Animated Solution for Chemistry - Organic Chemistry: The major product of the following reaction is

When you encounter a molecule where an ether oxygen is attached directly to a carbon-carbon double bond, you are looking at an enol ether. These compounds are fascinating because the oxygen atom significantly alters the reactivity of the double bond. In this problem, we are treating 1-ethoxy-3-ethylcyclohexene with aqueous sulfuric acid ($H_{2}S{O}_4$). Let's break down the mechanism step-by-step to see how this enol ether transforms into a ketone.

Protonation and Resonance Stabilization

The first step in any acid-catalyzed reaction involving a double bond is protonation. The $\pi$ electrons of the double bond act as a nucleophile and attack the $H^{+}$ ion. But regioselectivity is key here: which carbon gets the proton?

According to Markovnikov's rule, the proton will add to the carbon that results in the most stable carbocation. If the proton adds to C2, a positive charge develops on C1. This is highly favored because the adjacent oxygen atom can donate its lone pair of electrons to stabilize the positive charge through resonance. This creates a highly stable oxonium ion intermediate.

Nucleophilic Attack by Water

Now that we have a highly electrophilic carbon at C1 (stabilized by the oxygen), water molecules from the aqueous acidic solution will act as nucleophiles. A water molecule attacks C1, and after a rapid deprotonation step, we form a hemiacetal intermediate.

A hemiacetal is characterized by having both an ether group ($-OR$) and an alcohol group ($-OH$) attached to the exact same carbon atom.

Collapse of the Hemiacetal

Hemiacetals are generally unstable, especially under acidic conditions. The reaction does not stop here. The acidic environment protonates the ethoxy oxygen, turning it into an excellent leaving group (ethanol).

The lone pair of electrons on the hydroxyl oxygen swings down to form a strong carbon-oxygen double bond ($C=O$), simultaneously kicking out the ethanol molecule.

After the departure of ethanol, we are left with a protonated ketone. A final deprotonation step yields the neutral ketone. Throughout this entire mechanism, the ethyl group at position 3 of the cyclohexane ring remained completely unaffected. Therefore, the final major product is 3-ethylcyclohexanone.