Question

Treatment of 1,1 -diphenyl- 1,2 -epoxyethane with aqueous acid yields diphenylacetaldehyde as the major product. Propose a mechanism for the reaction.

Short answer

Epoxide protonation, nucleophilic attack by water, leads to diphenylacetaldehyde formation.

Step-by-step solution

Protonation of Epoxide

The mechanism starts with the protonation of the oxygen atom in the epoxide ring. This occurs through the reaction of the epoxide with a hydronium ion (H₃O⁺) present in the aqueous acid solution. The oxygen atom grabs a proton (H⁺) from the hydronium ion, forming an oxonium ion. This process makes the epoxide ring more susceptible to being opened.

Ring Opening through Nucleophilic Attack

After the epoxide is protonated, an intramolecular nucleophilic attack occurs. The more substituted carbon atom within the epoxide experiences nucleophilic attack due to greater carbocation stability preferences. Water acts as a nucleophile and attacks this carbon, breaking the C-O bond and leading to the formation of a carbocation intermediate.

Formation of Diphenylacetaldehyde

Once the ring is opened, the carbocation intermediate rearranges into a more stable form. A hydride shift occurs, which moves a hydrogen atom from the adjacent carbon to the carbocation, stabilizing it further. This shift and subsequent deprotonation step result in the formation of Diphenylacetaldehyde as the major product.

Key concepts

Protonation

In the world of organic chemistry, protonation is the process where a proton (H⁺) is added to an atom, most often oxygen or nitrogen. In this reaction, the epoxide's oxygen atom accepts a proton from a hydronium ion (H₃O⁺) present in the aqueous acid solution. This new oxonium ion becomes more positively charged, making the epoxide ring more susceptible to attack. The result is a tension-filled ring that's ready to pop open at any moment. This initial step is crucial as it sets the stage for the ring opening and further reactions.

Nucleophilic Attack

After protonation, the epoxide ring is primed for a nucleophilic attack. A nucleophile, like water in this case, searches for a positively charged region to attach to, like a magnet seeking metal. It attacks the more substituted carbon within the epoxide ring, which is more stable for forming carbocations. This clever "attack" breaks the carbon-oxygen bond and allows the ring to open up. It’s as if the nucleophile had the right key to unlock the door.

Carbocation Intermediate

Once the ring is opened by the nucleophilic attack, a carbocation intermediate is formed. A carbocation is a carbon atom with a positive charge, and it's crucial in rearrangements and further reactions. This intermediate is unstable, almost like a wobbly tower waiting to stabilize itself by finding a better position. The stability of carbocations is enhanced by shifts and rearrangements, which eventually lead to the next step in the reaction.

Hydride Shift

To stabilize the carbocation intermediate, a hydride shift can occur. This is where a hydrogen atom, along with its two electrons (forming a hydride ion), moves from an adjacent carbon to the carbocation. This shift reduces the overall energy of the molecule, leading to a more stable carbocation. These shifts are like a strategic chess move, positioning pieces for the maximum stability and advantage.

Diphenylacetaldehyde Formation

The final step is the formation of diphenylacetaldehyde, the major product of this reaction. After the hydride shift, the carbocation becomes stable enough to settle into a form that can lose a proton, resulting in the carbonyl structure that characterizes diphenylacetaldehyde. This process shows how a series of well-planned moves and adjustments can lead to a successful outcome in chemical synthesis.