Question

Which of the reactions is used in the conversion of a ketone into hydrocarbon ?

• A. Reimer-Tiemann reaction
• B. Wolff-Kishner reduction
• C. Aldol condensation
• D. Stephen reaction

Hint:

Remember the two famous reactions used to convert aldehydes and ketones into hydrocarbons: 1. Wolff-Kishner Reduction : \(NH_2NH_2/KOH,\ Heat\) (Basic medium) 2. Clemmensen Reduction : \(Zn(Hg)/HCl\) (Acidic medium) Both reactions remove the oxygen atom of the carbonyl group completely and convert the compound into the corresponding hydrocarbon.

Answer 1

The Correct Option is B

Concept: Ketones contain the carbonyl functional group \((\gt C=O)\). In many organic reactions, it is required to completely remove the oxygen atom of the carbonyl group and convert the ketone into the corresponding hydrocarbon. Such transformations are known as reductions of carbonyl compounds. Two important reactions commonly used for this purpose are Clemmensen reduction and Wolff-Kishner reduction. In the Wolff-Kishner reduction, a ketone or aldehyde is first converted into a hydrazone by reaction with hydrazine \((NH_2NH_2)\). The hydrazone is then heated with a strong base such as KOH, resulting in the complete removal of oxygen and formation of the corresponding hydrocarbon. Therefore, this reaction is specifically known for converting ketones into hydrocarbons.

Step 1: Understanding what transformation is being asked in the question. The question asks for a reaction that converts a ketone into a hydrocarbon. A ketone has the general structure \[ R-CO-R' \] whereas a hydrocarbon contains only carbon and hydrogen atoms. Therefore, the required reaction must completely eliminate the oxygen atom present in the carbonyl group and replace it effectively with hydrogen atoms. For example, \[ CH_3COCH_3 \longrightarrow CH_3CH_2CH_3 \] In this conversion, acetone is transformed into propane by complete reduction of the carbonyl group. Hence, we need a reaction known for complete reduction of ketones.

Step 2: Examining Option (A) : Reimer-Tiemann reaction. The Reimer-Tiemann reaction is a reaction of phenols with chloroform in the presence of alkali. Its primary purpose is the introduction of a formyl group \((-CHO)\) into an aromatic ring. A typical reaction is \[ \text{Phenol} \xrightarrow[\text{NaOH}]{CHCl_3} \text{Salicylaldehyde} \] Thus, this reaction is used for the preparation of aromatic aldehydes and not for converting ketones into hydrocarbons. Therefore, Option (A) is incorrect.

Step 3: Examining Option (B) : Wolff-Kishner reduction. In the Wolff-Kishner reduction, a ketone reacts with hydrazine to form a hydrazone. \[ R_2C=O + NH_2NH_2 \longrightarrow R_2C=NNH_2 \] The hydrazone is then heated with KOH. During this process, nitrogen gas is eliminated and the carbonyl carbon gets reduced. \[ R_2C=NNH_2 \xrightarrow[\text{Heat}]{KOH} R_2CH_2 + N_2 \] As a result, the ketone is converted directly into the corresponding hydrocarbon. For example, \[ CH_3COCH_3 \xrightarrow[NH_2NH_2]{KOH,\ Heat} CH_3CH_2CH_3 \] This is exactly the conversion asked in the question. Therefore, Option (B) is correct.

Step 4: Examining Option (C) : Aldol condensation. Aldol condensation occurs between aldehydes or ketones containing \(\alpha\)-hydrogen atoms. The reaction produces \(\beta\)-hydroxy aldehydes or \(\beta\)-hydroxy ketones, which may further dehydrate to form \(\alpha,\beta\)-unsaturated compounds. For example, \[ 2CH_3CHO \longrightarrow CH_3CH(OH)CH_2CHO \] Since this reaction forms larger carbon skeletons rather than reducing ketones to hydrocarbons, it cannot be the required answer. Hence, Option (C) is incorrect.

Step 5: Examining Option (D) : Stephen reaction. The Stephen reaction, also known as Stephen aldehyde synthesis, converts nitriles into aldehydes. The general transformation is \[ R-CN \longrightarrow R-CHO \] This reaction is useful for preparing aldehydes from nitriles but has no role in converting ketones into hydrocarbons. Therefore, Option (D) is also incorrect.

Step 6: Selecting the correct reaction based on the analysis of all options. After examining all four reactions, we observe that only the Wolff-Kishner reduction specifically performs the complete reduction of the carbonyl group of a ketone and converts it into a hydrocarbon. Thus, \[ R-CO-R' \longrightarrow R-CH_2-R' \] is achieved by the Wolff-Kishner reduction. Hence, the correct option is Option (B). \[ \boxed{\text{Wolff-Kishner reduction}} \]