Question

When ethylene is alkylated by isobutane in the presence of acid, there is obtained not neohexane, \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCH}_{2} \mathrm{CH}_{3}\), but chiefly 2,3 -dimethylbutane. Account in detail for the formation of this product.

Short answer

The acid-catalyzed alkylation of ethylene and isobutane primarily forms 2,3-dimethylbutane instead of neohexane due to the stability of intermediates that form during the reaction. Protonation of ethylene creates a secondary carbocation, which is more stable than a primary carbocation. When this carbocation attacks isobutane, another stable carbocation intermediate forms. Finally, deprotonation of this carbocation results in 2,3-dimethylbutane. Overall, the stability of the intermediates favors the formation of 2,3-dimethylbutane.

Step-by-step solution

Isobutane structure

Isobutane, also known as 2-methylpropane, has the chemical formula \(\left(\text{CH}_{3}\right)_{3}\text{CH}\) and the following structure. ``` CH3 | CH3 – C – CH3 ```

Ethylene structure

Ethylene, also known as ethene, is an alkene with the chemical formula \(\text{CH}_{2}=\text{CH}_{2}\) and has the following structure. ``` CH2=CH2 ``` #Step 2: Acid-catalyzed reaction between ethylene and isobutane#

Protonation of ethylene

When the acid catalyst is introduced, it provides protons that can protonate the alkene (ethylene). Protonation of ethylene forms a carbocation intermediate: ``` CH3 | C+ – CH3 | CH3 ``` The resulting carbocation is a secondary carbocation, which is more stable than a primary carbocation.

Formation of carbocation intermediate

With the ethylene protonated and the isobutane as a nucleophile, the carbocation can attack the isobutane, forming a new carbon-carbon bond. This process results in another carbocation intermediate: ``` CH2 | CH3 – C – CH3 | CH3 – C+ – H ``` #Step 3: Deprotonation to form 2,3-dimethylbutane#

Deprotonation of carbocation intermediate

The carbocation intermediate can be deprotonated to form a stable alkane. In this case, the loss of a proton leads to the formation of 2,3-dimethylbutane: ``` CH2 | CH3 – C – CH3 | CH3 – C – H ``` The overall reaction sequence leads to the formation of 2,3-dimethylbutane as the primary product, instead of neohexane.

Key concepts

Alkylation

In organic chemistry, alkylation is a fundamental reaction where an alkyl group is transferred from one molecule to another. This is often used to modify the structure and properties of organic compounds. The process typically involves:

• An alkylating agent, usually an alkyl halide or alkene.
• A catalyst that activates the reaction.
• Formation of a new carbon-carbon bond.

In the given exercise, ethylene undergoes alkylation with isobutane. This reaction is facilitated by an acid catalyst, which aids in transferring the alkyl group from isobutane to ethylene. The key role of the acid is to initiate the formation of a carbocation, making the alkene more reactive. This enables the transfer of an alkyl group, resulting in the formation of a different product—2,3-dimethylbutane.

Carbocation Stability

Carbocations are ions with a positively charged carbon atom. These intermediates are crucial in many organic reactions, including alkylation. The stability of a carbocation is important because it influences the reaction pathway and products formed. Factors that affect carbocation stability include:

• Hyperconjugation: The delocalization of electrons in adjacent sigma bonds to the empty p-orbital of the carbocation.
• Inductive effect: Electron-donating groups can stabilize the carbocation by reducing the positive charge.
• Resonance: If a carbocation can spread its charge over multiple atoms through resonance, it is more stable.

In the reaction between ethylene and isobutane, the formation of a secondary carbocation is crucial. Secondary carbocations are more stable than primary carbocations due to greater hyperconjugation and inductive effects from the adjacent alkyl groups. This stability favors the formation of certain products, like 2,3-dimethylbutane, over others like neohexane.

Acid-Catalyzed Reactions

Acid-catalyzed reactions are a common method in organic chemistry to speed up reactions that involve the formation of carbocations. The acid provides protons, which are essential in protonating alkenes or alcohols, making them more reactive. The mechanism generally involves:

• Protonation of the starting material, creating a carbocation intermediary.
• Nucleophilic attack by another molecule, forming a new bond.
• Deprotonation, resulting in the final product.

In the context of the ethylene and isobutane reaction, the acid catalyst starts by protonating ethylene to form a carbocation. This form of carbocation is a reactive species that can readily undergo nucleophilic attack by isobutane. Finally, deprotonation occurs, leading to the stable product of 2,3-dimethylbutane. The choice of proton source and its strength can significantly affect the reaction's efficiency and outcome.