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

These comprehensive RBSE Class 12 Chemistry Notes Chapter 12 Aldehydes, Ketones and Carboxylic Acids will give a brief overview of all the concepts.

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

→ Oxidation of primary (1°) and secondary (2°) alcohols with acidified K₂Cr₂O₇ or aqueous or alkaline KMnO₄ give aldehydes and catalytic dehydrogenation of primary and secondary alcohols give ketones.

→ Some other reagents have been developed for selective oxidation of 1° alcohols to aldehydes such as:

• Collin's reagent (CrO₃.2C₅H₅N).
• Corey's reagent (Pyridinium chlorochromate, PCC (CrO₃.C₅H₅N, HCl).
• Pyridinium dichromate, PDC i.e., (C₅H₅NH⁺)₂ Cr₂O₇^2-
• Jone's reagent : It is a solution of chromium- trioxide (CrO₃) inH₂SO₄ in aqueous acetone.

→ Oppenauer oxidation of secondary alcohols with aluminium tert-butoxide in presence of excess of acetone gives ketones in good yield without the danger of being further oxidised to carboxylic acids.

→ Dry distillation of calcium salts of fatty acids or passing the vapours of fatty acids over heated MnO at 573 K gives aldehydes and ketones.

→ Aldehydes and ketones can be prepared by the hydration of alkynes in presence of dil H₂SO₄ and HgSQ₄ as catalyst.

→ Aldehydes and ketones can be prepared by hydroboration-oxidation of alkynes.

 

→ Alkenes react with O₃ to form ozonide which upon subsequent cleavage with zinc dust and H₂O give aldehydes and ketones.

→ Gem-dihalides containing two halogens on the same carbon upon alkaline hydrolysis give the corresponding aldehydes or ketones.

→ Acid chlorides are easily converted into aldehydes by catalytic hydrogenation or reduction in presence of Pd-BaSO₄ (Rosenmund reduction).

→ When an ethereal solution of an alkyl cyanides is reduced with SnCl₄ in presence of HCl gas, imine hydrochloride is precipitated which upon hydrolysis with boiling H₂O gives aldehydes (Stephen reduction).

→ Alkyl benzenes can be oxidised to benzaldehyde with a solution of chromyl chloride in CCl₄ or CS₂ (Etard Reaction).

→ Aromatic aldehydes, can also be prepared by the oxidation of toluene and its derivatives with CrO₃ in (CH₃C0)₂O.

→ Side chain chlorination of toluene gives benzal chloride which upon hydrolysis gives benzaldehyde (Commercial preparation).

→ Benzene and its derivatives are converted to benzaldehyde or substituted benzaldehyde by treatment with CO and HCl in presence of anhydrous AlCl₃ or Cu₂Cl₂ as catalyst (Gattermann-Koch aldehyde synthesis).

→ Aldehydes can be prepared from hydrocyanic acid (HCN) on treatment with Grignard reagent subse¬quently followed by hydrolysis give an aldehyde. Phenolic aldehydes are prepared by treating the phenol with chloroform in aqueous NaOH solution at about 343 K. (Reimer-Tiemann reaction).

→ Ketones can be easily prepared by the action of a suitable dialkyl cadmium on acid chloride.

→ Both aliphatic and aromatic ketones can be prepared by the action of a suitable Grignard reagent on an alkyl or arylnitrile followed by acid hydrolysis of the intermediate addition product.

→ Friedel Craft's reaction is a typical example of electrophilic substitution reactions.

→ Fries rearrangement of phenyl esters with anhydrous AlCl₃ in presence of CS₂ gives a mixture of o—and p-hydroxy ketones.

→ Boiling points of aldehydes and ketones are higher than those of corresponding alcohols.

→ Due to polar carbonyl group, aldehydes and ketones undergo nucleophilic addition reactions. Reactivity orders are (i) CH₂O > CH₃CHO > (CH₃)₂ CO > [(CH₃)₂CH] CO > [(CH₃)₃C]₂ CO-decreases as the steric hindrance and +1 effect of the alkyl group increases.

→ Two molecules of an aldehyde or a ketone containing an a-Hydrogen combine each other to form aldols. This reaction is called aldol condensation.

→ Aldol condensation between two different aldehydes or two ketones or between one aldehyde and one ketone is called cross aldol condensation.

→ Aldehydes which do not contain a-hydrogens on treatment with cone, alkali undergo disproportionation to form the corresponding acid and the alcohol. This reaction is known as Cannizzaro reaction.

→ On heating with an ethanolic solution of KCN, two molecules of an aromatic aldehyde condense to form benzoins. This reaction is called Benzoin condensation.

→ Acidic or alkaline hydrolysis of esters, anhydrides, amides and acid chloride gives carboxylic acids.

→ Oxidation of methyl ketones with sodium hypohalite gives caboxylic acids with one C-atom less than the corresponding ketones. This reaction is called haloform reaction.

→ The boiling points of carboxylic acids are higher than those of alcohols with comparable molecular masses.

→ The greater stability of carboxylate ion is responsible for the acidic character of carboxylic acids.

→ Effect of substituents on the acid strength of alipha-tic carboxylic acids.

• Acid strength decreases as the +1 effect of the alkyl group increases.
  HCOOH > CH₃COOH > (CH₃)₂ CHCOOH > (CH₃)₃CCOOH
• Acidic strength decreases as the -I effect of the halogen decreases.
  FCH₂COOH > ClCH₂COOH > Br CH₂COOH > I CH₂COOH > CH₃COOH
• Acidic strength decreases as the number of halogen atoms decreases.
  CI₃CCOOH > Cl₂CHCOOH > ClCH₂COOH > CH₃COOH
• Acidic strength decreases as the distance of the halogen atom from the carboxyl group increases.

→ Electron withdrawing group (EWG) stabilizes the carboxylate ion by dispersing the negative charge and hence increase the acidic strength of the carboxylic acid.

→ Electron donating group (EDG) destabilizes the caboxylate ion by intensifying the negative charge and hence decreases the acidity of the carboxylic acid.

→ Carboxylic acids react with active metals such as Na, K, Ca, Mg, Zn etc to from their respective salts liberating hydrogen gas.

→ Being stronger acids than cabonic acid, carboxylic acids decompose carbonates and bicarbonates evolving CO₂ with brisk effervescences.

→ Carboxylic acids neutralize alkalies forming salt and water.

→ The — OH group of carboxylic acids can be replaced by atoms/ groups like Cl, OR', NH₂ and OOCR' to form acid chlorides, ester, amides and anhydries respectively. These derivatives are colletively known as functional derivatives of carboxylic acids.

→ When carboxylic acids are heated with alcohols or phenols in presence of cone. H₂SO₄ or dry HCl gas esters are formed. This reaction is called esterification reaction.