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Give simple chemical tests to distinguish between the following pairs of compounds : (i) Propanal and Propanone (ii) Acetophenone and Benzophenone (iii) Phenol and Benzoic acid (iv) Benzoic acid Ethyl benzoate (v) Pentan -2- one and Pentan -3- one (vi) Benzaldehyde and Acetophenone (vii) Ethanal and Propanal. |
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Answer» Solution :(i) Propanal `(CH_(3)CH_(2)CHO` and Propanone `(CH_(3)COCH_(3))` : 1. Tollen.s reagent test : Propanal being an aldehyde reduces Tollen.s reagent but propanone being a ketone does not reduce Tollen.s reagent. `underset("Propanal")(CH_(3)CH_(2)CHO)+2[Ag(NH_(3))_(2)]^(+)+3OH^(-)rarr underset("Silver mirror")(CH_(3)CH_(2)COO^(-)+Agdarr)+4NH_(3)+2H_(2)O` `underset("Propanone")(CH_(3)COCH_(3))+[Ag(NH_(3))_(2)]^(+)rarr"No silver mirror"` 2. Iodoform test : Propanone is a methyl ketone, therefore, it gives iodoform test with `I_(2)//NaOH(NaOI)`, but propanal does not. `underset("Propanone")(CH_(3)COCH_(3))+3NaOIrarr CH_(3)COONa +underset("(yellow ppt.)")underset("Iodoform")(2NaOH+CHI_(3))` `underset("Propanal")(CH_(3)CH_(2)CHO)overset(NaOI)rarr"No yellow ppt."` (ii) Acetophenone `(C_(6)H_(5)COCH_(3))` and Benzophenone `(C_(6)H_(5)COC_(6)H_(5))` : The two compounds can be distinguished by performing iodoform test. Acetophenone contains `CH_(3)CO-` group, hence, it gives iodoform test. As there is no `CH_(3)CO-` group in benzophenone, it does not respond to iodoform test. `underset("Acetophenone")(C_(6)H_(5)COCH_(3))+3NaOIoverset(I_(2)//NaOH)rarr underset("Sodium benzoate")(C_(6)H_(5)COONa)+2NaOH+underset("yellow ppt.")underset("Iodoform/")(CHI_(3))` `underset("Benzophenone")(C_(6)H_(5)COC_(6)H_(5))+NaOI rarr" No yellow ppt. of "CHI_(3)` (iii) Phenol `(C_(6)H_(5)OH)` and Benzoic acid `(C_(6)H_(5)COOH)`: The two compounds may be distinguished by the following tests : 1. `FeCl_(3)` test : Phenol gives a voilet colour with neutral `FeCl_(3)` but benzoic acid gives buff coloured ppt. of ferric benzoate. `underset("Phenol")(C_(6)H_(5)OH)+FeCl_(3)rarr "Violet colour"` `underset("Benzoic acid")(3C_(6)H_(5)COOH)+FeCl_(3)rarrunderset("(Buff coloured ppt.)")underset("Ferric benzoate")((C_(6)H_(5)COO)_(3)Fe)+3HCL` 2. Sodium bicarbonate test : Phenol is a weaker acid than benzoic acid while benzoic acid decomposes sodium bicarbonate, phenol does not. `underset("Benzoic acid")(C_(6)H_(5)COOH)+NaHCO_(3)rarrunderset("Sodium benzoate")(C_(6)H_(5)COONa)+CO_(2)uarr+H_(2)O` `underset("Phenol")(C_(6)H_(5)OH)+NaHCO_(3)rarr"No reaction"` (iv) Benzoic acid `(C_(6)H_(5)COOH)` and Ethyl benzoate `(C_(6)H_(5)COOC_(2)H_(5))` : 1. Iodoform test : Ethyl benzoate on boiling with excess of `NaOH` solution gives ethyl alcohol along with sodium acetate which on heating with more of `NaOH and I_(2)(NaOI)` gives a yellow ppt. of Iodoform. Benzoic acid does not give this test. `underset("(Ethyl benzoate)")(C_(6)H_(5)COOC_(2)H_(5))+NaOH overset("Boli")rarr C_(6)H_(5)COONa+underset("Ethyl alcohol")(C_(2)H_(5)OH)` `C_(2)H_(5)OH+4I_(2)+6NaOH overset(Delta)rarr HCOONA+5NaI+5H_(2)O+underset("Iodoform")(CHI_(3))` `underset("Benzoic acid")(C_(6)H_(5)COOH)overset(I_(2)//NaOH)underset(NaOI)rarr "No yellow ppt. of "CHI_(3)` 2. Sodium bicarbonate test : Benzoic acid gives effervescence with `NaHCO_(3)` solution while ethyl benzoate does not. `underset("Benzoic acid")(C_(6)H_(5)COOH)+NaHCO_(3)rarr C_(6)H_(5)COONa+CO_(2)uarr+H_(2)O` `underset("Ethyl benzoate")(C_(6)H_(5)COOC_(2)H_(5))+NaHCO_(3)rarr"No effervescence"` (v) Pentan -2- ONE `(CH_(3)COCH_(2)CH_(2)CH_(3))` and Pentan -3- one `(CH_(3)CH_(2)COCH_(2)CH_(3))` : Iodoform test : Pentan -2- one is a methyl ketone, therefore, it gives iodoform test with `NaOI`. Pentan -2- one is not a methyl ketone, hence, it does not respond to iodoform test. `underset("Pentan -2- one")(CH_(3)COCH_(2)CH_(2)CH_(3))+3NaOI rarr underset("Sodium butanoate")(CH_(2)CH_(2)CH_(2)COONa)+underset("(yellow ppt.)")underset("Iodoform")(2NaOH+CHI_(3))` `CH_(3)CH_(2)COCH_(2)CH_(3)+3 NaOI" rarr "No formation of iodoform"` (vi) Benzaldehyde `(C_(6)H_(5)CHO)` and Acetophenone `(C_(6)H_(5)COCH_(3))` : 1. Iodoform test : Acetophenone is a methyl ketone. It responds to idoform test while benzaldehyde does not. `underset("Acetophenone")(C_(6)H_(5)COCH_(3))+3NaOIrarr C_(6)H_(5)COONa+2NaOH+underset("(yellow ppt.)")underset("Iodoform")(CHI_(3)darr)` `underset("Benzaldehyde")(C_(6)H_(5)CHO)+3NaOIrarr"No yellow ppt."` 2. Tollens. reagent test : Benzaldehyde being an aldehyde reduces Tollens. reagent whereas acetophenone does not. `underset("Benzaldehyde")(C_(6)H_(5)CHO)+2[Ag(NH_(3))_(2)]^(+)+3OH^(-)rarr C_(6)H_(5)COO^(-)+2H_(2)O+underset("Silver mirror")(4NH_(3)+2Agdarr)` `underset("Benzophenone")(C_(6)H_(5)COCH_(3))overset("Tollens.")underset("reagent")rarr "No formation of silver mirror"` (vii) Ethanal `(CH_(3)CHO)` and Propanal `(CH_(3)CH_(2)CHO)` : The two compounds can be distinguished by iodoform test : `underset("Ethanal")(CH_(3)CHO)+3NaOIrarr underset("Sodium methanoate")(HCOONa)+2NaOH+underset("(yellow ppt.)")underset("Iodoform")(CHI_(3))` `underset("Propanal")(CH_(3)CH_(2)CHO)+3NaOIrarr"Noyellow ppt."` |
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