Benzaldehyde Formula, Preparation, Properties, uses, and Tests

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(MnO). In the presence of vanadium pentoxide(VO) at 350°C, oxidation of toluene by air oxygen also yields benzaldehyde.

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.

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.

benzaldehyde-cyanohydrin

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:

Grignard-reagent benzaldehyde

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

C6H5CHO + NH2OH C6H5CH = NOH  + H2O

Reaction with Hydrazine: It reacts with Hydrazine(N2H4to 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.

Benzaldehyde-dimethyl-acetal

Mixed aldol condensation: αhydrogen 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 αhydrogen containing aldehyde or ketone. This reaction is called mixed aldol condensation. This reaction is also called claisen reaction or Claisen – Schmidt condensation.

Cannizaro Reactions: a αhydrogen 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.

benzaldehyde-Reaction

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

benzaldehyde-Nitration

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

benzaldehyde-Sulphonation

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.

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