Aldehydes are the organic compounds in which carbonyl group is attached to one hydrogen atom and one alkyl or aryl group.
Where R can be an alkyl or aryl group.
Preparation of Aldehydes
Aldehyde from acyl chloride
Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.
Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.
This reaction is called Stephen reaction.
Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes:
Similarly, esters are also reduced to aldehydes with DIBAL-H.
Aldehyde from hydrocarbons
Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.
By side chain chlorination followed by hydrolysis
Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.
Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.
This reaction is called Etard reaction.
Gattermann – Koch reaction
When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.
This reaction is known as Gatterman-Koch reaction.
Physical properties of aldehydes
- Aldehydes and ketones have higher boiling points than hydrocarbons and ethers of similar molecular masses due to weak molecular association arising out of dipole-dipole interactions.
- The boiling points of aldehydes and ketones are lower than alcohols of similar molecular masses due to the absence of intermolecular hydrogen bonding.
- Methanal is a gas at room temperature while ethanal is a volatile liquid, and other aldehydes and ketones are either liquid or solid at room temperature.
- The solubility of aldehydes and ketones in water decreases with an increase in the length of the alkyl chain. The lower members of aldehydes and ketones like methanal, ethanal, and propanone are miscible in water in all proportions as they form hydrogen bonds with water.
- Aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc.
- The lower aldehydes have sharp pungent odours, and as the size of the molecule increases, the odour becomes less pungent and more fragrant.
- Many naturally occurring aldehydes and ketones are used in the blending of perfumes and flavouring agents.
Chemical Reactions of aldehyde
Nucleophilic addition reactions
Due to partial positive charge on carbonyl carbon, aldehydes and ketones pose more attraction to the coming nucleophile to add to the carbonyl carbon. Therefore the reactions of aldehydes and ketones are nucleophilic addition reactions.
(i) Mechanism of nucleophilic addition reactions
A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon . The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced. This intermediate captures a proton from the reaction medium to give the electrically neutral product. The net result is addition of Nu– and H+ across the carbon oxygen double bond.
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.
nucleophilic addition-elimination reactions
Addition of hydrogen cyanide (HCN) to aldehyde
Aldehydes and ketones react with hydrogen cyanide (HCN) to yield cyanohydrins. This reaction occurs very slowly with pure HCN. Therefore, it is catalysed by a base and the generated cyanide ion (CN-) being a stronger nucleophile readily adds to carbonyl compounds to yield corresponding cyanohydrin.
Addition of sodium hydrogensulphite to aldehyde
This reaction is used for the separation and purification of aldehydes and ketones. This is because the addition compound formed above is water soluble and can be converted back to the original carbonyl compound by treating it with dilute mineral acid or alkali.
Addition of Grignard reagents to aldehyde
Addition of alcohols to aldehyde
Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals, which further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal as shown in the reaction.
Addition of ammonia and its derivatives to aldehyde
Nucleophiles, such as ammonia and its derivatives H2N-Z add to the carbonyl group of aldehydes and ketones. The reaction is reversible and catalysed by acid. The equilibrium favours the product formation due to rapid dehydration of the intermediate to form >C=N-Z.
Z = Alkyl, aryl, OH, NH2, C6H5NH, NHCONH2, etc.
Reduction of aldehyde to alcohols
Aldehydes and ketones are reduced to primary and secondary alcohols respectively by sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4)
Reduction of aldehyde to hydrocarbons
The carbonyl group of aldehydes and ketones is reduced to CH2 group on treatment with zinc-amalgam and concentrated hydrochloric acid [Clemmensen reduction] or with hydrazine followed by heating with sodium or potassium hydroxide in high boiling solvent such as ethylene glycol (Wolff-Kishner reduction).
Clemmensen reduction of aldehyde
Wolff-Kishner reduction of aldehyde
Oxidation of aldehyde
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromate, etc. Even mild oxidising agents, mainly Tollens’ reagent and Fehlings’ reagent also oxidise aldehydes.
Tollens’ test of aldehyde
On warming an aldehyde with freshly prepared ammoniacal silver nitrate solution (Tollens’ reagent), a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. The reaction occurs in alkaline medium.
Fehling’s test of aldehyde
Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.
Reactions due to alpha-hydrogen
Acidity of α-hydrogens of aldehydes and ketones: The aldehydes and ketones undergo a number of reactions due to the acidic nature of α-hydrogen. The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.
Aldol condensation of aldehyde
Aldehydes and ketones containing atleast one α -hydrogen undergo self condensation reactions in the presence of dilute alkali to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively which on heating in the presence of H+ gives α, β-unsaturated aldehydes or ketones.
Cross aldol condensation of aldehyde
When aldol condensation is carried out between two different aldehydes and / or ketones, it is called cross aldol condensation. If both of them contain α-hydrogen atoms, it gives a mixture of four products
Cannizzaro reaction of aldehyde
Aldehydes which do not have an α -hydrogen atom undergo self-oxidation and reduction (disproportionation) reaction on treatment with concentrated alkali to form alcohol and salt of acid.
Electrophilic substitution reaction of aldehyde
Aromatic aldehyde and ketones undergo electrophilic substitution at the meta position. Carbonyl group shows + R effect, therefore acts as a deactivating and meta directing group.
Example of electrophilic substitution reaction:
Claisen condensation reaction of aldehyde
When a base-catalyzed cross aldol condensation between an aromatic aldehyde and an aliphatic aldehyde or a ketone is called Claisen Schmidt condensation or simply Claisen reaction. For example, Benzaldehyde reacts with Acetaldehyde to form 3-Phenylprop-2-en-1-al.