Benzaldehyde Formula, Preparation, Properties, uses, and Tests

Benzaldehyde Preparation

Toluene: Benzaldehyde is obtained from oxidation of toluene by chromyl chloride CrO₂Cl₂. This reaction is called Etard’s Reaction.

C₆H₅CH₃ + 2CrO₂Cl₂ → C₆H₅CH(OCrOHCl₂)₂ → C₆H₅CHO

Benzaldehyde is also obtained by oxidation of acidic solution of toluene’s manganese dioxide(MnO₂). In the presence of vanadium pentoxide(V₂O₅) at 350°C, oxidation of toluene by air oxygen also yields benzaldehyde.

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Benzyl Chloride: benzyl chloride(C₇H₇Cl) gets benzaldehyde on heating with lead nitrate solution in the atmosphere of carbon dioxide gas.

2C₆H₅CH₂Cl + Pb(NO₃)₂ → 2C₆H₅CHO + PbCl₂ + 2HNO₂

Preparation Method

To make benzaldehyde in the laboratory, take 10 grams of benzene chloride, 8 gram lead nitrate and about 50 ml of water in a round flask. Attaches a reflux condenser to the flask. CO₂ gas flows into the flask. So that the atmosphere of CO₂ remains in the flask.

Heats the flask for about 6 hours in this condition. The mixture is then cooled and extracted by ether. Benzaldehyde enters the ether layer. Separating the ether layer and adding a saturated solution of sodium bisulfite(NaHSO₃) to it.

Benzoic acid to benzaldehyde

Preparation of Benzaldehyde.

Crystals of benzaldehyde sodium bisulfite are obtained. They are filtered and dried. Benzaldehyde is released upon heating these crystals with sulfuric acid. It is extracted by ether. Distill the ether solution and separate the ether. After benign distillation of the remaining fluid, pure benzaldehyde is obtained at 179°C.

Benzal Chloride: Benzaldehyde is obtained by reaction of benzal chloride(C₇H₆Cl₂) with aqueous NaOH or KOH.

C₆H₅CHCl₂ → C₆H₅CH(OH)₂ → C₆H₅CHO

Benzene Alcohol: benzaldehyde can be obtained by oxidation of benzene alcohol by acidic potassium dichromate.

C₆H₅CH₂OH + O → C₆H₅CHO + H₂O

Benzaldehyde is also obtained when the vapors of benzene alcohol are precipitated at 300°C heated copper.

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Benzoyl Chloride: benzaldehyde is obtained from rosenmund reduction of Benzoyl chloride.

C₆H₅COCl + H₂ → C₆H₅CHO + HCl

Benzene: benzaldehyde is also obtained from benzene by gattermann koch aldehyde synthesis.

C₆H₆ + CO → C₆H₅CHO + HCl

Physical Properties

• Benzaldehyde is a colorless liquid. Its boiling point is 179°C.
• It smells like bitter almonds.
• It is less soluble in water and more soluble in alcohol and ether.

Chemical Properties

Reduction: The reduction of benzaldehyde by H₂ and Ni, Na-Hg and water, Na-Hg and alcohol, LiAlH₄ or NaBH₄ gives benzene alcohol.

C₆H₅CHO + 2H → C₆H₅CH₂OH

Toluene is formed when zinc mercury is reduced by amalgam and concentrated HCl. This reaction is called clemmensen reduction reaction.

C₆H₅CHO + 4H → C₆H₅CH₃ + H₂O

Addition of Hydrogen cyanide: Reaction with Hydrogen cyanide gives Benzaldehyde Cyanohydrin additive product.

Addition of sodium bisulfite: It reacts with sodium bisulfite(NaHSO₃) to give a additive product[benzaldehyde sodium bisulfite(C₇H₇NaO₄S)].

C₆H₅CHO + 4H → C₆H₅CH₃ + H₂O

Reaction with grignard reagents: It reacts with grignard reagents to form additive which forms secondary alcohol upon water decomposition.

Example:

Reaction with hydraxyl amine: It reacts with hydraxyl amine to form oxime.

C₆H₅CHO + NH₂OH → C₆H₅CH = NOH  + H₂O

Reaction with Hydrazine: It reacts with Hydrazine(N₂H₄) to form hydrazone. The reaction of benzaldehyde with phenylhydrazine and 2,4-dinitrophenylhydrazine test is similar.

C₆H₅CHO + NH₂NH₂ → C₆H₅CH = N – NH₂ + H₂O

Reaction with phosphorus penta chloride: The reaction of benzaldehyde from phosphorus penta chloride(PCl₅) forms benzal chloride(C₆H₅CHCl₂).

C₆H₅CHO + PCl₅ → C₆H₅CHCl₂ + POCl₃

Oxidation: benzaldehyde is oxidized by acidic potassium dicromate, acidic potassium permagnate, nitric acid or tollens reagent. The oxidation of benzaldehyde gives benzoic acid.

C₆H₅CHO + O → C₆H₅COOH

C₆H₅CHO + Ag₂O → C₆H₅COOH + 2Ag

It is not oxidized by the fahling solution.

Reaction with ammonia: It reacts with ammonia to form hydrobenzamide.

3C₆H₅CHO + 2NH₃ → (C₆H₅CH = N)₂CHC₆H₅ + 3H₂O

Reaction with primary amines: benzaldehyde reacts with primary alifatic and aromatic amines to form imines with common names, also called schiffes bases.

Example:

C₆H₅CHO + C₆H₅NH₂ → C₆H₅ – CH = N – C₆H₅ + H₂O

Reaction with alcohols: In the presence of dry hydrogen chloride gas, it reacts with alcohols to form acetal.

Mixed aldol condensation: α–hydrogen atom is not present in benzaldehyde. Hence it does not exhibit aldol condensation.

This condensation forms aldehyde or ketone by reacting in the presence of dilute alkali with another α–hydrogen containing aldehyde or ketone. This reaction is called mixed aldol condensation. This reaction is also called claisen reaction or Claisen – Schmidt condensation.

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Cannizaro Reactions: a α–hydrogen is not present in benzaldehyde. Hence, it exhibits cannizaro reactions. By heating it with concentrated NaOH or KOH, benzene alcohol and sodium benzoate are obtained.

2 C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa

Benzaldehyde Cannizzaro Reaction

Cannizzaro’s Reaction|| Reaction Mechanism || Internal and Cross Cannizzaro’s Reaction || 𝜶 hydrogen

Perkin reaction: Cinnamic acid is obtained by heating benzaldehyde with acetic anhydride and sodium acetate. This reaction is called perkin reaction.

C₆H₅CHO + (CH₃CO)₂O → C₆H₅CH = CHCOOH + CH₃COOH

Benzoin condensation: Benzoaldehyde is obtained by reacting with an alcohol solution of NaCN or KCN. This reaction is called Benzoin condensation.

2 C₆H₅CHO → C₆H₅ – CH(OH) – CO – C₆H₅

Reaction with halogens: In the absence of catalyst, it reacts with chlorine to form benzoyl chloride.

C₆H₅CHO + Cl₂ → C₆H₅COCl + HCl

It reacts with halogens in the presence of a halogen carrier to form a meta substitution product.

Nitration: This forms meta nitro benzaldehyde when heated with concentrated HNO₃ and concentrated H₂SO₄.

Sulphonation: This meta benzaldehyde makes sulphonic acid when heated with gentle sulfuric acid.

Benzaldehyde Uses

Benz aldehyde is used in the manufacture of aromatic substances, in the manufacture of dyes, and in the synthesis of other carbonic compounds.

Benzaldehyde Tests

• The smell of benzaldehyde is bitter almond-like.
• It gives the following general tests of the aldehyde group.

• schiffs test: pink color is obtained by adding schiffs reagent.
• tollens’ Tests: On heating it with ammonium silver nitrate solution ie tollens’ reagent, a silver mirror is obtained.
• It also gives positive results in 2,4-dinitrophenylhydrazine test and NaHSO3 test. But it does not give the test of fehling solution because It does not contain α-hydrogen.