The reaction, `RC-=CR underset("Lindlar's catalyst")overset(H_(2))to`Gives the main product as

Options

A. cis-alkene
B. trans-alkene
C. alkane
D. none of these

Answer

To solve the question regarding the reaction of an alkyne with hydrogen gas in the presence of Lindlar's catalyst, we can follow these steps:

### Step 1: Understand the Reaction
The reaction involves an alkyne (RC≡CR) reacting with hydrogen gas (H₂) in the presence of Lindlar's catalyst. Lindlar's catalyst is a specific catalyst used for the partial hydrogenation of alkynes.

**Hint:** Remember that Lindlar's catalyst is designed to selectively reduce alkynes to alkenes without further reducing them to alkanes.

### Step 2: Identify the Type of Product Formed
When an alkyne is treated with hydrogen gas in the presence of Lindlar's catalyst, it is selectively reduced to a cis-alkene. This is because the catalyst is poisoned, which prevents complete hydrogenation to an alkane.

**Hint:** Focus on the definition of cis and trans isomers. Cis means that the substituents are on the same side of the double bond.

### Step 3: Determine the Main Product
The main product of the reaction will be a cis-alkene. In this case, the alkyne (RC≡CR) will yield a cis-alkene (cis-RC=CR) as the main product.

**Hint:** Recall that the presence of the catalyst and the conditions of the reaction favor the formation of the cis isomer.

### Step 4: Evaluate the Options
Given the options:
- A) cis-alkene
- B) trans-alkene
- C) alkane
- D) none of these

Since we established that the main product is a cis-alkene, the correct answer is option A.

**Hint:** Always compare your findings with the provided options to ensure that your answer matches one of them.

### Final Answer
The main product of the reaction `RC≡CR` with hydrogen gas in the presence of Lindlar's catalyst is **cis-alkene**.