Question

The compound which gives the most stable carbonium ion on dehydration is (a) CC(C)CO (b) CC(C)(C)O (c) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{CH}_{2} \mathrm{OH}\) (d) CC(C)O

Short answer

(b) CC(C)(C)O produces the most stable carbonium ion.

Step-by-step solution

Understand Carbonium Ion Stability

Carbonium ions (carbocations) are positively charged ions ( abla^+) that are stabilized by the structural features around them. The stability order is tertiary > secondary > primary. A tertiary carbonium ion has three alkyl groups attached, providing more electron-donating effects and hyperconjugation, leading to increased stability.

Analyze Each Compound's Alcohol Group

Examine the given alcohol compounds and identify the potential carbocations after dehydration (loss of water). The formed carbocation will be at the carbon that was bonded to the hydroxyl group (OH).

Determine the Classification of Formed Carbocations

For (a) and (d): Dehydrating CC(C)CO and CC(C)O would form secondary carbocations. For (b): Dehydrating CC(C)(C)O forms a tertiary carbocation as it has three methyl groups around the positively charged carbon. For (c): Dehydrating ext{CH}_3- ext{CH}_2- ext{CH}_2- ext{OH} ext{ would create a primary carbocation.}

Compare the Stability of the Formed Carbocations

Compare the stability based on their classifications: tertiary carbocations are the most stable, followed by secondary, and then primary. CC(C)(C)O is the only compound that leads to a tertiary carbocation, which is the most stable among the given options.

Key concepts

Alcohol Dehydration

When you hear the term "alcohol dehydration," it refers to removing a water molecule from an alcohol. This reaction occurs under specific conditions, usually in the presence of an acid catalyst. With the removal of water, an alcohol transforms into an alkene through a process called a dehydration reaction.
The mechanism generally involves three main steps:

• Protonation of the alcohol group: The hydroxyl group (OH) in the alcohol is converted to a better leaving group by protonation. This step usually requires an acid like sulfuric acid.
• Loss of water: The protonated alcohol then loses a water molecule, leaving behind a carbocation (a positively charged ion). This carbocation plays a crucial role in the next step.
• Formation of an alkene: The unstable carbocation is transformed into a more stable product by the loss of a proton from a neighboring carbon, resulting in the formation of an alkene.

Understanding the dehydration reaction allows chemists to predict the formation of different products. In terms of stability, the nature of the carbocation formed during dehydration is key to deciding the final result, which leads us to exploring tertiary carbocations.

Tertiary Carbocation

One important term to understand is the "tertiary carbocation." This refers to a carbocation where the charged carbon atom is connected to three other carbon groups. These groups donate electrons towards the positive charge, which helps stabilize the ion.
Carbocations are generally ranked based on how many alkyl groups surround the positively charged carbon:

• Tertiary: Three carbon groups around the charged carbon. Most stable.
• Secondary: Two carbon groups around the charged carbon. Moderately stable.
• Primary: One carbon group around the charged carbon. Least stable.

The stability of a tertiary carbocation comes from two main factors: hyperconjugation and the inductive effect. These effects both promote electron donation towards the carbocation, which diminishes the positive charge and makes the carbocation less reactive.
In the context of the exercise, only the compound with the tertiary carbocation will give the most stable product during a dehydration reaction.

Electronic Effects in Organic Chemistry

Understanding the role of electronic effects in organic chemistry provides a powerful tool for predicting molecular behavior. Two key electronic effects are the inductive effect and hyperconjugation, both of which influence carbocation stability.

• Inductive Effect: This arises from the polarization of sigma () bonds within a molecule. Alkyl groups are electron-donating through this effect, which helps stabilize positively charged ions like carbocations. The more alkyl groups attached to the charged carbon, the more stable the carbocation tends to be.
• Hyperconjugation: This involves the overlap of electron clouds. Specifically, it's the interaction of the sigma bonds of the alkyl groups with the empty p-orbital of the carbocation. This stabilizing interaction reduces the energy of the system and thus enhances the stability of the carbocation.

By applying these electronic effects, chemists can reason why certain carbocations are more stable than others. A tertiary carbocation showcases both strong inductive effects and extensive hyperconjugation, making it a highly stable intermediate compared to secondary or primary carbocations. Understanding these principles is crucial for mastering concepts in organic chemistry and predicting the outcome of chemical reactions.