CBSE Class 12 Chemistry Notes Chapter 12 Aldehydes, Ketones and Carboxylic Acids

Aldehyde ketone and carboxylic acid notes cover vital organic compounds containing the carbonyl functional group, where a carbon atom is double-bonded to oxygen. Aldehydes feature the carbonyl group at the chain end, while ketones have it internally. Carboxylic acids possess a carboxyl group. These notes provide a clear roadmap for mastering reactions, IUPAC naming, and chemical properties.

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Understanding Aldehyde Ketone and Carboxylic Acid Notes Class 12 for Board Exams

Organic chemistry often feels like a giant puzzle. When you dive into aldehyde ketone and carboxylic acid notes, you're looking at the heart of organic synthesis. These compounds are everywhere, from the fragrance of vanilla to the tang of vinegar. To ace your exams, you need to understand how the polar nature of the carbonyl group dictates every reaction you'll study. We've designed these notes to help you visualize the molecular changes, making it easier to predict products during your JEE or Board preparations.

Classification and Structure of the Carbonyl Group

The carbonyl group consists of a carbon atom double-bonded to an oxygen atom. In aldehydes (R-CHO), the carbonyl carbon is bonded to at least one hydrogen atom. However, in ketones (R-CO-R'), the carbonyl carbon is bonded to two carbon atoms. The carbon atom in the carbonyl group is sp^2 hybridized. This creates a trigonal planar structure. The bond angles are approximately 120 degrees. Because oxygen is more electronegative than carbon, the C=O bond is highly polar. You'll find that this polarity makes the carbon electrophilic and the oxygen nucleophilic.

IUPAC Nomenclature and Common Names

Naming these compounds is your first step toward mastery. For aldehydes, we replace the "-e" of the parent alkane with "-al." For example, ethane becomes ethanal. If the aldehyde group is attached to a ring, we use the suffix "carbaldehyde." Ketones are different. You replace the "-e" with "-one." Propanone is the simplest ketone. When you use aldehyde ketone and carboxylic acid notes handwritten styles, it’s helpful to draw the structures next to the names. Carboxylic acids use the suffix "-oic acid." Methanoic acid is the IUPAC name for formic acid. Always number the chain starting from the carbonyl carbon to keep things consistent.

Preparation of Aldehydes and Ketones

You can prepare these compounds through several reliable methods. Oxidation of primary alcohols yields aldehydes, while secondary alcohols give you ketones. Use acidified K_2Cr_2O_7 or KMnO_4 for this. Dehydrogenation of alcohols over heated copper at 573K is another great choice. We also use the Rosenmund reduction. This involves the hydrogenation of acyl chlorides over a catalyst like Pd/BaSO_4. Don't forget the Stephen reaction! Here, nitriles are reduced to imines with stannous chloride and then hydrolyzed. Ozonolysis of alkenes also provides a pathway to both aldehydes and ketones depending on the alkene structure.

Advanced Synthesis for Aldehyde Ketone and Carboxylic Acid Notes JEE

For competitive exams, you must know the Etard reaction. This uses chromyl chloride (CrO_2Cl_2) to oxidize toluene to benzaldehyde. Gatterman-Koch reaction is another favorite. It treats benzene with CO and HCl in the presence of anhydrous AlCl_3. Friedel-Crafts acylation is the primary way to make aromatic ketones. You react benzene with an acyl chloride. Grignard reagents are also vital tools. Adding a Grignard reagent to a nitrile followed by hydrolysis produces a ketone. These specific reactions often appear in aldehyde ketone and carboxylic acid notes pdf downloads used by toppers.

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Physical Properties of Carbonyl Compounds

Methanal is a gas at room temperature. Most other lower aldehydes and ketones are liquid. Their boiling points are higher than non-polar hydrocarbons of similar mass. This happens because of dipole-dipole interactions. However, they don't have boiling points as high as alcohols. Alcohols can form intermolecular hydrogen bonds, but aldehydes and ketones can't. Small molecules like acetone and formaldehyde mix well with water. They form hydrogen bonds with water molecules. As the carbon chain grows longer, solubility decreases rapidly. This is due to the increase in the hydrophobic nature of the alkyl part.

Chemical Reactions: Nucleophilic Addition

The most common reaction is nucleophilic addition. Since the carbonyl carbon is electron-deficient, nucleophiles attack it easily. We see this in the addition of HCN to form cyanohydrins. Sodium hydrogensulphite (NaHSO_3) adds to aldehydes and methyl ketones to form crystalline solids. This is actually used for purification. Reacting with Grignard reagents is a staple. It produces alcohols after hydrolysis. Addition of alcohols to aldehydes forms acetals. Ketones form ketals under similar conditions. These reactions are reversible. They require an acid catalyst to proceed effectively.

Oxidation and Reduction Processes

Aldehydes are very easy to oxidize. They turn into carboxylic acids even with mild agents. Tollen’s test is a classic example. It uses ammoniacal silver nitrate to produce a silver mirror. Fehling’s test uses copper ions. A red precipitate of cuprous oxide forms if an aldehyde is present. Ketones don't react with these mild reagents. They require harsh conditions to oxidize. Reduction is also possible. Clemmensen reduction uses Zn-Hg and HCl to turn C=O into CH_2. Wolff-Kishner reduction uses hydrazine and KOH. Both methods effectively remove the oxygen atom entirely.

Understanding the Alpha-Hydrogen Reactivity

The hydrogen atoms on the carbon adjacent to the carbonyl group are acidic. This leads to the Aldol condensation. Aldehydes or ketones with at least one alpha-hydrogen undergo this in dilute alkali. The result is a beta-hydroxy aldehyde or ketone. If there are no alpha-hydrogens, like in benzaldehyde, Cannizzaro reaction occurs. Here, one molecule is reduced to alcohol while the other is oxidized to a salt of carboxylic acid. This disproportionation is a key topic in aldehyde ketone and carboxylic acid notes.

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Carboxylic Acids: Structure and Importance

Carboxylic acids contain the -COOH group. The bonds are in one plane. The carboxyl carbon is less electrophilic than the carbonyl carbon. This is due to resonance stabilization from the lone pairs on the hydroxyl oxygen. You can prepare these by oxidizing primary alcohols or alkylbenzenes. Hydrolysis of nitriles and amides also works. Grignard reagents reacting with dry ice (CO_2) followed by acidification is a common laboratory method. These acids have higher boiling points than even alcohols. They form stable dimers through hydrogen bonding in the vapor phase.

Chemical Properties of Carboxylic Acids

These compounds are acidic because the carboxylate ion is resonance-stabilized. They react with metals to release hydrogen gas. They also react with alkalis to form salts. With alcohols, they undergo esterification in the presence of an acid catalyst. Phosphorus pentachloride (PCl_5) converts them into acid chlorides. Reaction with ammonia produces amides. A unique reaction is the Hell-Volhard-Zelinsky (HVZ) reaction. It uses red phosphorus and halogen to substitute an alpha-hydrogen with a halogen atom. Decarboxylation occurs when sodium salts of acids are heated with soda lime.

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CBSE Class 12 Chemistry Notes Chapter 12 FAQs

1. What is the main difference between an aldehyde and a ketone?

Aldehydes have the carbonyl group at the end of the carbon chain (R-CHO), while ketones have it in the middle (R-CO-R).

2. How do you distinguish between aldehydes and ketones?

Use Tollen’s reagent or Fehling’s solution; aldehydes will form a silver mirror or red precipitate, whereas ketones will not react.

3. Why do carboxylic acids have higher boiling points than alcohols?

Carboxylic acids form stronger intermolecular hydrogen bonds and exist as dimers, requiring more energy to break than the single bonds in alcohols.

4. What is the use of the Rosenmund reduction?

It is specifically used to prepare aldehydes from acyl chlorides using a palladium catalyst poisoned by barium sulfate.

5. Which compound undergoes the Cannizzaro reaction?

Aldehydes that lack an alpha-hydrogen, such as formaldehyde or benzaldehyde, undergo this reaction in the presence of concentrated alkali.