Solutions-Class-12-Chemistry-Chapter-12-Aldehydes, Ketones and Carboxylic acids-Maharashtra Board

(c) 4, 6

Explanation:

• In structure (A), the oxygen atom has two lone pairs of electrons, which means there are 4 unshared electrons.
• In structure (B), the oxygen atom has three lone pairs of electrons, totalling 6 unshared electrons.

(c) hydrazones

(d) addition - elimination reaction

(d) acid group

(d) Fehling solution

The compounds in which a C=O group is attached to either two aryl groups or one aryl and one alkyl group are called aromatic ketones.

Example :

No. Phenyl acetic acid is not an aromatic carboxylic acid.

The aqueous solution of formaldehyde (40%) is known as formalin.

Ethane, formaldehyde, methyl alcohol.

The cooling agent used in the above reaction is dry ice (O = C = O).

Tollen's reagent oxidises acetaldehyde to acetic acid (carboxylate ion) and Ag⁺ in Tollen's reagent complex are reduced to silver. In this reaction, oxidation and reduction takes place simultaneously hence, it is a redox reaction.

• In acetic acid, methyl group is an electron donating group. The acetate ion formed gets destabilized due to electron releasing effect of methyl group (+ I effect) which is higher than that of H-atom in the corresponding formic acid.
• As a result, acetic acid dissociates to a lesser extent. Thus decreasing the acidity of acetic acid.

CH₃-COOH + H₂O ⇌ CH₃-COO- + H₃O⁺

• Formic acid having lower pK_a value than acetic acid. Hence, formic acid is a stronger acid than acetic acid.

Reactivity of aldehydes and ketones is due to the polarity of carbonyl group which results in electrophilicity of carbon. In general, aldehydes are more reactive than ketones toward nucleophilic attack. This can be well explained in terms of both the electronic effects and steric effect.

(i) Influence of electronic effects:

• Alkyl groups have an electron-donating inductive effect (+I). A ketone has two electron-donating alkyl groups bonded to carbonyl carbon which are responsible for decreasing its positive polarity and electrophilicity.
• In contrast, aldehydes have only one electron-donating group bonded to the carbonyl carbon. This makes aldehydes more electrophilic than ketones.

(ii) Steric effects:

• Two bulky alkyl groups in ketone come in the way of the incoming nucleophile. This is called steric hindrance to nucleophilic attack.
• On the other hand, nucleophile can easily attack the carbonyl carbon in aldehyde because it has one alkyl group and is less crowded or sterically less hindered. Hence, aldehydes are more easily attacked by nucleophiles.

(a) Action of dilute NaOH :

(b) Action of dilute HCL :

The increasing order of acidity will be (1) < (3) < (2).

Acidity depends on mainly two factors :

(i) ease of proton release

(ii) stability of conjugate base formed.

In example (3) the ether O exerts a -I effect and is closer to -COOH group than in (2) (I effect diminishes). Also the conjugate base formed will be stabilized by the same -I effect by delocalization of charge.

Cannizzaro reaction :

• Aldehydes which do not have a-hydrogen atom, on heating with concentrated alkali (50% aqueous or ethanolic solution of NaOH or KOH) undergo self oxidation and reduction reaction or redox reaction. This self redox reaction or disproportionation reaction is called Cannizzaro reaction.
• In this reaction one molecule of the aldehyde is oxidised to carboxylic acid while the second molecule of the aldehyde is reduced to alcohol (carboxylic acid formed, reacts with alkali, NaOH and forms a salt R—COONa).
• When formaldehyde (methanal) is heated with 50% NaOH solution, methanol (reduction product) and sodium formate (oxidation product) are formed.

Similarly, benzaldehyde gives Cannizzaro reaction.

• Ketones and aldehydes like acetaldehyde, propionaldehyde, etc. having α-H atom do not give Cannizzaro reaction.

Nitriles are reduced to imine hydrochloride by stannous chloride in presence of hydrochloric acid which on acid hydrolysis give corresponding aldehydes. This reaction is called Stephen reaction.

Examples :

(a) IUPAC name : 3-Methylcyclohexanone

(b) IUPAC name : Ethanedioic acid

Step 1: Converting - Br group to - CN

Step 2: Converting - CN group to - CONH₂

Step 3: Converting - CONH₂ group to - COOH