Question

1.Explain how you could use the simple laboratory tests performed in this experiment to distinguish bet the following compounds

a. 2-pentanone and 3-pentanone

b. 3-pentanone and pentanal

c. Benzalehyde and acetophenone

d. Acetaldehyde and propionaldehyde

2. In the separation of t-butyl chloride from the reaction mixture, what is the purpose of the following washes?

a. cold water b. sodium bicarbonate c. water

3. Benzyl chloride, though a primary halide, readily forms a precipitate when treated with 2% ethanolic silver nitrate. Explain.

Answer 1

1.

a) Iodoform test.

2-pentanone gives positive result in iodoform test. It gives a pale yellow precipitate.

When Iodine solution is added to a small amount of aldehyde or ketone, followed by addition of sodium hydroxide solution, then aldehydes and ketones containing CH₃C=O group give a pale yellow precipitate of triiodomethane.

3-pentanone doesn't give any yellow precipitate. It gives negative iodoform test.

b. Tollen's test- aldehydes give a silver mirror with Tollen's reagent (positive result) whereas ketones don't show any reaction (negative result).

pentanal would give a positive result with Tollen's reagent. A silver mirror would appear along the walls of the test tube holdig the sample. But, 3-pentanone will not show any silver mirror with Tollen's reagent.

(note: Tollen's reagent contains [Ag(NH₃)₂]⁺ which is made by adding a drop of sodium hydroxide solution to silver nitrate solution that would give a precipitate of silver oxide Ag₂O. Then just enough dil.ammonia solution is added to redisolve the preipitate. This gives ammonical silver which is the tollen's reagent. cAldehydes reduce the diamminesilver(I) ion to metallic silver. )

Fehling's test can also be used. Pentanal gives positive Fehling's test (Aldehydes give brick red precipitate with Fehling's reagent).

C. Benzaldehyde gives positive result in Fehling's test (brick red precipitate is formed) and Tollen's test (silver mirror formed) whereas acetophenone gives negative result for these tests.

Acetophenone gives positive result in iodoform test. It gives a pale yellow precipitate of triiodoform. But benzaldehyde gives negative iodoform test.

d. Acetaldehyde gives positive result in iodoform test. It gives a pale yellow precipitate of triiodoform. But propionaldehyde gives negative iodoform test.

2.

a) t-butyl chloride is very less soluble in water. When the crude compoud is wahed with cold water, all water-soluble compounds(either solvent or reactant) of the reaction-mixture.

b) In the preparation of t-butyl chloride, conc. HCl is added. So, the reaction-mixture contains HCl. Hence, it is washed with sodium bicarbonate to neutralize the excess HCl used in the reaction.

c) The reaction-mixture is again washed with water so that any remaining water-soluble compounds can be removed.

3. Though a primary alkyl halide, benzyl chloride can lose the halide ion instantly by SN1 mechanism because it can form a stable benzyl carbocation (its resonance stabilied by the benzene ring). The chloride ion thus formed, reacts with silver ion from silver nitrate to give a white precipitate due to the formation of silver chloride.