Aldehydes, Ketones and Carboxylic Acids
Preparation of Aldehydes and Ketones
Those organic compounds in which the carbonyl group (>C=O) is bonded to a carbon atom and a hydrogen atom are called aldehydes. In Ketones, carbonyl group is bonded to two alkyl groups.
In Aldehydes and Ketones, the carbonyl carbon atom is sp2 hybridised and forms three sigma bonds.
Aldehydes are known as alkanals and ketones as alkanones in IUPAC system.
Aldehydes and ketones are prepared by the oxidation of alcohols (Primary alcohols give aldehydes and secondary alcohols give ketones), by catalytic dehydrogenation of alcohols, by ozonolysis of Alkenes and by hydration of Alkynes.
Only aldehydes can be prepared from acyl chlorides (Rosenmund’s reduction), from Nitriles and esters (Stephen reaction), by Etard reaction, by Gattermann–Koch Reaction etc.
Only ketones can be prepared from acyl chlorides, From Nitriles, by Friedel Craft Acylation.
In Rosenmund’s reduction, acid chlorides are reduced to the corresponding aldehydes by passing hydrogen gas through boiling xylene solution of the acid chloride in the presence of Pd catalyst supported over BaSO4 and partially poisoned by the addition of sulphur or quinoline.
By Rosenmund’s reduction, formaldehyde cannot be prepared, since formyl chloride, HCOCl, is unstable at room temperature.
Physical and Chemical Properties of Aldehydes and Ketones
The boiling points of aldehydes and ketones are higher than those of hydrocarbons and ethers of comparable molecular masses.
Lower aldehydes and ketones containing upto four carbon atoms are soluble in water due to Hydrogen-bonding between the polar carbonyl group and the water molecules.
Aldehydes and ketones give Nucleophilic addition reaction with various compounds, Clemmensen Reduction, Wolff-Kishner reduction, Haloform reaction, Aldol condensation and Cannizzaro reaction.
The reduction of aldehydes and ketones to the corresponding hydrocarbons on reaction with amalgamated zinc and concentrated hydrochloric acid is called Clemmensen Reduction.
In Wolff-Kishner reduction reaction, aldehydes and ketones are reduced to the corresponding hydrocarbon by treatment with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol.
Iodoform reaction with sodium hypoiodite is used for the detection of CH3CO group or CH3CH(OH) group. Yellow colored Iodoform is produced during the reaction.
In Aldol condensation, two molecules of aldehydes or ketones containing -hydrogen atoms on treating with a dilute NaOH undergo condensation to form -hydroxyaldehydes (aldol) or -hydroxyketones. Aldol condensation between two different aldehydes or ketones is called cross aldol condensation.
Aldehydes which do not have - hydrogen atoms, on treatment with concentrated alkali solution undergo disproportionation reaction, i.e., self oxidation-reduction. As a result, one molecule of the aldehyde is reduced to the corresponding alcohol while other molecule is oxidized to the salt of corresponding carboxylic acid. This is called Cannizzaro reaction.
Aldehydes are easily oxidised as compared to ketones on treating with common oxidising agent. Ketones are generally oxidised under vigorous conditions.
Formaldehyde on reaction with ammonia yield hexamethylene diamine, which is used as urinary antiseptic under the name of urotropine.
Organic compounds having –COOH functional group are called carboxylic acids. A carboxyl group consists of a carbonyl group attached to the hydroxyl group.
The common names of the carboxylic acid end with the suffix ‘ic’. In the IUPAC system, carboxylic acid is named as alkanoic acid. In the –COOH group, the C atom is attached to the two O atoms; one by a single bond and the other by a double bond. Actually C atom and two O atoms are sp2 hybridised which make structures linear.
Carboxylic acids are prepared by oxidation of primary alcohols and aldehydes, from Esters, from Acid chloride on hydrolysis with water, from nitriles and amides, from Grignard reagent, from alkylbenzene and from acid anhydride.
Carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water.
The higher carboxylic acids are practically insoluble in water due to increased hydrophobic interactions of the hydrocarbon part.
The boiling points of carboxylic acids are much higher than those of hydrocarbons, aldehydes, ketones and alcohols of comparable molecular masses due to association of carboxylic acid molecules through intermolecular hydrogen bonding.
The carboxylic acids liberate hydrogen on reaction with electropositive metals like sodium.
Strength of acid is indicated by pKa value, which is given as, pKa = – logKa
Smaller the pKa value, stronger is the acid.
Carboxylic acids are weaker acids than mineral acids but stronger acids than phenols because the carboxylate ion is more stabilised (it has two equivalent resonating structures) as compared to phenoxide ion (it has non-equivalent resonating structures).
Electron withdrawing groups stabilize the carboxylate anion and increase the acidity.
Electron donating groups destabilize the carboxylate anion and decrease the acidity.
Carboxylic acids undergo esterification reaction to give esters on reaction with alcohols.
Carboxylic acids can be converted to the corresponding acid chlorides by treating them with phosphorus pentachloride, phosphorus trichloride or thionyl chloride.
The process of removal of a molecule of CO2 from a carboxylic acid is called decarboxylation. The reaction of an aliphatic carboxylic acid containing -hydrogen with Cl2 or Br2 in presence of a small amount of red phosphorus to give -halo acids is called Hell-Volhard Zelinsky reaction.
Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane.
Carboxylic acids undergo Kolbe’s electrolysis, Halogenation and Ring Substitution reaction.
Acetic acid, formic acid, benzoic acid, oxalic acid are highly useful compounds.