Question

Addition of tert-butylbenzene to the strongly acidic solvent \(\mathrm{HF} / \mathrm{SbF}_{5}\) followed by aqueous workup gives benzene. Propose a mechanism for this dealkylation reaction. What is the other product of the reaction?

Short answer

Answer: The products of the reaction are benzene and tert-butanol. The overall mechanism involves protonation of tert-butylbenzene, rearrangement of the tert-butyl cation, deprotonation to form benzene and tert-butyl cation, and finally, the reaction of the tert-butyl cation with water to form tert-butanol.

Step-by-step solution

Protonation of tert-butylbenzene

In the presence of a strong Lewis acid like SbF5, the HF can protonate the tert-butylbenzene. This leads to the formation of a tert-butyl cation and a benzene ring with a negative charge. $$\mathrm{C(CH_3)_3C_6H_6 + HF \xrightarrow{SbF_5} [C(CH_3)_3C_6H_6H]^{+} + F^{-}}$$

tert-Butyl cation rearrangement

The tert-butyl cation is not stable, and it will undergo a rearrangement to form a more stable carbocation, i.e., 3° carbocation, by transferring a hydride (H-) to the positively charged carbon. The result will be a positive charge on the former tert-butyl carbon that carries the benzene ring. The benzene ring's negative charge will now move to create a bond with the carbocation. $$\mathrm{[C(CH_3)_3C_6H_6H]^{+} \xrightarrow{} [C(CH_3)_3C_6H_7]^{+}}$$

Deprotonation

In this step, a water molecule from the aqueous workup will abstract a proton from the carbocation intermediate, and the electrons from the bond between hydrogen and carbon will now form a double bond, generating benzene and the other product (tert-butyl cation). $$\mathrm{[C(CH_3)_3C_6H_7]^{+} + H_2O \xrightarrow{} C(CH_3)_3^{+} + C_6H_6 + H_3O^{+}}$$

Termination

The tert-butyl cation will react with water or another nucleophile present in the workup solution to form a neutral compound. For example, it can react with water to form tert-butanol: $$\mathrm{C(CH_3)_3^{+} + H_2O \xrightarrow{} C(CH_3)_3OH}$$ So, the other product of the reaction is tert-butanol, and the overall mechanism results in the dealkylation of tert-butylbenzene, yielding benzene and tert-butanol.

Key concepts

Tert-Butylbenzene Dealkylation

Dealkylation is a common type of organic reaction that involves the removal of an alkyl group from a molecule. Specifically, tert-butylbenzene dealkylation is a process where the tert-butyl group is removed from benzene. This transformation is particularly interesting because the tert-butyl group is quite bulky and its removal significantly alters the chemical properties of the original molecule.

During the reaction with a strong Lewis acid, such as antimony pentafluoride (SbF₅), and hydrofluoric acid (HF), the tert-butyl group is protonated, creating a positive charge that facilitates its separation from the benzene ring. This process often requires an acidic environment to proceed. When tert-butylbenzene is treated with these reagents, a cation is formed, which ultimately leads to the formation of benzene and another byproduct, typically an alcohol like tert-butanol, upon aqueous workup. It's the elegance of this reaction that it not only dealkylates but also repurposes the detached group into a useful compound.

Carbocation Rearrangement

In organic chemistry, the stability of carbocations plays a pivotal role in determining the path and outcome of many reactions. For example, in the dealkylation of tert-butylbenzene, a carbocation intermediate is formed. These intermediates are positively charged carbon species that can undergo rearrangement to attain greater stability.

Carbocations prefer to be in the most stable form possible, and this often involves a shift of a hydride or an alkyl group from an adjacent carbon atom. Among carbocations, tertiary (3°) carbocations are more stable than secondary (2°) or primary (1°) carbocations due to hyperconjugation and inductive effects. In the context of the exercise, a rearrangement occurs as the initially formed tert-butyl cation, which is a primary carbocation, rearranges to a more stable tertiary carbocation. This rearrangement is an example of the inherent 'self-correcting' nature of certain organic intermediates, seeking out forms that are lower in potential energy.

Organic Reaction Mechanisms

Understanding organic reaction mechanisms is crucial for predicting the behavior of organic compounds in chemical reactions. A mechanism provides a step-by-step description of the changes at the molecular level that take place during a chemical reaction. These mechanisms involve the breaking and forming of bonds, and they often proceed through various intermediate states.

Key to dissecting these mechanisms is recognizing the role of electron movement, represented by curved arrows in reaction diagrams. These arrows indicate where electrons are coming from and where they are going, thus mapping out the entire process from reactants to products. Familiarity with fundamental reaction types, such as additions, eliminations, substitutions, and rearrangements, is also essential. In the provided exercise, a series of such steps - protonation, carbocation rearrangement, and deprotonation - collectively explain how tert-butylbenzene is transformed into benzene and tert-butanol. Through studying mechanisms like this, students gain insights into the 'choreography' of atoms and electrons that define the field of organic chemistry.