Alehydes, Ketones and Carboxylic Acid

Transcription

1 Alehydes, Ketones and Carboxylic Acid Aldehydes and Ketones: Introduction Aldedydes and ketones are organic compounds that contain carbon-oxygen doule bonds. The general formula for aldehydes is O C R H The general formula for ketones is O C R R Aldedydes and ketones are well-known for their fragrance and flavour. They are used as solvents and as starting material for the synthesis of various other products.

2 Aldehydes and Ketones: Nomenclature The common names of aldehydes are derived from the names of the corresponding carboxylic acids by replacing -ic acid by -aldehyde. In the IUPAC system, aldehydes are named by replacing the -e of the corresponding alkane with al. In case of branched chain aldehydes, the longest chain carrying the aldehyde group is considered the parent structure and is numbered starting from the carbon of the aldehyde gropu. When the aldehyde is attached to a ring system, the suffix carbaldehyde is added after the full name of the ring. In the common system, ketones are named simply by adding the suffix ketone to the name of two alkyl groups. A ketone on which the acyl group is attached to a benzene ring is named a phenone. In the IUPAC system, ketones are named by replacing e of the corresponding alkane with one. In certain poly-functional compounds, the presence of a carbonyl group can be indicated by the prefix oxo with a number to show its position in a molecule. Aldehydes and Ketones: Structure of the Carbonyl Group and Physical Properties In carbonyl group, the carbon-oxygen double bond is polar in nature due to the electronegativity difference between carbon and oxygen. At room temperature, formaldehyde is a gas and acetaldehyde is a volatile liquid. Other aldehydes and ketones are liquids or solids at room temperature.

3 Aldehydes and ketones have higher boiling points, than hydrocarbons and ethers of comparable molecular weights. Lower aldehydes and ketones such as formaldehyde, acetaldehyde an acetone, are miscible in water in all proportions because of their ability to form hydrogen bonding with water. The lower aldehydes and ketones have a characteristic pungent odour. With an increase in the size of the molecule, the odour becomes less pungent and more fragrant. Aldehydes and Ketones: Chemical Reactions I The c = o groups govern the chemistry of aldehydes and ketones by providing a site for nucleophilic addition and by increasing the acidity of the hydrogen atoms to the α-carbon. The typical reactions of aldehydes and ketones are nucleophilic addition reactions. The reactivity of the carbonyl group towards nucleophilic addition depends mainly upon the electronic factor and the steric factor. Aldehyde reacts with alcohol in the presence of anhydrous hydrogen chloride to form an unstable intermediate known as hemiacetal. Aldehydes and ketones react with primary amines to form substituted imines. Aldehydes and ketones react with hydroxylamine to form the corresponding oximes. Aldehydes and ketones react with hydrazine and its derivatives to form the corresponding hydrazones. Oximes and hydrazones are used to detect and purify aldehydes and ketones. Carboxylic Acids: Nomenclature Organic compounds that contain the carboxyl functional group, - COOH, are called carboxylic acids.

4 Higher members of aliphatic carboxylic acids are known as fatty acids. In the common system, aromatic acids are named as the derivatives of benzoic acid, C 6 H 5 COOH. In the IUPAC system, carboxylic acids are named by replacing the ending e of the corresponding alkane by -oic acid to form alkanoic acid. Compounds with more than one carboxyl group are named by retaining the ending e of the alkane, and adding the multiplicative prefix di, tri, etc. to the term oic acid. Carboxylic Acids: Preparation On oxidation, primary alcohols or aldehydes, gives carboxylic acids with same unmer of carbon atoms. Aldehydes can also be oxidised to the corresponding carboxylic acids even with mild oxidising agents such as Tollen s reagent or Fehling s reagent. The oxidation of alkyl benzene is one of the most useful methods for preparing an aromatic carboxylic acid. The carbonation of Grignard reagents and hydrolysis of nitriles can be used for converting alkyl halides into the corresponding carboxylic acids having more than one carbon atom than the parent alkyl halide. The hydrolysis of acid derivatives such as amides, acid chlorides, anhydrides and esters yields carboxylic acids. Carboxylic Acids: Properties I The first four members of carboxylic acids are fairly soluble in water due to the ability of the carboxyl group to form hydrogen bonds with water molecules. Higher carboxylic acids are virtually insoluble in water due to the greater influence of the hydrocarbon part.

5 The boiling points of carboxylic acids are higher than that of aldehydes, ketones and alcohols of comparable molecular weights. Unlike phenols, carboxylic acids react even with weaker bases such as carbonates and bicarbonates to form the corresponding salts with the evolution of carbon dioxide. The reaction of carboxylic acids with carbonates and bicarbonates is used to detect the presence of the carboxyl group in an organic compound. Every carboxylic acid has its characteristic Ka, which indicates how strong an acid is. Electron-withdrawing groups increase the acidity of carboxylic acids by stabilising the anion through the dispersal of the negative charge on the carboxylate ion by the inductive or resonance effects. Electron-donating groups, decrease the acidity by destabilising the anion by intensifying the negative charge. Carboxylic Acids: Properties II The reaction of carboxylic acids with weak bases is used to detect the presence of the carboxyl group in an organic compound. Carboxylic acids react with alcohols in the presence of a mineral acid catalyst to form esters. The reaction is reversible and is known as esterification. The esterification of carboxylic acids with alcohols is a nucleophilic acyl substitution reaction. When carboxylic acids are treated with phosphorous halides or thionyl chloride, the OH group is replaced y a chlorine atom to form acid chlorides. Carboxylic acids, on reduction with lithium aluminium hydride or diborane, give primary alcohols. When heated with soda lime, the sodium salts of carboxylic acids lose carbon dioxide to form hydrocarbons. This reaction is known as decarboxylation. Carboxylic acids with an alpha hydrogen react with chlorine or bromine in the presence of a small amount of red phosphorus to give α-halocarboxylic acids. This reaction is known as the Hell-Volhard-Zelinsky reaction.

6 Carboxylic acids do not undergo the Friedel Crafts reaction because the carboxyl group is deactivating and the catalyst, aluminium chloride, which is a Lewis acid, gets bonded to the carboxyl group strongly.