Bromoetherification, the addition of the elements of Br and...

Name: Problem 62, Chapter 10: Organic Chemistry
Uploaded: 2019-10-22T05:00:51-08:00
Duration: 4 min 45 s
Channel: Ian Kaigh
Description: Bromoetherification, the addition of the elements of Br and OR to a double bond, is a common method for constructing rings containing oxygen atoms. This reaction has been used in the synthesis of the polyether antibiotic monensin (Problem 18.34 ). Draw a stepwise mechanism for the following intramolecular bromoetherification reaction.

Bromoetherification, the addition of the elements of Br and OR to a double bond, is a common method for constructing rings containing oxygen atoms. This reaction has been used in the synthesis of the polyether antibiotic monensin (Problem 18.34 ). Draw a stepwise mechanism for the following intramolecular bromoetherification reaction.

Key Concepts

Bromoetherification

Bromoetherification is a reaction where a bromine atom and an oxygen-based group (OR) add across a carbon–carbon double bond. This reaction is especially useful for constructing oxygen-containing rings, as it forms a new cyclic ether structure through the simultaneous introduction of a halogen and an alkoxy group.

Electrophilic Addition Mechanism

The electrophilic addition mechanism involves the attack of an electron-rich double bond by an electrophile, in this case bromine. The alkene donates electron density to generate a reactive intermediate, setting the stage for subsequent bond-forming events.

Halonium Ion Formation

During bromoetherification, the initial interaction between the alkene and bromine leads to the formation of a cyclic bromonium ion. This three-membered ring intermediate, where the bromine carries a positive charge, is a key feature that directs the course of the reaction by providing a site for nucleophilic attack.

Intramolecular Nucleophilic Attack

In the subsequent step, an intramolecular nucleophile—typically an oxygen atom from an alcohol or ether group—attacks the electrophilic carbon of the bromonium ion. This nucleophilic attack results in the opening of the cyclic intermediate and the formation of a ring structure, effectively locking in the stereochemistry and establishing the new C–O bond.

Transcript

00:02 This is the answer to chapter 10, problem number 77 from the smith organic chemistry textbook.

00:10 This problem says draw a stepwise mechanism for the following reaction.

00:15 This reaction combines two processes together, the opening of an epoxide ring with a nucleophile, and the addition of an electrophile to a carbon -carbon -double bond.

00:25 And we're given a hint to begin the mechanism by protonating the epoxy ring.

00:30 Okay.

00:31 Okay.

00:32 And so let's do that.

00:34 So the first step, as the problem hopefully tells us, is going to be protonation of this epoxide by sulfuric acid.

00:46 So that's going to look like this.

01:00 Okay.

01:00 So we have our protonated epoxide now.

01:04 It's going to have only one lone pair on that oxygen as well as a positive charge.

01:12 And so two things are going to happen at once sort of well not really two things at once but the electrons of this double bond are going to attack that carbon the more substituted carbon of the epoxide so this bond is going to break and the electrons will revert to the oxygen there so from there we've now formed our five -membered ring and so here it is.

01:48 And we're going to have a methyl group here.

01:52 And then an alcohol here, which is the remains of the epoxide.

02:02 And our carbocation is going to be located on this tertiary carbon.

02:12 And so we were told that this reaction is done with solar.

02:17 Ferg acid in water...

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