Catalytic hydrogenation involves addition of hydrogen to a gt C = Clt or -C `-=` C- bond in presence of a metal catalyst such as Ni, Pt, Pd, etc. These are called heterogeneous catalysts. Both `H_2` and alkene/alkyne get adsorbed on the surface of the metal , and syn-addition of hydrogen atoms takes place. The relative rates of hydrogenation follows the order:
`-C-=C- gt gt C=C lt gt C= o lt C_6H_6`
If sodium or lithium in liquid ammonia is used, anti-addition of H-atoms occurs across `-C-=C-`bond. These reducing agents can also be used to reduce double bonds of benzene rings and conjugated dienes but not of isolated dienes.
Which of the following alkenes will react most readily with `H_2` under catalytic hydrogeneration conditions ?

Options

A.
B.
C.
D.

Explanation

To determine which alkene will react most readily with H₂ under catalytic hydrogenation conditions, we need to analyze the structure of the alkenes provided in the options. The key factors influencing the rate of hydrogenation include steric hindrance and the degree of substitution of the double bond.

### Step-by-Step Solution:

1. **Understanding the Reaction**: Catalytic hydrogenation involves the addition of hydrogen (H₂) across a double bond (C=C) in the presence of a metal catalyst (like Ni, Pt, or Pd). The reaction occurs via syn-addition, meaning both hydrogen atoms add to the same side of the double bond.

2. **Identify the Alkene Structures**: Look at the provided alkene options. The structure of each alkene will determine how easily it can undergo hydrogenation.

3. **Evaluate Steric Hindrance**:
- Alkenes with fewer bulky groups (R groups) attached to the double bond will have less steric hindrance and will react more readily.
- For example, a terminal alkene (like propene) will generally react faster than a more substituted alkene (like 2-butene) because the terminal alkene has fewer substituents around the double bond.

4. **Degree of Substitution**:
- The rate of hydrogenation decreases with an increase in the number of substituents on the double bond.
- For example, a monosubstituted alkene will react faster than a disubstituted or trisubstituted alkene.

5. **Comparing the Options**:
- If the options include alkenes such as ethene (no substituents), propene (one substituent), and 2-butene (two substituents), you would expect ethene to react the fastest, followed by propene, and then 2-butene.
- If there are more complex alkenes with multiple substituents, they will generally be slower to react.

6. **Conclusion**: Based on the analysis of steric hindrance and degree of substitution, the alkene that is least substituted (or has the least steric hindrance) will react most readily with H₂ under catalytic hydrogenation conditions.

### Final Answer:
The alkene that will react most readily with H₂ under catalytic hydrogenation conditions is the one with the least steric hindrance and the lowest degree of substitution.

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