Question

The incorrect option for the following reaction sequences is \(\mathrm{P} \frac{\mathrm{H}_{2} / \mathrm{Pd}}{\mathrm{leq}} \mathrm{Q} \frac{\mathrm{O}_{3}}{\mathrm{Zn}, \mathrm{H}_{2} \mathrm{O}}{\longrightarrow} \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{O}\) (Only product) (A) \(\mathrm{P}\) may be \(\mathrm{H}_{3} \mathrm{C}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) (B) \(\mathrm{P}\) may be \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH}=\mathrm{CH}_{2}\) (C) \(Q\) is cis 2-butene (D) \(\mathrm{Q}\) is trans 2 -butene

Short answer

The incorrect option for the given reaction sequence is (B) P may be H2C=CH-CH=CH2 (1,3-Butadiene).

Step-by-step solution

Analyze option A

P may be H3C-C≡C-CH3 (2-Butyne) Examine what would happen if P were 2-butyne: 1) Reduction of 2-butyne(H3C-C≡C-CH3) with H2/Pd would result in cis-2-butene: H3C-C≡C-CH3 -> H3C-CH=CH-CH3 (cis-2-butene) 2) Then, ozonolysis of cis-2-butene with O3 / Zn, H2O would result in the given product CH3-CH=O: cis-2-butene -> CH3-CH=O Option A is a correct possibility for P and the steps in the reaction sequence.

Analyze option B

P may be H2C=CH-CH=CH2 (1,3-Butadiene) Examine what would happen if P were 1,3-butadiene: 1) Reduction of 1,3-butadiene with H2/Pd would result in butane: H2C=CH-CH=CH2 -> H3C-CH2-CH2-CH3 (Butane) 2) Then, ozonolysis of butane with O3 / Zn, H2O would not result in the given product CH3-CH=O since butane does not have any double bonds for the desired ozonolysis reaction to occur. This means option B would be the incorrect option. But we will still examine options C and D to confirm our understanding of the reaction.

Analyze option C

Q is cis 2-butene. This option is implying that the intermediate compound in the reaction is cis 2-Butene. As we have already seen in step 1, if P is 2-butyne, the intermediate formed after the hydrogenation step is indeed cis-2-butene, and this is a valid option for Q.

Analyze option D

Q is trans 2-butene. This option is implying that the intermediate compound in the reaction is trans 2-Butene. The reaction of P (2-Butyne) with H2/Pd results in cis-2-butene, not trans-2-butene. However, if another starting material P were used, it could potentially form trans-2-butene as the intermediate. This option does not provide sufficient information to definitively be labeled as incorrect. Conclusion: Based on the analysis, the incorrect option for the given reaction sequence is: (B) P may be H2C=CH-CH=CH2 (1,3-Butadiene)

Key concepts

Reduction Reactions

Reduction reactions are a fundamental concept in organic chemistry where hydrogen is added, or oxygen is removed, from a molecule. These reactions are vital in organic synthesis and are used to convert a variety of functional groups into more reduced forms.

One common type of reduction reaction involves the addition of hydrogen ( H_2 ) to alkenes and alkynes, processes known as hydrogenation, which we'll explore further.

Reduction reactions typically require a catalyst such as palladium (Pd), nickel (Ni), or platinum (Pt) to facilitate the addition of hydrogen to unsaturated carbon-carbon bonds.

The choice of catalyst can influence the stereochemistry, or 3D arrangement, of the resulting product. For example, using Pd results in syn-addition, leading to cis-alkenes from alkynes.

• Hydrogenation: Adds hydrogen in the presence of a catalyst.
• Decreases the number of bonds to oxygen while increasing bonds to hydrogen.

This type of reaction is essential in converting unsaturated compounds into saturated ones, making it widely applied in the production of edible fats and in petrochemical refining.

Ozonolysis

Ozonolysis is a powerful oxidative technique used to cleave carbon-carbon double bonds ( C=C ). It involves the reaction of ozone ( O_3 ) with alkenes or alkynes to form ozonides, which are then reduced to carbonyl compounds using a reducing agent such as zinc ( Zn ) and water ( H_2O ).

This reaction allows chemists to determine the structure of unknown compounds by breaking alkenes and alkynes into smaller, more manageable pieces.

After the reaction, ozonides are often not isolated; instead, they are directly converted to aldehydes, ketones, or carboxylic acids depending on the reaction conditions.

• Starts with ozone to oxidize the double or triple bonds.
• Uses reducing agents like Zn and H_2O for final reduction.

Ozonolysis is particularly useful in organic synthesis for constructing complex molecules and elucidating structures.

Hydrogenation

Hydrogenation is a specific type of reduction reaction whereby hydrogen is added to unsaturated carbon-carbon bonds like those in alkenes and alkynes. This process typically requires the presence of a catalyst, often palladium ( Pd ), to proceed efficiently.

During hydrogenation, alkenes are converted into alkanes by the addition of hydrogen across the double bond. For alkynes, they first convert to alkenes and potentially to alkanes with prolonged hydrogenation.

Syn addition, where hydrogen atoms add to the same side of the molecule, often results in the formation of cis-alkenes from alkynes.

• Requires catalysts like Ni , Pt , or Pd .
• Useful in converting unsaturated to saturated compounds.

Hydrogenation is widely used industrially in the production of butter and margarine where unsaturated fats are converted into saturated fats for improved consistency and shelf-life.

Alkynes

Alkynes are hydrocarbons with one or more carbon-carbon triple bonds ( C≡C ). They are structurally different from alkenes due to the presence of this stronger, more rigid triple bond.

Alkynes can undergo various transformations, including hydrogenation and ozonolysis, to form alkenes or smaller carbonyl compounds.

These reactions are pivotal in organic synthesis for constructing complex molecules or simplifying structures by breaking them down.

• Alkynes can be reduced to alkenes and eventually to alkanes.
• They have higher energy and reactivity compared to alkenes.

Alkynes are essential intermediates in the synthesis of numerous synthetic materials, pharmaceuticals, and agrochemicals.

Alkenes

Alkenes are hydrocarbons featuring one or more carbon-carbon double bonds ( C=C ). Double bonds create areas of high electron density, making alkenes more reactive than alkanes.

Alkenes can participate in a variety of chemical reactions, including hydrogenation, halogenation, and ozonolysis. Each adds new functional groups to the double bond, affecting the molecule's properties and uses.

During hydrogenation, alkenes convert into alkanes by the addition of hydrogen. In ozonolysis, the double bonds are cleaved, resulting in two separate carbonyl-containing compounds.

• Serve as intermediates in forming polymers like polyethylene.
• Very reactive due to the presence of double bonds.

Alkenes are critical building blocks in petrochemical production and serve as starting materials for alcohols, plastics, and numerous other compounds.