Preparation of Benzaldehyde – JEE Important Topic

Benzaldehyde is an aromatic aldehyde with the molecular formula C₆H₅CHO and is also known as the oil of bitter almonds. It is a major component of bitter almonds in the form of its glucoside named amygdalin (C₂₀H₂₇O₁₁N).  On boiling with dilute acids, amygdalin will hydrolyze into benzaldehyde, glucose, and hydrogen cyanide. Molecular weight of benzaldehyde is 106.12 $\dfrac{g}{mol}$. It was first extracted by a French pharmacist known as Martres in 1803.

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Structure of Benzaldehyde

Benzaldehyde structure contains a simple aldehydic group attached to a benzene ring as follows.

Chemical Structure of Benzaldehyde

Methods of Preparation of Benzaldehyde

Laboratory Method

Benzaldehyde is prepared in the laboratory by boiling benzoyl chloride (C₆H₅CH₂Cl) with copper nitrate (Cu(NO₃)₂) or lead nitrate (Pb(NO₃)₂) in the presence of CO₂.

$\begin{align} &\underset{\text { Benzoyl chloride} }{2 \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{2} \mathrm{Cl}}+\underset{\underset{\mathrm{Pb}(\mathrm{NO_3})_2}{or}}{\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}} \underset{CO_2}{\xrightarrow{Heat}}\underset{Benzaldehyde}{2\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}+\mathrm{CuCl}{ }_{2}+2 \mathrm{HNO}_{2} \end{align}$

By Hydrolysis of Benzal Chloride

Hydrolysis of benzal chloride produces benzaldehyde where hydrolysis is done by aqueous caustic soda or milk of lime.

Hydrolysis of Benzal Chloride

By Rosenmund Reaction

This reaction is also called catalytic reduction of benzoyl chloride; in this process, hydrogen is passed through the solution of benzoyl chloride in xylene till the evolution of HCl stops in the solution. This is done in the presence of a palladium catalyst poisoned by BaSO₄.

$\underset{\text{Benzoyl chloride}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}}+\mathrm{H}_{2} \underset{\text { (Boiling xylene) }}{\xrightarrow{\mathrm{PdBaSO}_{4}, \mathrm{~S}}} \underset{\text{Benzaldehyde}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}+\mathrm{HCl}$

In order to prevent the further reduction of aldehyde (benzaldehyde) to respective alcohol, the catalyst is poisoned with a small amount of sulphur or quinoline.

Benzoyl chloride can also be reduced to benzaldehyde by treating it with Lithium tri-tert-butoxyaluminum hydride LiAlH[OC(CH₃)₃]₃ at -78⁰C as follows:

$\underset{\text{Benzoyl chloride}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COCl}}+\mathrm{H}_{2} \underset{\text { (Boiling xylene) }}{\xrightarrow{LiAlH[OC(CH_3)_3]_3~\text{di ethyl ether}, -78^{\circ}C}} \underset{\text{Benzaldehyde}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}$

Dry distillation of a mixture of calcium benzoate and calcium formate will form benzaldehyde as the main product. The other possible products of this reaction are diphenyl ketone and formaldehyde.

Preparation of Benzaldehyde from Calcium Benzoate and Calcium Formate

By Oxidation of Benzyl Alcohol

Treating benzyl alcohol with dilute HNO₃, acidic potassium dichromate (or chromic anhydride) in acetic anhydride, or with the presence of a copper catalyst at 350⁰C will give:

Oxidation of Benzyl Alcohol

This method is used in the commercial production of benzaldehyde.

By Oxidation of Toluene

Toluene is treated with oxygen in presence of vanadium pentoxide (V₂O₅) at 350⁰C to form benzaldehyde.

Oxidation of Toluene

For commercial purposes, oxidation of toluene can also be carried out in the presence of air diluted with nitrogen at 500⁰C in presence of oxides of Mn, Mo, or Zr as a catalyst. Here, nitrogen is used to prevent complete oxidation.

By Etard’s reaction

The reaction of chromyl chloride dissolved in CS₂ or CCl₄ with toluene in CS₂ will form a brown-coloured chromium complex product, which forms benzaldehyde on decomposition with water. This method is used in laboratories for the preparation of benzaldehyde.

Etard’s Reaction

From toluene, benzaldehyde is also formed by oxidation of toluene with chromic oxide in acetic anhydride. This reaction will form benzylidene diacetate, which causes the production of benzaldehyde on hydrolysis with acid or alkali.

$\underset{\text{Toluene}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}} \underset{\left(\mathrm{CH}_{3} \mathrm{CO}\right)_{2}}{\xrightarrow{\text{CrO}_3,[\text{O}]}} \underset{\text{Benzylidene diacetate}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}\left(\mathrm{OCOCH} \mathrm{CO}_{3}\right)_{2}} \underset{{(\text { Hydrolysis })} }{\xrightarrow{\mathrm{OH}^{-} / \mathrm{H}_{2} \mathrm{O}}}\underset{\text{Benzaldehyde}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}+2 \mathrm{CH}_{3} \mathrm{COOH}$

Gattermann-Koch Aldehyde Synthesis

Carbon monoxide and HCl gas are passed into the ether solution of benzene in presence of anhydrous aluminium chloride and cuprous chloride under high pressure of 90 atm to produce benzaldehyde.

Gattermann-Koch Benzaldehyde Synthesis

Stephen’s Reduction

Partial reduction of phenyl cyanide with stannous chloride through the flow of HCl gas into the ethereal solution, followed by hydrolysis of aldimine stannic chloride with H₂O, yields benzaldehyde.

$\underset{\text{phenyl cyanide}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CN}}+\mathrm{HCl} \underset{\mathrm{Ether}}{\xrightarrow{\mathrm{HCl}/ \text{SnCl}_{2}}} \underset{\text{Aldimine complex}}{\left[\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}=\mathrm{NH}\right]_{2} \mathrm{SnCl}_{6}} \underset{\text{Boil}}{\xrightarrow{\text { H}_2\text{O}}} \underset{\text{Benzaldehyde}}{2 \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}$

By Ozonolysis of Styrene

Styrene reacts with O₃ on hydrolysis and gives benzaldehyde. 

Ozonolysis of Styrene

Grignard Reaction

Treatment of phenyl magnesium bromide with reagent ethyl formate can produce benzaldehyde.

Formation of Benzaldehyde using Grignard Reagent

Physical Properties of Benzaldehyde

• Colourless oily liquid with a boiling point of 179⁰C.

• Smells like bitter almonds.

• Sparingly soluble in water but highly soluble in organic solvents.

• Poisonous in nature.

Some Important Chemical Reactions of Benzaldehyde

Oxidation of Benzaldehyde

Oxidation of benzaldehyde will produce benzoic acid. Thus, to prepare benzoic acid from benzaldehyde, we need to use oxidising agents like acidified K₂Cr₂O₇, alkaline KMnO₄ or dil. HNO₃.

$\underset{\text{Benzaldehyde}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}} \xrightarrow{[\mathrm{O}]} \underset{\text{Benzoic acid}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}}$

Clemmensen’s Reduction

Benzaldehyde can be reduced to toluene through the use of amalgamated zinc in presence of con. HCl.

$\underset{\text { Benzaldehyde }}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}+4 \mathrm{H} \underset{\text { conc.HCl }}{\xrightarrow{Zn-Hg}} \underset{\text{Toluene}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}}+\mathrm{H}_{2} \mathrm{O}$

Wolff-Kishner Reduction

Benzaldehyde can be reduced to hydrocarbon toluene in presence of excess hydrazine and a strong base such as KOH or potassium tert. butoxide, by heating at 200 degree Celsius.

$\underset{\text { Benzaldehyde }}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}}+4 \mathrm{H} \underset{200^{\circ}C}{\xrightarrow{H_2NNH_2/KOH, \text{glycol}}} \underset{\text{Toluene}}{\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}}+\mathrm{H}_{2} \mathrm{O}+\mathrm{N}_2$

Nitration of Benzaldehyde

Benzene ring of benzaldehyde can undergo nitration to produce m-nitrobenzaldehyde.

Nitration of Benzaldehyde

We know that –CHO is a deactivating group as it is an electron withdrawing group, which pulls out the electron from a benzene ring. It decreases the electron density at ortho and para position of the benzene ring rather than at meta position. Thus, the meta position has more electron density compared to others which will lead to an attack on it by electrophile, like NO₂. 

Resonance Effect of –CHO on Benzene Ring

Uses of Benzaldehyde

• Used in perfumery.

• Used in the manufacture of different types of dyes.

• Used in the manufacture of benzoic acid, cinnamic acid, Schiff’s base, benzoyl chloride, etc.

• Used as a flavouring agent for almond extract in many food items.

Conclusion

Benzaldehyde is an important organic compound due to its wide range of usage in food, pharmaceuticals, cosmetics etc. It can undergo numerous types of reactions which makes it a major component of many organic reactions. Benzaldehyde is a colourless oily liquid with a boiling point of 179⁰C. It has a smell like bitter almonds. It is sparingly soluble in water but highly soluble in organic solvents. Benzaldehyde is poisonous in nature. Benzaldehyde also has many industrial uses and is used as a reactant for many economical reactions.