The product (A) formed in the following reaction sequence is : C H _ 3 - C ≡ C H [ (i i) H _ 2 / N i ](i) H g ^ 2 +, H _ 2 S O _ 4 (A)$$\mathrm {C H} _ {3} - \mathrm {C} \equiv \mathrm {C H} \xrightarrow [ (\mathrm {i i}) \mathrm {H} _ {2} / \mathrm {N i} ]{(\mathrm {i}) \mathrm {H} g ^ {2 +}, \mathrm {H} _ {2} \mathrm {S O} _ {4}} (A)$$

Core Logic

Step 1: Free radical side-chain halogenation of toluene. Toluene reacts with Cl₂$Cl_2$ in the presence of light (hν$h\nu$) to undergo substitution on the methyl group. Under typical conditions intended to yield an aldehyde later, di-chlorination occurs forming benzal chloride (A).

Step 2: Hydrolysis. Benzal chloride upon hydrolysis with H₂O$H_2O$ at 373 K yields a gem-diol intermediate which is unstable and loses water to form Benzaldehyde (B).

Step 1: Identify Structures

(A) = Benzal chloride (C₆H₅CHCl₂$C_6H_5CHCl_2$) (B) = Benzaldehyde (C₆H₅CHO$C_6H_5CHO$)

Pattern Recognition

Toluene Cl₂, hν$\xrightarrow{Cl_2, h\nu}$ targets the side chain. If the next step is hydrolysis to an aldehyde, you must have stopped at the gem-dihalide stage (CHCl₂$CHCl_2$).

Chapter Mix

Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids Class 11 Chemistry: Hydrocarbons

Core Logic

Let's analyze the transformation happening in each reaction:

(A) CH₃(CH₂)₅COOC₂H₅ arrow CH₃(CH₂)₅CHO$CH_3(CH_2)_5COOC_2H_5 \rightarrow CH_3(CH_2)_5CHO$ An ester is reduced to an aldehyde. This is a selective reduction achieved using DIBAL-H (Diisobutylaluminium hydride) followed by hydrolysis. Thus, (A) arrow$\rightarrow$ (IV).

(B) C₆H₅COC₆H₅ arrow C₆H₅CH₂C₆H₅$C_6H_5COC_6H_5 \rightarrow C_6H_5CH_2C_6H_5$ A ketone (carbonyl group >C=O$>C=O$) is fully reduced to an alkane (>CH₂$>CH_2$) methylene group. This is the Clemmensen reduction, which uses Zinc amalgam and concentrated HCl. Thus, (B) arrow$\rightarrow$ (II).

(C) C₆H₅CHO arrow C₆H₅CH(OH)CH₃$C_6H_5CHO \rightarrow C_6H_5CH(OH)CH_3$ Benzaldehyde (aldehyde) is converted into a secondary alcohol with an extra methyl group. This is a nucleophilic addition of a Grignard reagent (CH₃MgBr$CH_3MgBr$) followed by hydrolysis. Thus, (C) arrow$\rightarrow$ (I).

(D) CH₃COCH₂COOC₂H₅ arrow CH₃CH(OH)CH₂COOC₂H₅$CH_3COCH_2COOC_2H_5 \rightarrow CH_3CH(OH)CH_2COOC_2H_5$ A ketone group is reduced to a secondary alcohol while the ester group remains intact. NaBH₄$NaBH_4$ is a mild reducing agent that reduces aldehydes and ketones but generally does not touch esters. Thus, (D) arrow$\rightarrow$ (III).

Pattern Recognition

Ester arrow$\rightarrow$ Aldehyde = DIBAL-H Ketone arrow$\rightarrow$ Alkane = Clemmensen (Zn(Hg)/HCl$Zn(Hg)/HCl$) or Wolff-Kishner Carbonyl arrow$\rightarrow$ Alcohol with carbon chain extension = Grignard Reagent Ketone arrow$\rightarrow$ Alcohol (leaving ester intact) = NaBH₄$NaBH_4$

Chapter Mix

Class 12 Chemistry: Aldehydes, Ketones and Carboxylic Acids

Core Logic

The reaction sequence proceeds in three distinct steps from the starting material, styrene (Ph-CH=CH₂$Ph-CH=CH_2$).

Step 1: Acid-catalyzed Hydration Styrene reacts with H₂O, H^+$H_2O, H^+$ to undergo electrophilic addition. Protonation yields the more stable secondary benzylic carbocation. Attack by water followed by deprotonation gives 1-phenylethanol (Ph-CH(OH)-CH₃$Ph-CH(OH)-CH_3$).

Step 2: Oxidation 1-phenylethanol is a secondary alcohol. Treatment with chromium trioxide (CrO₃$CrO_3$, Jones reagent condition) oxidizes the secondary alcohol to a ketone. This yields acetophenone (Ph-CO-CH₃$Ph-CO-CH_3$).

Step 3: Wolff-Kishner Reduction Acetophenone is treated with hydrazine (NH₂-NH₂$NH_2-NH_2$) and a strong base (KOH$KOH$) under heating. This is the classic Wolff-Kishner reduction, which completely reduces the carbonyl group (C=O$C=O$) to a methylene group (-CH₂-$-CH_2-$). The final product is ethylbenzene (Ph-CH₂-CH₃$Ph-CH_2-CH_3$).

Step 1: Overall Reaction Pathway

The final product A is ethylbenzene.

Chapter Mix

Class 11 Chemistry: Hydrocarbons Class 12 Chemistry: Alcohols Phenols and Ethers Class 12 Chemistry: Aldehydes Ketones and Carboxylic Acids

Core Logic

Fehling's test is a mild oxidizing test used primarily to distinguish aliphatic aldehydes from ketones and aromatic aldehydes.

• Aliphatic aldehydes (like methanal, acetaldehyde, cyclohexane carbaldehyde) give a positive Fehling's test (formation of red-brown precipitate of Cu₂O$Cu_2O$).
• Aromatic aldehydes (like benzaldehyde, 4-nitrobenzaldehyde) lack alpha-hydrogens in a purely aliphatic environment and are not sufficiently easily oxidized to give a positive Fehling's test.
• Ketones (like acetone, acetophenone) generally do not give a positive Fehling's test (except α$\alpha$-hydroxy ketones).

Step 1: Evaluation of Given Compounds

• Benzaldehyde: Aromatic aldehyde arrow$\rightarrow$ Negative
• Acetaldehyde (CH₃CHO$CH_3CHO$): Aliphatic aldehyde arrow$\rightarrow$ Positive
• Acetone: Ketone arrow$\rightarrow$ Negative
• Acetophenone: Ketone arrow$\rightarrow$ Negative
• Methanal (HCHO$HCHO$): Aliphatic aldehyde arrow$\rightarrow$ Positive
• 4-nitrobenzaldehyde: Aromatic aldehyde arrow$\rightarrow$ Negative
• Cyclohexane carbaldehyde: Aliphatic aldehyde arrow$\rightarrow$ Positive

The compounds giving a positive test are Acetaldehyde, Methanal, and Cyclohexane carbaldehyde.

Pattern Recognition

Tollens' reagent oxidizes ALL aldehydes (aliphatic + aromatic). Fehling's reagent is weaker and only oxidizes ALIPHATIC aldehydes.

Chapter Mix

Class 12 Chemistry: Aldehydes Ketones and Carboxylic Acids

Core Logic

m-chlorobenzaldehyde lacks alpha-hydrogen atoms. Therefore, when treated with concentrated base like 50%$50\%$ KOH, it undergoes a disproportionation redox reaction known as the Cannizzaro reaction.

Two molecules of the aldehyde react: one gets oxidized to the corresponding carboxylate ion (m-chlorobenzoate ion), and the other gets reduced to the corresponding alcohol (m-chlorobenzyl alcohol).

Step 1: Reaction

The reaction proceeds as:

Pattern Recognition

No α$\alpha$-hydrogen + Conc. Alkali (50%$50\%$ KOH/NaOH) = Cannizzaro (Oxidation to salt of carboxylic acid + Reduction to primary alcohol).

Chapter Mix

Class 12 Chemistry: Aldehydes Ketones and Carboxylic Acids