Question

Solvolysis of the following bicyclic compound in acetic acid gives a mixture of products, two of which are shown. The leaving group is the anion of a sulfonic acid, \(\mathrm{ArSO}_{3} \mathrm{H}\). A sulfonic acid is a strong acid, and its anion, \(\mathrm{ArSO}_{3}{ }^{-}\), is a weak base and a good leaving group. Propose a mechanism for this reaction. \(\mathrm{O}\)

Short answer

Question: Propose a mechanism for the solvolysis of a bicyclic compound in acetic acid, considering the role of acetic acid and the leaving group in the process. Answer: The solvolysis mechanism involves the formation of a tertiary alkyl sulfonate by attaching the leaving group (ArSO3-) to the bridgehead carbon. Acetic acid acts as a weak acid and facilitates the dissociation of this leaving group, forming a carbocation intermediate. Two pathways can lead to products formation: a direct attack at the bridgehead carbon forming a product with an acetate group next to the double bond, and an attack at the allylic carbon leading to a product with an acetate group away from the double bond due to resonance stabilization. Both pathways occur simultaneously, resulting in the formation of two given products.

Step-by-step solution

Identify the reactive sites and draw the leaving group attached

The bicyclic compound contains a double bond and a bridgehead carbon. The leaving group is the anion of a sulfonic acid, which we will represent as ArSO3-. We can attach the leaving group to the bridgehead carbon, as it can form a proper bond with ArSO3-. The starting compound turns into a tertiary alkyl sulfonate.

Understand the role of acetic acid in the solvolysis process

Acetic acid is a weak acid and a polar solvent. In solvolysis, the role of the solvent is to facilitate the dissociation of the leaving group, thus becoming the nucleophile. The solvent will first break the bond between ArSO3- and the bridgehead carbon, creating a carbocation intermediate.

Determine the possible pathways for the products formation

There are two pathways for the carbocation intermediate to be attacked by the nucleophile (acetate ion): a) Direct attack at the bridgehead carbon: This results in the product with the acetate group next to the double bond. b) Attack at the allylic carbon: This results in the formation of a new carbocation, which is more stable than the carbocation generated at the bridgehead carbon. This is due to resonance stabilization (allylic carbocations are more stable than tertiary carbocations). After an acetate ion attacks the allylic carbocation, one of the double bonds in the double bond system shifts to form a new double bond. Consequently, the product with the acetate group away from the double bond is formed. Both of these pathways will occur simultaneously, leading to the formation of the two given products.

Key concepts

Carbocation Intermediates

In the realm of organic chemistry, carbocation intermediates play a pivotal role in many reaction mechanisms. A carbocation is essentially a carbon atom bearing a positive charge. This charge results from the carbon having only three bonds instead of its usual four, leaving it one electron short of a complete octet.

During the solvolysis reaction of our example, the leaving group - a sulfonic acid derivative - departs, bestowing upon the molecule a positively charged carbon. This intermediate is highly reactive and is the centerpiece in the reaction, seeking to regain stability through various means, one of which is nucleophilic attack. The stability of carbocations increases as they move from being primary, secondary, to tertiary, with tertiary being the most stable due to inductive and hyperconjugative effects from the surrounding alkyl groups.

Sulfonic Acid as Leaving Group

A leaving group's ability to detach from the main molecule is a critical feature that influences many chemical reactions. Sulfonic acid derivatives, with the general formula ArSO3H, are known for being strong acids. Their conjugate bases, ArSO3-, are excellent leaving groups due to their relative stability after dissociation.

In our solvolysis example, the anion of a sulfonic acid abandons the bicyclic molecule, inducing the formation of a carbocation. This departure is key to the reaction's progression, since the ability of the sulfonate to disassociate without causing undue destabilization encourages the process, allowing the reaction to move forward to create the end products.

Nucleophilic Attack

Nucleophilic attack is the bread and butter of organic reaction mechanisms. A nucleophile, which is a species rich in electrons, seeks an electrophilic center such as a carbocation, and donates a pair of electrons to form a new covalent bond.

In the solvolysis reaction described, the nucleophile is an acetate ion, derived from acetic acid. This ion targets the electron-deficient carbocation - the aftereffect of the sulfonic acid group's exodus. This nucleophilic attack is the decisive step that leads to the formation of the new product, wherein the acetate ion becomes covalently bonded to the previously electron-deficient carbon.

Resonance Stabilization

Resonance is an important concept when discussing the stability of molecules or intermediates in organic chemistry. A species is resonance-stabilized when its electrons can be delocalized over two or more structures or resonance forms.

In the context of solvolysis, the formation of a carbocation intermediate adjacent to a double bond (allylic position) can result in resonance stabilization. This means that the positive charge is not localized on a single carbon atom but can be spread over a series of adjacent atoms. This dispersal of charge increases the stability of the intermediate and influences the pathway of the reaction, favoring the formation of a more stable intermediate which can lead to different products, as seen in our example with the nucleophilic attack occurring not just at the bridgehead but also at the allylic position.