. How will you prepare the following compounds starting with benzene?


(a) How will you prepare the following compounds starting with benzene?

(i) Benzaldehyde

(ii) Acetophenone

(b) Give chemical tests to distinguish between the following pairs of compounds:

(i) Ethanal and Propanal

(ii) Benzaldehyde and Acetophenone

(iii) Phenol and Benzoic acid

                                                                              OR

(a) Explain the mechanism of nucleophilic attack on a carbonyl group of an aldehyde or a ketone

(b) How would you bring about the following conversions?

(i) Propanone to Propene

(ii) Ethanol to 3-hydroxybutanal

(iii) Benzaldehyde to Benzophenone

Best Answer

Benzaldehyde

(b) (i) Ethanol and propanal

      Iodoform test: Ethanol will give iodoform test positive while propanal will not

      Iodoform test

(ii) Benzaldehyde and Acetophenone

     Iodoform test: Treat both the compounds with I2 and NaOH. A yellow ppt. of iodoform will be obtained in acetophenone

Benzaldehyde and Acetophenone

(iii) Phenol and Benzoic acid

      Neutral FeCl3 test: Add few drops of FeCl3 solution to both the samples of the compounds. A violet colouration will be obtained in case of phenol

      Phenol and Benzoic acid.

(a) 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 hybridized orbitals of carbonyl carbon. The hybridization of carbon changes from sp2 to sp3 in this process and a tetrahedral intermediate is produced. The intermediate captures a proton from the medium to give the neutral product

Propanone to Propene, Ethanol to 3-hydroxybutanal, Benzaldehyde to Benzophenone

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