Question

When 3 -methylbutan-2-ol is treated with HBr, the following reaction takes place: Give a mechanism for this reaction. (Hint: The secondary carbocation formed in step II rearranges to a more stable tertiary carbocation by a hydride ion shift from 3rd carbon atom.

Short answer

The mechanism involves protonation, carbocation formation, hydride shift, and bromide ion attack.

Step-by-step solution

Protonation of the Alcohol Group

In the first step, the hydroxyl group in 3-methylbutan-2-ol is protonated by the acid, HBr. This occurs when the lone pair of electrons in the oxygen atom of the OH group attacks the nearby proton (H⁺) from HBr, forming water (a good leaving group) and creating an oxonium ion.

Formation of a Secondary Carbocation

Once the hydroxyl group is protonated, water (now H₂O) leaves, resulting in the formation of a secondary carbocation at the 2nd carbon. This carbocation is unstable, which leads to the next step in the reaction mechanism.

Hydride Shift to Form a More Stable Carbocation

To stabilize the carbocation, a hydride ion (H⁻) from the adjacent carbon atom (3rd carbon) shifts to the carbon with the positive charge. This results in the formation of a more stable tertiary carbocation.

Attack by Bromide Ion

Finally, the bromide ion (Br⁻) generated from the ionization of HBr attacks the positively charged carbon of the tertiary carbocation. This results in the formation of 2-bromo-2-methylbutane as the final product.

Key concepts

Carbocation Rearrangement

In organic chemistry, carbocation rearrangement is a crucial concept that helps us understand how molecular reconfigurations lead to more stable structures during a reaction. A carbocation is a positively charged carbon atom, which often forms during intermediate stages of reactions involving OH groups or halogens. When such carbocations are formed, they can sometimes rearrange themselves in the molecule to increase their stability.

This rearrangement involves shifting the carbocation from a less stable position to a more stable one, such as from a secondary to a tertiary carbon. The instinct behind this is to reach a configuration where the charge is better balanced, which usually happens in structures where the positively charged carbon is surrounded by more alkyl groups. Alkyl groups donate electron density through their sigma bonds, stabilizing the positive charge on the carbon. Understanding carbocation rearrangement is essential, as it can influence the outcome of the reaction significantly, leading to the formation of major products.

Hydride Shift

A hydride shift is a specific type of rearrangement that occurs when a hydride ion, a hydrogen atom with its bonding pair of electrons, moves from one carbon to an adjacent positively charged carbon atom. This shift generally occurs to stabilize a carbocation by forming a more stable intermediate.

Think of a hydride shift as a strategy the molecule uses to enhance its stability. When a secondary carbocation forms and an adjacent more stable configuration (like a tertiary carbocation) is possible, the hydride shift allows this adjustment to happen. The carbon atom with the initial positive charge captures the hydride ion (H⁻), balancing itself, while the hydride donor becomes positively charged in turn.

• Stability increase: It results from moving the charge to a carbon surrounded by more alkyl groups.
• Practical Result: Leads often to the production of different major products than otherwise expected.

In practice, this process helps in generating more stable and preferable products, therefore is a key maneuver in many synthetic organic reactions. Hydride shifts are essential in creating rearrangement reactions and are an interesting tool for chemists to exploit in complex syntheses.

Tertiary Carbocation Formation

A tertiary carbocation is one of the most stable types of carbocations, primarily due to its chemical environment. It forms when the positively charged carbon atom is connected to three other carbon groups, typically making it more stable than secondary or primary carbocations.

This increased stability is due to hyperconjugation and the inductive effect from surrounding alkyl groups that donate electron density, offsetting the positive charge on the carbocation. As a rule of thumb:

• More Alkyl Groups = Greater Stability
• Tertiary > Secondary > Primary in terms of stability

When a secondary carbocation transforms into a tertiary one through a mechanism like a hydride shift, it can significantly boost the overall stability, creating a favorable pathway for the completion of a reaction. Recognizing when and how tertiary carbocations form aids in predicting reaction pathways and outcomes, as these stable intermediates are often a driving force for the progression of chemical reactions.