(a) Give chemical tests to distinguish between

(i) Propanol and propanone

(ii) Benzaldehyde and acetophenone

(b) Arrange the following compounds in an increasing order of their property as indicated:

(i) Acetaldeyde, Acetone, Methyl tert-butyl ketone (reactivity towards HCN)

(ii) Benzoic acid, 3, 4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength).

(iii) CH₃CH₂CH (Br) COOH, CH₃CH (Br) CH₂COOH, (CH₃)₂CHCOOH (acid strength)

(a)

(i) Propanone gives iodoform test while propanol does not give this test.

(ii) Benzaldehyde (C₆H₅CHO) and acetophenone (C₆H₅COCH₃) can be distinguished by iodoform test.

Acetophenone, being a methyl ketone on treatment with I₂/NaOH undergoes iodoform reaction to give a yellow ppt. of iodoform. On the other hand, benzaldehyde does not give this test.

(b)

(i) Methyl tert-butyl ketone < Acetone < Acetaldehyde

When HCN reacts with a compound, the attacking species is a nucleophile, CN^-.Therefore, as the negative charge on the compound increases, its reactivity with HCN decreases. In the given compounds, the +I effect increases as shown below. It can be observed that steric hindrance also increases in the same.

(ii) 4-Methoxybenzoic acid < Benzoic acid < 3, 4-Dinitrobenzoic acid

Electron-donating groups decrease the strengths of acids, while electron-withdrawing groups increase the strengths of acids. As methoxy group is an electron-donating group, 4-ethoxybenzoic acid is a weaker acid than benzoic acid. Nitro group is an electron-withdrawing group and will increase the strengths of acids.

(iii) (CH₃)₂ CHCOOH < CH₃CH(Br)CH₂COOH < CH₃CH₂CH(Br)COOH

After losing a proton, carboxylic acids gain a negative charge as shown:

R – COOH ---> R - COO^- + H⁺

Now, any group that will help stabilise the negative charge will increase the stability of the carboxyl ion and as a result, will increase the strength of the acid. Thus, groups having +I effect will decrease the strength of the acids and groups having -I effect will increase the strength of the acids. In the given compounds, -CH₃ group has +I effect and Br^- group has -I effect. Thus, acids containing Br^- are stronger.

The -I effect grows weaker as distance increases. Hence, CH₃CH(Br)CH₂COOH is a weaker acid than CH₃CH₂CH(Br)COOH.