Terminal alkynes react with Bra and water to yield bromo ketones. For example: (FIGURE CAN'T COP

step 1 In problem 9 .46, we are asked to react the molecule given with bromine and H2O and get the prod

Terminal alkynes react with Bra and water to yield bromo...

Key Concepts

Electrophilic Addition Mechanism

This concept involves the attack of an electrophile on an electron?rich unsaturated multiple bond (such as a triple or double bond). In such reactions, the ? electrons from the unsaturation interact with an electrophilic species, leading to the formation of an intermediate that can then react further. It is fundamental in organic reactions where the addition of atoms or groups occurs across a multiple bond.

Bromonium Ion Intermediate

When a halogen like bromine is added to an unsaturated bond, the reaction often proceeds through a cyclic bromonium ion intermediate. This three?membered ring structure temporarily stabilizes the positive charge and directs the subsequent nucleophilic attack, typically resulting in anti addition and controlling the regiochemistry of the reaction.

Nucleophilic Attack by Water

In reactions where water is present along with a halogen, after the formation of the halonium ion, water acts as a nucleophile and attacks the electrophilic carbon center. This step is crucial for opening the cyclic intermediate and leads to the incorporation of a hydroxyl group in the product, ultimately resulting in halohydrin formation.

Tautomerization to Ketone

Following the addition steps, the initially formed enolic species can undergo tautomerization, a rearrangement that converts the enol form to a more stable keto form. This process is key in reactions where the final product is a bromo ketone, as the keto form is generally thermodynamically favored.

Halohydrin Formation

This reaction is analogous to the formation of halohydrins from alkenes, where a halogen and a nucleophile (such as water) add across a multiple bond. The process involves the initial formation of a halonium ion followed by nucleophilic attack, leading to products that contain both a halogen and a hydroxyl group. The analogy highlights similar mechanistic steps between alkene and alkyne additions under electrophilic conditions.

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