Question

A less stable carbonium ion rearranges to more stable carbonium ion. During this rearrangement, the migrating atom or group leaves as a (1) Free radical (2) Carbene (3) Positively charged ion (4) Negatively charged ion

Short answer

Positively charged ion (3).

Step-by-step solution

- Understand Carbonium Ion Stability

Carbonium ions are positively charged ions (carbocations). Their stability depends on factors such as the number of alkyl groups attached and the presence of electron-donating or withdrawing groups.

- Concept of Rearrangement

In the rearrangement process, a less stable carbonium ion changes into a more stable one. This occurs typically through the migration of an atom or group from one carbon to another.

- Identify the Migrating Species

During this rearrangement, the migrating species can either carry away or donate positive or negative charge. Observe that carbonium ions (carbocations) specifically involve positively charged ions.

- Analyze the Options

Review the given options: Free radical (neutral), Carbene (neutral with lone pairs), Positively charged ion (cation), Negatively charged ion (anion). Since carbocations are positively charged, the migrating group leaves as a positively charged ion.

Key concepts

carbocations

Carbocations, also known as carbonium ions, are positively charged ions with a carbon atom bearing the positive charge. The stability of a carbocation is influenced by various factors:

• Alkyl groups: The presence of alkyl groups increases stability through hyperconjugation and inductive effects. Tertiary carbocations (where the positively charged carbon is attached to three alkyl groups) are more stable than secondary or primary carbocations.
• Resonance: Carbocations stabilized by resonance are more stable. An example is the allylic carbocation, where the positive charge is delocalized over multiple atoms.
• Inductive effects: Electron-donating groups (EDGs) adjacent to the positively charged carbon push electron density towards it, increasing stability. Conversely, electron-withdrawing groups (EWGs) decrease stability.

Stability is crucial because it dictates how readily a carbocation will form and how it behaves in chemical reactions. Understanding these principles helps predict reaction pathways and the formation of intermediates.

molecular rearrangement

Molecular rearrangement is a process where a molecule changes its structure to become more stable. In the context of carbocations:

• Rearrangement types: Common rearrangements include hydride shifts (movement of a hydrogen atom with a pair of electrons) and alkyl shifts (movement of an alkyl group).
• Driving force: The primary driving force for rearrangement is the conversion to a more stable carbocation. For example, a primary carbocation can rearrange to form a more stable secondary or tertiary carbocation.
• Migration: During rearrangement, the migrating atom or group usually shifts to an adjacent carbon atom to facilitate the stabilization process.

This concept is important in organic synthesis and mechanisms because it impacts the final products of a reaction. Understanding how and why rearrangements occur helps in controlling and predicting reaction outcomes.

chemical reactions

Chemical reactions involve the making and breaking of chemical bonds, leading to the transformation of reactants into products. In reactions involving carbocations:

• Formation: Carbocations typically form when a bond to a leaving group breaks, resulting in a positively charged carbon. This often occurs in reactions like the S_N1 mechanism.
• Role of rearrangement: Once formed, carbocations may undergo rearrangement to achieve greater stability before proceeding to react further. This step is crucial in determining the overall product of the reaction.
• Reaction pathways: The stability of carbocations influences potential reaction pathways and intermediates. More stable carbocations are more likely to persist and lead to the major product.

Understanding these chemical principles is fundamental to mastering organic chemistry. Each concept interrelates, helping predict and rationalize the diverse behaviors observed in chemical reactions.