Question

The pinacol reartangement is general for all glycols. In the rearrangeanent of piracol, a symmetrical diol, equivalent carbocations are formed no matter which - Ol1 becomes protonated and leaves. Studies of unsymmetrical vicinal diols reveal that the OH group that becomes protomated and leaves is the one that gives rise to the more stable carboration. For example, treatment of 2 -methyd 1,2 propaumediol wìth cold concentrated sulfuric acid gives a 3 " carbocation. Subsequerat migration of hydride ion (H: -) followed by transfer of a proton from the new cation to solvent gines 2-methylporopanil.

Short answer

Answer: The major product formed in this reaction is 2-methylpropanal.

Step-by-step solution

Write down the given reactants and reagent

We are given 2-methyl-1,2-propanediol as the reactant and cold concentrated sulfuric acid as the reagent for the pinacol rearrangement.

Understand the pinacol rearrangement mechanism

In the pinacol rearrangement, an OH group of the vicinal diol becomes protonated and leaves, forming a carbocation. This is typically followed by the migration of a substituent group to form a new carbocation. Then, the new cation transfers a proton to the solvent, yielding the final product.

Identify the OH group that becomes protonated and leaves

For the given reactant (2-methyl-1,2-propanediol) we must evaluate which OH group will lead to a more stable carbocation when protonated. The choice will be between a 2° carbocation (from the OH bound to C1) and a 3° carbocation (from the OH bound to C2). The more stable carbocation is the 3° carbocation, hence the OH group on C2 will be protonated and will leave.

Predict the migration of the hydride ion and formation of a new cation

After the departure of the protonated OH group from C2, the adjacent hydride ion (H:-) from C1 will migrate to the C2 carbocation, forming a new carbocation at C1.

Determine the proton transfer to the solvent to yield the final product

Finally, the new carbocation at C1 will undergo deprotonation in the presence of the solvent (acting as a base), resulting in the formation of the final product, 2-methylpropanal.

Key concepts

Carbocation Stability

Carbocation stability is a crucial concept in organic chemistry, especially when it comes to rearrangement reactions like the pinacol rearrangement. A carbocation is a positively charged carbon atom with only six electrons in its outer shell, making it highly reactive and unstable.
However, certain carbocations are more stable than others, and their stability can significantly influence the outcome of a reaction. Several factors affect carbocation stability:

• Hyperconjugation: This phenomenon involves neighboring C–H or C–C bonds that can donate electron density to the carbocation, stabilizing it.
• Inductive effect: Electron-withdrawing or -donating groups can stabilize or destabilize a carbocation.
• Resonance: Delocalization of the positive charge through resonance can significantly increase stability.

In the pinacol rearrangement, a more stable carbocation (such as a 3° carbocation) is preferred. This affects which alcohol group will undergo protonation and departure during the reaction.

Vicinal Diols

Vicinal diols are compounds that feature two hydroxyl groups (-OH) attached to adjacent carbon atoms. They play an important role in pinacol rearrangement, where these diols undergo a specific transformation to yield carbonyl-containing compounds. The presence of these two hydroxyl groups makes the molecule susceptible to rearrangement.

• In the presence of an acid, one -OH group can be protonated.
• Following protonation, a water molecule leaves, forming a carbocation.

This reaction is primarily initiated by the interaction of the vicinal diol's -OH group with the acidic environment, triggering the sequence of transformations that constitute the rearrangement.

Protonation

Protonation is the addition of a proton ( H⁺) to an atom, ion, or molecule. It is a key step in many organic reactions, including the pinacol rearrangement, where it plays a crucial role in converting a vicinal diol into a more reactive intermediate.
This process begins the rearrangement by activating one of the hydroxyl groups of the vicinal diol. Examples of protonation include:

• Adding a proton to the -OH group of the vicinal diol, turning it into a water molecule that can easily leave the molecular structure.
• Formation of an oxonium ion, which is highly unstable.

This leads to a carbocation formation as water departs, setting the stage for subsequent steps like hydride migration.

Hydride Shift

The hydride shift is an essential step in many rearrangements, including the pinacol rearrangement. After a hydroxyl group has been protonated and leaves, a carbocation is formed.
To stabilize this carbocation, a hydride (essentially a hydrogen atom with two electrons, H^-) migrates to the positively charged carbon. This migration is crucial for several reasons:

• It helps form a more stable structure by moving the positive charge to a more favorable position.
• It can lead to the formation of a new carbocation that is more stable than the first one.
• Helps in forming the final product after a subsequent proton transfer to the solvent.

The hydride shift ensures that the carbocation rearrangement is energetically favorable, aligning with the stability priorities of the molecule.