Reactions of Aldehydes and Ketones, Chemistry tutorial

Introduction:

Aldehydes and ketones are the reactive organic compounds which characteristically undergo nucleophilic addition or condensation reactions. This is because of the difference in electronegative between carbon and oxygen atom. The carbonyl carbon atom is the nucleophilic site as it is electron deficient due to the difference in electronegativity between carbon and oxygen.

Carbonyl Carbon consists of a nucleophilic site:

The susceptibility of carbonyl carbon to nucleophilic attack is decreased by its attachment to the electron liberating alkyl or aryl groups that reduces the degree of positive charge on the carbon. As well, the increase in steric hindrance regarding the carbon through the bulky hydrocarbon groups obstructs the approach of the attacking nucleophile and contributes to the reduction in this reactivity. Therefore, aldehydes are more reactive than ketones as they have only one hydrocarbon group. The carbonyl carbon in the aldehydes is more positive and less sterically hindered than that in ketones, therefore more susceptible to the nucleophilic attack.

The order of reactivity of carbonyl compounds to nucleophilic attack is as:

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Fig: Order of reactivity of carbonyl compounds

Alkyl groups donate electrons through inductive effects whereas aryl (Ar) groups do so through the resonance.

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Fig: Alkyl groups donating electrons

Nucleophilic Addition:

Addition of Grignard Reagents:

A Grignard reagent consists of a formula RMgX, here 'X' is a halogen and 'R' is an alkyl or aryl group. For illustration: CH3CH2MgBr.

Aldehydes and ketones react by Grignard reagents to form alcohols. This reaction is extremely helpful for the preparation of all the three classes of alcohols. The product formed based on the beginning carbonyl compound as illustrated below:

Methanal → 1o alcohol

Aldehydes → 2o alcohol

Ketones → 3o alcohol

=> Mechanism:

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Fig: Addition of Grignard Reagents

In the preparation of tertiary alcohols, aqueous ammonium chloride is employed for the hydrolysis as dilute acid brings about dehydration of the alcohol to yield the alkene that is a removal product.

For illustration:

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Fig: Example of Addition of Grignard Reagents

Addition of hydrogen cyanide, HCN:

The Aldehydes and ketones undergo addition by hydrogen cyanide to outcome a class of compounds known as 2-hydroxyalkanonitriles (that is, cyanohydrins).

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Fig: Addition of hydrogen cyanide

Addition of sodium hydrogensulphite NaHSO3:

Aldehydes and ketones react reversibly by means of surplus 40 percent hydrogensuphite in excess at room temperature to yield the carbonyl hydrogensulphite that is isolated as colourless crystals.

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Fig: Addition of sodium hydrogensulphite

The addition of hydrogensulphite to carbonyl compounds is generally employed to separate the carbonyl compounds from a mixture of organic compounds. The carbonyl hydrogensulphite is treated by an acid or an alkali to regenerate the free carbonyl compound. This reaction is extremely good technique for the purification and separation of appropriate carbonyl compounds from the non carbonyl compounds.

The hydrogenosulphate reacts by potassium cyanide or sodium cyanide to provide 2-hydroxyalkanonitrile.

Addition of Alcohols:

Aldehydes undergo addition by the excess anhydrous alcohol in the presence of a little dry hydrogen chloride catalyst to prepare 1, 1- dialkoxyl alkanes (Acetals).

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Fig: Addition of Alcohols

The carbonyl compound (acetal) reacts by two equivalents of an alcohol.

In the absence of a catalyst, the aldehyde and alcohol exist in the equilibrium with a kind of compound termed as 1-alkoxyalcohol (that is, hemiacetal).

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Fig: Aldehyde and alcohol exist in equilibrium 

The Ketone equivalents of such derivatives are termed as ketals and they are hard to prepare through this process.

Reduction using Lithium tetrahydridoaluminate (III):

Aldehydes and ketones, whenever reduced by using the Lithium tetrahydridoaluminate (III) in Ethoxy ethane at 0°C provide primary and secondary alcohols correspondingly.

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Fig: Reduction using Lithium tetrahydridoaluminate

Lithium tetrahydridoborate (III), LiBH4 -a milder reducing agent- in Ethoxy ethane or tetra hydrofuran is sometimes employed as an alternative reagent.

As well, Sodium tetrahydridoborate (III) Na+ BH4¯, dissolved in water or methanal, as it is insoluble in Ethoxy ethane behaves in an identical manner to Lithium tetra hydridoaluminate (III) however it is milder in its action.

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Fig: Reduction by Lithium tetrahydridoaluminate

Addition of Ammonia:

Aliphatic aldehydes react by gaseous ammonia on passing the latter via a dry solution of Ethoxy ethane to make a white precipitate of the aldehyde ammonia. Aldehydes ammonia is not stable.

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Fig: Addition of Ammonia

Condensation Reaction (Addition - Elimination Reactions):

Aldehydes and ketones react by the derivatives of ammonia like hydroxlamine, NH2OH, hydrazine, NH2NH2 or 2, 4 - dinitrophenylhydrazine C6H5N2H3 (NO2)2 to give a compound having an imine group, >C=N- by the removal of water molecules.

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Fig: Condensation Reaction

Reaction with hydroxylamine, NH2OH:

Hydroxylamine in the form of one of its more stable salts, (that is, hydroxylamine hydrochloride HONH+ 4Cl-) reacts by the aldehydes and ketones to form OXIMES that are crystalline solids. Free hydroxylamine is discharged through reacting hydroxylamine hydrochloride by sodium hydroxide or sodium ethanate.

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Fig: Reaction with hydroxylamine

Reaction with hydrazine, NH2NH2:

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Fig: Reaction with hydrazine

Reaction with Phenylhydrazine, C6H5NHNH2:

Phenylhydrazine is a colourless and poisonous liquid which condenses by the carbonyl compound dissolved in the aqueous alcohol on warming to provide phenyl hydrazones.

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Fig: Reaction with Phenylhydrazine

The phenylhydrazone derivatives of simple aliphatic aldehydes and ketones are oils or solids by low melting points whereas aromatic carbonyl compounds derivatives are the crystalline solids.

Reaction with 2, 4-dinitrophenyl hydrazine (2, 4-C6H3(NO2)2NHNH2)-Brady's Reagent:

Brady's reagent is employed for the detection of carbonyl functional group.

Aldehydes and ketones react by 2, 4-dinitrophenyl hydrazine to provide the 2, 4-dinitrophenyl hydrazone derivatives.

Reaction by Semicarbazide - NH2NHCONH2:

Aldehydes and ketones react by an aqueous solution of Semicarbazide hydrochloride and sodium ethanoate in the cold to provide a colourless crystals-Semicarbazide.

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Fig: Reaction with Semicarbazide

Reaction with primary amines:

The primary amines condense readily with aldehydes and ketones to form the Schiff bases.

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Fig: Reaction with primary amines

Other reactions of Aldehydes and Ketones:

Chlorination:

a) By PCl5:

The Aliphatic and simple aromatic carbonyl compounds react under dry conditions by PCl5 to form dichlorohydrocarbon. In this reaction, the oxygen atom of the carbonyl compound is substituted by two chlorine atoms.

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Fig: Aldehydes and Ketones with PCl5

Lower dichlorohydrocarbons are mainly colourless liquids.

b) By Chlorine gas (Trichloroethanal formation):

Whenever surplus chlorine is bubbled via ethanal, the methyl hydrogen atoms are substituted by chlorine to yield a colourless, oily liquid, Trichloroethanal known as chloral.

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Fig: Aldehydes and Ketones with Chlorine

The Chloral reacts exothermically with water to make a stable crystalline hydrate.

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Fig: Stable crystalline hydrate

As well, if chlorine is bubbled via warm propanone, successive replacement of the methyl hydrogen occurs, providing a mixture of chloropropanones, example: CH3COCH2Cl, ClCH2COCH2Cl, Cl2CHCOCH2Cl and so forth.

Trihalomethane (Iodoform) Reaction:

Aldehydes or ketones react by the Iodoform reagent -iodine in aqueous sodium hydroxide solution to provide a carboxylate and triiodomethane-a yellow precipitate.

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Fig: Trihalomethane Reaction

Aldol Condensation:

Aldehydes and ketones having at least one α-hydrogen atom- the carbon atom linked to the carbonyl carbon should contain at least one hydrogen-undergo Aldol condensation reaction.

For illustration, ethanal, CH3CHO, Methanal, HCHO and benzene carbaldehyde, C6H5CHO can't undergo this reaction as they don't have α-hydrogen atom. The reaction is base catalyzed.

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Fig: Aldol Condensation

Cannizzaro Reaction:

The above reaction is a direct contrast to the Aldol condensation. It just applies to carbonyl compounds which don't have α- hydrogen atoms. It is thus limited to compounds in which the carbonyl compound is linked to a tertiary alkyl carbon atom. It is as well base catalyzed. For illustration, if an appropriate aldehyde is treated by a concentrated aqueous solution (40 to 60%) alkali at room temperature, it goes via concurrent oxidation and reduction to outcome the suitable salt of the carboxylic acid and an alcohol.

2C6H5CHO + NaOH → C6H5COONa + C6H5CH2 OH

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Fig: Mechanism of Cannizzaro Reaction

Silver mirror Test:

Tollen's reagent - a solution of silver ions in the aqueous ammonia is frequently utilized as a test for the presence of aldehydes.

Aldehydes are mixed by freshly made Tollen's reagent in the clean test tube and positioned in a water bath at around 60° C. A 'silver mirror' is developed within the test tube. The aldehydes are oxidized through Tollen's reagent to provide carboxylates.

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Fig: Silver mirror Test

Fehling's Test:

Fehling's solution is the mixture of equal volume of copper (II) tetraoxosulphate (VI) solution (that is, Fehling solution A) and sodium hydroxide and potassium sodium tartarate solution (that is, Fehling's solution B).

Fehling solution is a weak oxidizing agent that reacts by an aldehyde to provide a carboxylate.

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Fig: Fehling's Test

Whenever an aldehyde is heated by Fehling's solution, the deep blue solution changes to green and a reddish brown precipitate of copper (I) oxide is acquired.

Schiff test:

Aldehydes restore the red coloration of Schiff reagent, whereas ketones either cause no colour change at all or only very slowly. Simple aliphatic aldehydes, provides a positive result in one minute, whereas more complex ones might take thirty minute. A number of aromatic aldehydes provide negative result.

Uses of Aldehydes and Ketones:

1) In making plastics: They are significant raw materials for preparing plastic. For illustration: Urea-methanal. It is white in colour and it is an outstanding electrical insulator which is resistant to the chemical attack. It is broadly employed in electrical industry for preparing plugs, sockets and casing for the electrical appliances. Phenol methanal is the other helpful polymer in the plastic industry.

Propane is employed as a raw material for making a plastic generally termed as Perspex. The Perspex is highly transparent and it is broadly employed as replacement for glass.

2) Used as solvent: An illustration is propane, a significant solvent in industries and in the laboratories.

3) Used as a preservative: In the laboratory 40 percent methanal aqueous solution is broadly employed as a preservative for the biological specimens.

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