Aldehydes, Ketones and Carboxylic Acids Class 12 Chapter Notes

Must remember

• Carbonyl group C=O is polar: carbon is electrophilic and oxygen is nucleophilic; most aldehyde/ketone reactions begin with nucleophilic attack on carbonyl carbon.
• Reactivity towards nucleophilic addition: HCHO > RCHO > ArCHO > R2CO > ArCOR > Ar2CO approximately; steric effect and +I/+R electron donation reduce electrophilicity.
• Aldehydes oxidise easily to carboxylic acids and give Tollens’ silver mirror and Fehling’s red precipitate; ordinary ketones do not, except alpha-hydroxy ketones/reducing sugars in special cases.
• Reduction map: aldehyde → 1° alcohol, ketone → 2° alcohol using NaBH4/LiAlH4; carbonyl → methylene using Clemmensen or Wolff-Kishner reduction.
• Aldol condensation requires at least one alpha-hydrogen; Cannizzaro reaction requires aldehyde with no alpha-hydrogen in concentrated alkali.
• Haloform test is given by methyl ketones, CH3CO-R, and compounds oxidisable to them such as CH3CH(OH)-R; yellow CHI3 confirms iodoform test.
• Carboxylic acids are stronger than phenols because the carboxylate ion has two equivalent resonance structures, while phenoxide has less effective charge delocalisation.
• Electron-withdrawing groups increase acid strength of carboxylic acids: ClCH2COOH > CH3COOH; effect decreases with distance: alpha > beta > gamma.
• For conversions, identify the functional group level: alcohol/aldehyde/ketone/acid/acid chloride/ester/amide, then choose oxidation, reduction, substitution or addition route.

Key definitions and exam terms

Carbonyl group

The functional group C=O present in aldehydes and ketones; it is planar, polar and undergoes nucleophilic addition.

Aldehyde

Organic compound containing -CHO group, where carbonyl carbon is bonded to at least one hydrogen; general formula R-CHO.

Ketone

Organic compound containing >C=O group bonded to two carbon groups; general formula R-CO-R'.

Nucleophilic addition

Reaction in which a nucleophile attacks electrophilic carbonyl carbon, followed by protonation or elimination depending on reagent.

Alpha-hydrogen

Hydrogen attached to the carbon next to a carbonyl carbon; required for aldol condensation and alpha-halogenation.

Aldol condensation

Base-catalysed reaction of aldehydes/ketones having alpha-H to form beta-hydroxy carbonyl compounds, which may dehydrate to alpha,beta-unsaturated compounds.

Cannizzaro reaction

Disproportionation of aldehydes without alpha-H in concentrated alkali to give one molecule of alcohol and one molecule of carboxylate salt.

Carboxylic acid

Compound containing -COOH group; acidic due to formation of resonance-stabilised carboxylate ion.

Esterification

Reaction of a carboxylic acid with alcohol in presence of acid catalyst to form ester and water; reversible equilibrium reaction.

Carbonyl group: core idea behind the whole chapter

The most important concept is the polarity of C=O. Oxygen is more electronegative, so the carbonyl carbon carries partial positive charge and becomes the site for nucleophilic attack. Aldehydes are generally more reactive than ketones because an aldehyde has one alkyl/aryl group and one small hydrogen, while ketones have two carbon groups that create more steric hindrance and donate electron density by +I effect. Aromatic carbonyl compounds are often less reactive because the carbonyl group can participate in resonance with the benzene ring, reducing the positive character of carbonyl carbon. After nucleophile attack, the C=O pi bond breaks to give an alkoxide intermediate; protonation gives alcohol-type addition products. Many named reactions in this chapter are variations of this idea: addition of HCN gives cyanohydrin, NH2OH gives oxime, hydrazine gives hydrazone, semicarbazide gives semicarbazone, and 2,4-DNP gives orange-red precipitate. For board revision, connect every reagent to the atom that attacks: CN-, hydride H-, ammonia derivative nitrogen, or enolate carbon.

Aldehydes vs ketones vs carboxylic acids: quick comparison

Revision process for solving conversion questions

1. —
2. —
3. —
4. —
5. —
6. —
7. —
8. —

Important preparations and reaction map

Aldehydes and ketones preparation: oxidation/dehydrogenation of alcohols gives aldehydes from 1° alcohols and ketones from 2° alcohols. Acid chlorides undergo Rosenmund reduction to aldehydes. Nitriles with SnCl2/HCl followed by hydrolysis give aldehydes by Stephen reaction. Aromatic aldehydes can be made by Gattermann-Koch reaction from benzene using CO + HCl in presence of AlCl3/CuCl. Ketones are prepared by Friedel-Crafts acylation of arenes or by reacting acid chlorides with dialkyl cadmium. Carboxylic acids preparation: oxidation of primary alcohols/aldehydes, hydrolysis of nitriles/amides, carbonation of Grignard reagent followed by acid hydrolysis, and hydrolysis of esters/acid chlorides. Remember: Grignard route adds one carbon because CO2 contributes the carboxyl carbon.

Named reactions and tests: compact board table

Acidity trends in carboxylic acids

Short worked mini-examples

• {"title":"Example 1: Identify reagent for reduction","bodyMarkdown":"Convert propanone to propane. First change >C=O to -CH2-, not alcohol. Use Clemmensen reduction Zn-Hg/HCl or Wolff-Kishner NH2NH2/KOH, heat. Product: CH3COCH3 → CH3CH2CH3."}
• {"title":"Example 2: Predict aldol product","bodyMarkdown":"Ethanal has alpha-H, so in dilute NaOH two molecules give 3-hydroxybutanal: 2CH3CHO → CH3CH(OH)CH2CHO. On heating, it dehydrates to but-2-enal."}
• {"title":"Example 3: Distinguish ethanal and propanone","bodyMarkdown":"Use Tollens’ reagent: ethanal gives silver mirror, propanone does not. Caution: both can give iodoform test, so iodoform cannot distinguish this pair."}

Exam tips and board traps

Trap 1: Do not write Fehling’s test as positive for all aldehydes; aromatic aldehydes like benzaldehyde usually fail Fehling’s test but give Tollens’ test. Trap 2: Aldol needs alpha-H; benzaldehyde has no alpha-H, so it gives Cannizzaro, not aldol. Trap 3: Iodoform test is not exclusive to methyl ketones; ethanol and ethanal also give it because they form the required CH3CO- unit. Trap 4: NaBH4 reduces aldehydes/ketones but normally not carboxylic acids and esters in board-level conditions; LiAlH4 is stronger. Marking cue: In conversions, write reagent, condition and product structure. For named reactions, include the special condition such as conc. NaOH for Cannizzaro, dilute NaOH for aldol, and SOCl2 for acid chloride preparation.

Practice Questions with Solutions

• Q: Why are aldehydes more reactive than ketones towards nucleophilic addition? A: Aldehydes have less steric hindrance and fewer electron-donating alkyl groups, so carbonyl carbon is more electrophilic.
• Q: Which test distinguishes benzaldehyde and acetophenone? A: Tollens’ test: benzaldehyde gives silver mirror; acetophenone does not.
• Q: What is the product when acetic acid reacts with SOCl2? A: Acetyl chloride, CH3COCl, along with SO2 and HCl gases.
• Q: Which reaction will benzaldehyde undergo in concentrated NaOH: aldol or Cannizzaro? A: Cannizzaro reaction, because benzaldehyde has no alpha-hydrogen.

Frequently Asked Questions