b) Write any two chemical reactions to distinguish between Ethanal and
Propanone.
3+2=5
Answers
Answer:
Ethanal and propanone are an aldehyde and ketone respectively. These two compounds can be distinguished by using mild oxidizing agents as aldehydes and ketones have different oxidation reactions.
The two tests to distinguish them are Fehling’s test and Tollen’s test.
Fehling’s test consists of a reagent which is two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartrate (Rochelle salt). These two solutions are mixed in equal amounts before the test. On heating an aldehyde like ethanal with Fehling’s reagent, a reddish-brown precipitate of Cu2O is obtained. Aldehydes are oxidised to their corresponding carboxylate anion. Ketones like propanone do not respond to this test. The reaction is given as
R—CHO + 2Cu2+ + 5OH-→ RCOO- + Cu2O + 3H2O
The second test is the Tollen’s test. In this test, warming an aldehyde with freshly prepared Tollens’ reagent (ammoniacal silver nitrate solution) in alkaline medium, a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. Ketones like propanone do not get oxidized.
R—CHO + 2[Ag(NH3)2]2+ + 3OH-→ R—COO- + 2Ag + 2H2O + 4NH3
(ii) Pentan-2-one is a methyl ketone and pentane-3-one is not. Methyl ketones are detected by iodoform reaction with sodium hypoiodite. Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to iodoform test. Pentan-2-one gets oxidized by NaOI to form iodoform as a precipitate. Pentan-3-one which does not contain a methyl group does not get oxidized by NaOI.
(b) The chemical formula of the given compounds are:
Benzoic acid – C6H5COOH, 4-Nitrobenzoic acid - C6H4(NO2)COOH, 3,4-Dinitrobenzoic acid – C6H4(NO2)2COOH, 4-Methoxybenzoic acid – C6H4(CH3)COOH
When considering the acid strength of the given compounds, substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects, both in the case of the given compounds. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base. This happens because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached. The presence of electron-withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron-donating groups decrease their acidity. The more the electron-withdrawing groups, the more the acid strength of the compound. –CH3 and –NO2 are electron-withdrawing groups, the –NO2 group has higher withdrawing power. Hence, the order of the given compounds are:
4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid.
OR
In the nucleophilic addition reaction of carbonyl groups, a nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp2 hybridised orbitals of carbonyl carbon. The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in ethanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance and hence it is less reactive than ethanal.
Aldehydes having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol). This reaction is known as aldol reaction. When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation.
Answer:
Ethanal and propanone are an aldehyde and ketone respectively. These two compounds can be distinguished by using mild oxidizing agents as aldehydes and ketones have different oxidation reactions.
The two tests to distinguish them are Fehling’s test and Tollen’s test.
Fehling’s test consists of a reagent which is two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartrate (Rochelle salt). These two solutions are mixed in equal amounts before the test. On heating an aldehyde like ethanal with Fehling’s reagent, a reddish-brown precipitate of Cu2O is obtained. Aldehydes are oxidised to their corresponding carboxylate anion. Ketones like propanone do not respond to this test. The reaction is given as
R—CHO + 2Cu2+ + 5OH-→ RCOO- + Cu2O + 3H2O
The second test is the Tollen’s test. In this test, warming an aldehyde with freshly prepared Tollens’ reagent ( ammoniacal silver nitrate solution) in alkaline medium, a bright silver mirror is produced due to the formation of silver metal. The aldehydes are oxidised to corresponding carboxylate anion. Ketones like propanone do not get oxidized.
R—CHO + 2[Ag(NH3)2]2+ + 3OH-→ R—COO- + 2Ag + 2H2O + 4NH3
(ii) Pentan-2-one is a methyl ketone and pentane-3-one is not. Methyl ketones are detected by iodoform reaction with sodium hypoiodite. Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to iodoform test. Pentan-2-one gets oxidized by NaOI to form iodoform as a precipitate. Pentan-3-one which does not contain a methyl group does not get oxidized by NaOI.
(b) The chemical formula of the given compounds are:
Benzoic acid – C6H5COOH, 4-Nitrobenzoic acid - C6H4(NO2)COOH, 3,4-Dinitrobenzoic acid – C6H4(NO2)2COOH, 4-Methoxybenzoic acid – C6H4(CH3)COOH
When considering the acid strength of the given compounds, substituents may affect the stability of the conjugate base and thus, also affect the acidity of the carboxylic acids. Electron withdrawing groups increase the acidity of carboxylic acids by stabilising the conjugate base through delocalisation of the negative charge by inductive and/or resonance effects, both in the case of the given compounds. Conversely, electron donating groups decrease the acidity by destabilising the conjugate base. This happens because of greater electronegativity of sp2 hybridised carbon to which carboxyl carbon is attached. The presence of electron-withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron-donating groups decrease their acidity. The more the electron-withdrawing groups, the more the acid strength of the compound. –CH3 and –NO2 are electron-withdrawing groups, the –NO2 group has higher withdrawing power. Hence, the order of the given compounds are:
4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid.
Explanation:
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