form amides, esters or anhydrides.
Ignoring Distance Effect
Electron-withdrawing groups increase acidity most when close to -COOH; the effect decreases with distance.
Confusing Esterification and Saponification
Esterification forms ester in acid medium; saponification hydrolyses ester in base.
Wrong Decarboxylation Carbon Count
Soda lime decarboxylation removes the carboxyl carbon, so the alkane has one carbon fewer than the acid salt.
Carboxylic acids donate a proton to form resonance-stabilised carboxylate ions.
Variables
RCOOH=Carboxylic acid
RCOO−=Carboxylate ion
Carboxylic acid reacts with alcohol in acidic medium to form ester.
Variables
RCOOR'=Ester
R'OH=Alcohol
Important Reactions & Uses
Overview
This topic consolidates the most testable reactions, qualitative tests and applications of aldehydes, ketones and carboxylic acids. Named reactions such as Rosenmund reduction, Stephen reaction, Etard reaction, Clemmensen reduction, Wolff-Kishner reduction, aldol condensation, Cannizzaro reaction, haloform reaction and HVZ reaction are essential for NEET. Laboratory tests include 2,4-DNP for carbonyl compounds, Tollens' and Fehling's tests for aldehydes, Schiff's test, sodium bisulphite addition, iodoform test for methyl ketones and NaHCO3 test for carboxylic acids. Uses are also important: formalin as disinfectant and preservative, acetone as solvent, benzaldehyde in perfumes, acetic acid in vinegar, benzoic acid as preservative and carboxylic acids in metabolism.
- 12,4-DNP confirms the presence of a carbonyl group but does not distinguish aldehyde from ketone.
- 2Tollens' reagent is ammoniacal silver nitrate, [Ag(NH3)2]+.
- 3Fehling's test is not generally given by aromatic aldehydes.
- 4Iodoform test requires CH3CO- group or a group oxidisable to CH3CO-.
- 5Schiff's reagent gives pink/magenta colour with aldehydes.
- 6Formaldehyde, acetaldehyde, acetone, acetic acid and benzoic acid are common NCERT examples with practical uses.
- 7Biologically, aldehydes and ketones occur in sugars, hormones and metabolic intermediates; carboxylic acids occur in amino acids, fatty acids and the Krebs cycle.
Test Memory
DNP detects carbonyl, Tollens tells aldehyde, Iodoform identifies methyl ketone, bicarbonate bubbles for acid.
Reduction Medium
Clemmensen is acidic: C comes with HCl. Wolff-Kishner is basic: K in Kishner reminds KOH.
Test Identification Example
A compound gives 2,4-DNP and Tollens' test but not iodoform test. It is likely an aldehyde without CH3CHO group, such as propanal or benzaldehyde.
Industrial Application Example
Acetone is widely used as a solvent because it dissolves many organic substances and evaporates quickly.
Biological Importance Example
Pyruvic acid, acetic acid derivatives, fatty acids and amino acids show the importance of carbonyl and carboxyl groups in metabolism.
Using 2,4-DNP to Distinguish Aldehyde and Ketone
2,4-DNP confirms carbonyl group in both aldehydes and ketones. Use Tollens', Fehling's or Schiff's for aldehydes.
Forgetting Ethanol Gives Iodoform Test
Ethanol is oxidised to acetaldehyde under test conditions, so it gives iodoform test even though it is not a ketone.
Assuming All Acids Give Same Strength of NaHCO3 Reaction
Carboxylic acids give brisk CO2 effervescence, but weakly acidic phenols usually do not react with NaHCO3.
Aldehyde is oxidised and Ag+ is reduced to metallic silver mirror.
Variables
RCHO=Aldehyde
Ag=Metallic silver deposit
Aliphatic aldehydes reduce Cu2+ to red precipitate of cuprous oxide.
Variables
Cu2O=Red precipitate
RCOO−=Carboxylate ion formed by oxidation
Formula Sheet
10Open-chain saturated aldehydes and ketones with one carbonyl group have the same molecular formula but different structures, making them functional isomers.
Variables
n=Number of carbon atoms in the molecule
This formula represents open-chain saturated monocarboxylic acids such as formic acid, acetic acid and propionic acid.
Variables
n=Number of carbon atoms in the acid
Oxygen is more electronegative than carbon, so the carbonyl carbon becomes electron deficient and is attacked by nucleophiles.
Variables
δ+=Partial positive charge
δ−=Partial negative charge
The carbonyl carbon is attached to one hydrogen and one alkyl, aryl or hydrogen group.
Variables
R=Alkyl, aryl or hydrogen group
CHO=Aldehydic functional group
The carbonyl carbon is bonded to two carbon groups, which may be same or different.
Variables
R, R'=Alkyl or aryl groups
CO=Carbonyl group
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NEET PYQs — Aldehydes, Ketones and Carboxylic Acids
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Compound P(C₈H₈O) gives a red-orange precipitate with 2,4-DNP reagent and does not reduce Fehling's reagent. On drastic oxidation with chromic acid, P gives an aromatic acidic product Q that produces effervescence on treatment with aq. NaHCO₃. Compound P and Q respectively are:
Fehling’s solution ‘A’ is:
Identify the major product obtained in the following reaction:
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