Question

Which one of the following alkenes will react faster with H₂ under catalytic hydrogenation conditions?(R=Alkyl substituent)

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is A

The reaction in question is catalytic hydrogenation of alkenes. The rate at which alkenes react with hydrogen depends on several factors, including steric hindrance and the degree of substitution of the double bond.

To determine which alkene will react the fastest, let's consider these factors:

1. Steric Hindrance: Hydrogenation is facilitated by less steric hindrance. Alkenes that are less substituted react faster because there is more space around the double bond for the catalyst to interact with the hydrogen gas.
2. Substitution of the Double Bond: Terminal alkenes (those with the double bond at the end of the carbon chain) are generally less substituted than internal alkenes and should therefore react more quickly under catalytic hydrogenation conditions.

Now, let's evaluate the given options based on these points:

• The first option features a terminal alkene, which is less hindered and usually reacts faster due to its lower substitution level.
• Other options, being more substituted or internal alkenes, are expected to react slower because they have greater steric hindrance.

Therefore, the terminal alkene option will undergo hydrogenation faster under the given conditions.

Conclusion: The alkene that will react the fastest with hydrogen under catalytic hydrogenation conditions is shown in the following image: