Question

Reactivity of alkenes towards IIX decreases in the order (1) Butene \(>\) propene \(>\) ethene (2) Butene \(>\) ethene \(>\) propene (3) Ethene \(>\) propene \(>\) butene (4) Propene \(>\) ethene \(>\) butene

Short answer

The order is (1) Butene > propene > ethene.

Step-by-step solution

- Identify the factors influencing reactivity

Understand that the reactivity of alkenes with IIX (where X can be any halogen) depends on the stability of the carbocation intermediate formed during the reaction. Carbocations are more stable with more alkyl groups attached to the positively charged carbon.

- Analyze the structure and stability

Consider the structures and the number of alkyl groups attached to the carbocation intermediates formed from each alkene: Butene forms a secondary (or potentially tertiary) carbocation, propene forms a secondary carbocation, and ethene forms a primary carbocation.

- Compare the stability of carbocations

Order the alkenes based on the stability of the carbocations they form: tertiary > secondary > primary. Therefore, butene, which can form the most stable carbocation, will be the most reactive, followed by propene, and then ethene.

- Choose the correct order

Match the correct order of decreasing reactivity: Butene > Propene > Ethene. This matches option (1).

Key concepts

Carbocation Stability

Understanding carbocation stability is key to predicting the reactivity of alkenes in chemical reactions. A carbocation is an ion with a positively charged carbon atom, and its stability depends on the number of alkyl groups attached to this carbon. More alkyl groups lead to greater stability. This is due to the electron-donating effect of alkyl groups, which helps to distribute the positive charge more effectively.

In our exercise, butene forms a secondary or potentially tertiary carbocation, propene forms a secondary carbocation, and ethene forms a primary carbocation. The primary carbocation is the least stable, while the tertiary carbocation is the most stable. Therefore, butene, which can form the most stable carbocation, is the most reactive towards halogens.

Alkyl Groups

Alkyl groups are groups of carbon and hydrogen atoms arranged in a chain. They play a crucial role in stabilizing carbocations. The more alkyl groups attached to the positively charged carbon, the more stable the carbocation is.

Alkyl groups are electron-releasing groups. They push electrons towards the positively charged carbon, reducing the positive charge's intensity. This makes the carbocation more stable. In the context of the problem, butene has more alkyl groups contributing to the stability of its carbocation compared to propene and ethene.

Here’s a quick summary:

• Primary carbocation: Very unstable, least number of alkyl groups.
• Secondary carbocation: Moderately stable, has two alkyl groups.
• Tertiary carbocation: Very stable, has three alkyl groups.

Chemistry Reactions

Chemistry reactions involving alkenes typically depend on the creation of carbocations. During the reaction with a halogen, for instance, the alkene provides a location (the double bond) where the halogen can add to.

The stability of the carbocation intermediate formed during this reaction dictates the rate of reaction. More stable carbocations form faster and thus react more quickly.

To understand the reactivity trend given in the problem:

• Butene forms a tertiary or secondary carbocation.
• Propene forms a secondary carbocation.
• Ethene forms a primary carbocation.

Because a tertiary carbocation is more stable than a secondary, and a secondary is more stable than a primary, the reactivity order is: Butene > Propene > Ethene.

This confirms that the correct answer to the exercise is: Butene > Propene > Ethene.