ALDEHYDES, KETONES AND CARBOXYLIC ACIDS

ALDEHYDES, KETONES AND CARBOXYLIC ACIDS

KEY POINTS

1	In aldehydes, the carbonyl group is bonded to a carbon and hydrogen while in the ketones, it is bonded to two carbon atoms. The carbonyl compounds in which carbonyl group is bonded to oxygen are known as carboxylic acids, and their derivatives (e.g. esters, anhydrides) while in compounds where carbon is attached to nitrogen and to halogens are called amides and acyl halides respectively.
2	Aldehydes, Ketones and Carboxylic acids are important classes of organic compounds containing carbonyl groups.        They are highly polar molecules.       They boil at higher temperatures than the corresponding hydrocarbons and weakly polar compounds such as ethers. Lower members are soluble in water because they can form H-bond with water. Higher members are insoluble in water due to large size of their hydrophobic group.
3	The carbonyl carbon atom is sp2 -hybridised and forms three sigma (σ) bonds. The π Electron cloud of >C=O is unsymmetrical. On the other hand, due to same electronegativity of the two carbon atoms, the π-electron of the >C=C< bond is symmetrical. The structure of the carbonyl group in aldehydes and Ketones is, not entirely adequately represented by >C=O, nor by the alternative>C+─ O-. The real structure or resonance hybrid lies somewhere between the following structure: its picture coming
4	The IUPAC names of open chain aliphatic aldehydes and ketones are derived from the names of the corresponding alkanes by replacing the ending –e with –al and –one respectively
5	General Methods of Preparation: (i)      Controlled oxidation of 1º alcohols : RCH2OH      PCC/ CrO3-->         RCHO RCH2OH         Cu/573 K-->           RCHO + H2
5	Preparation of Aldehydes and Ketones (i) By oxidation of alcohols Aldehydes and ketones are generally prepared by oxidation of primary and secondary alcohols, respectively. RCH2 OH → RCHO By hydration of alkynes: Ethyne on hydration with HgSO4/dil.H2SO4 at 333 K forms acetaldehyde.
6	By Rosenmund reduction: Hydrogenation of acyl chloride over palladium on barium sulphate gives aldehyde RCOCl + H2       Pd/BaSO4--->  RCHO + HCl
7	Stephen Reaction: Reduction of nitriles in presence of stannous chloride in presence of HCl gives imine which on hydrolysis gives corresponding aldehyde.
8	Nitriles are selectively reduced by DIBAL-H (Diisobutylaluminium hydride) to aldehydes.
9	From Hydrocarbons: (i)By oxidation of methyl benzene: Etard Reaction: Chromyl chloride (CrO2Cl2) oxidizes methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.
10	Benzene or its derivatives on treatment with carbon monoxide and HCl in presence of anhydrous aluminium chloride or cuprous chloride (CuCl) gives benzaldehyde or substituted benzaldehydes.
11	Oxidation of secondary alcohols in presence of oxidizing agent like K2Cr2O7/H2SO4, KMnO4, CrO3 gives ketones. R2CH(OH) → RCOR
12	Acyl chloride on treatment with dialkyl cadmium (prepared by reaction of cadmium chloride with Grignard reagent) gives ketone.
13	From nitriles: Nitriles on treatment with Grignard reagent followed by hydrolysis give ketones.
14	By Friedel Crafts acylation reaction: Benzene or substituted benzene on treatment with acid chloride in presence of anhydrous aluminium chloride forms ketone.
15	The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses. It is due to weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions. Also, their boiling points are lower than those of alcohols of similar molecular masses due to absence of intermolecular hydrogen bonding. The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bond with water.
16	Since aldehydes and ketones both possess the carbonyl functional group, they undergo similar chemical reactions Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. Electronically, aldehydes are more reactive than ketones because two alkyl groups reduce the electrophilicity of the carbonyl carbon more effectively than in former.   Order of reactivity of aldehydes and ketones towards nucleophilic addition is : (ii)    HCHO > CH3CHO > CH3CH2CHO. (iii)  HCHO > RCHO > R CO R. (iv)   ArCHO > Ar COR > Ar CO Ar.
17	Nucleophilic addition reactions of aldehydes and ketones: (a)Addition of hydrogen cyanide (HCN) to form cyanohydrins : (NaCN+HCl) CH3CHO+ HCN → CH3CH(OH)(CN)
18	(b)Addition of sodium hydrogensulphite (NaHSO3) to form bisulphate addition Compound
19	(c)Addition of Grignard reagent (RMgX) to form alcohol
20	(d)Addition of alcohol: (i) Aldehydes on addition of monohydric alcohol in presence of dry HCl forms hemiacetal and acetal.
21	(e)Addition of ammonia and its derivatives:
22	Reduction of aldehydes and ketones: (a) Reduction to alcohols: Aldehydes and ketones on catalytic hydrogenation in presence of Ni, Pt or Pd by using lithium aluminium hydride (LiAlH4) or sodium borohydride (NaBH4 ) forms primary andsecondary alcohols respectively.
23	Reduction to hydrocarbons: (i) Clemmensen reduction: Carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc amalgam and concentrated hydrochloric acid.
24	Oxidation of aldehydes and ketones: (i) Aldehydes are oxidized to acids in presence of mild oxidising agents HNO3, K2Cr2O7, KMnO4.
25	Ketones are oxidized under drastic conditions i.e. with powerful oxidising agents like HNO3, K2Cr2O7, KMnO4. at higher temperature.: ketones gives mixture of carboxylic acids
26	Test to distinguish aldehydes and ketones: 1) Tollen’s test: When an aldehyde is heated with Tollen’s reagent it forms silver mirror. . Tollen’s reagent is ammoniacal solution of AgNO3 Ketones do not form silver mirror and hence do not give this test.
27	Fehling’s test: When an aldehyde is heated with Fehling’s reagent it forms reddish brown precipitates of cuprous oxide. Fehling’s reagent: Fehling solution A (aqueous solution of CuSO4) + Fehling solution B (alkaline solution of sodium potassium tartarate) Ketones do not give this test.
28	ALDOL CONDENSATION :  Aldehydes and ketones having at least one α-hydrogen condense in the presence of dilute alkali  to form β-hydroxy aldehydes or β –hydroxyketones (aldol)(ketol). Cross aldol condensation: Aldol condensation between two different aldehydes and ketones is called aldol condensation. If both of them contain α -hydrogen atoms, it gives a mixture of four products.
28	(iii)Haloform reaction: Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom i.e. methyl ketones are oxidised by sodium hypohalite to sodium salts of corresponding carboxylic acids having one carbon atom less than that of carbonyl compound. The methyl group is converted to haloform.
28	Carboxylic Acids: Carboxylic acids are the compounds containing the carboxylfunctional group (-COOH).
29	Preparation of carboxylic acid: (i) From alcohols: Primary alcohols are readily oxidised to carboxylic acids with common oxidising agents such as potassium permanganate (KMnO4) in neutral, acidic oralkaline media or by potassium dichromate (K2Cr2O7) and chromium trioxide (CrO3) in acidic media.
30	(iii)From alkyl benzenes: Aromatic carboxylic acids can be prepared by vigorous oxidation of alkyl benzenes with chromic acid or acidic or alkaline potassium permanganate.
31	(v)From Nitriles: Nitriles on hydrolysis in presence of dilute acids or bases forms amide which on further hydrolysis gives carboxylic acid.
32	From Grignard reagent: Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids which on hydrolysis forms carboxylic acids.
33	From acyl halides and anhydrides: Acid chlorides when hydrolysed with water give carboxylic acids .On basic hydrolysis carboxylate ions are formed which on further acidification forms corresponding carboxylic acids. Anhydrides on hydrolysis forms corresponding acid(s) From esters: Acidic hydrolysis of esters gives directly carboxylic acids while basic hydrolysis gives carboxylates, which on acidification give corresponding carboxylic acids.
34	Physical properties of carboxylic acids: (i)Solubility: As the size of alky group increases solubility of carboxylic acid decreases because non-polar part of the acid increases (ii)Boiling points: Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to extensive association of carboxylic acid molecules through intermolecular hydrogen bonding.
35	Acidity of carboxylic acids: Carboxylic acids are more acidic than phenols. The strength of acid depends on extent of ionization which in turn depends on stability of anion formed. (i)Effect of electron donating substituents on the acidity of carboxylic acids: Electron donating substituent decreases stability of carboxylate ion by intensifying the negative charge and hence decreases acidity of carboxylic acids. (ii)Effect of electron withdrawing substituent on the acidity of carboxylic acids: Electronwithdrawing group increases the stability of carboxylate ion by delocalizing negativecharge and hence, increases acidity of carboxylic acid. The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3
36	Reaction of carboxylic acids: Reaction with Na metal NaOH, NaHCO3
37	Esterification: Carboxylic acids are esterified with alcohols in the presence of a mineral acid such as concentrated H2SO4 or HCl gas as a catalyst.  (iii) Carboxylic acids react with PCl5, PCl3 and SOCl2 to form acyl chlorides.
38	Reaction with ammonia (NH3): Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature gives amides.
39	(ii)Decarboxylation : Sodium or potassium salts of carboxylic acids on heating with sodalime (NaOH + CaO in ratio of 3:1) gives hydrocarbons which contain one carbon less thanthe parent acid.
40	Hell-Volhard-Zelinsky reaction: Carboxylic acids having an α -hydrogen are halogenated at the α -position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give α -halocarboxylic acids)
41	(ii)Ring substitution in aromatic acids: Aromatic carboxylic acids undergo electrophilicsubstitution reactions. Carboxyl group in benzoic acid is electron withdrawing group and is meta- directing.