Todd:Chem3x11 ToddL8

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Chem3x11 Lecture 8

Being constructed Sat May 19

This lecture is about how things add to alkenes, using bromine as an example.

(Back to the main teaching page)

Key concepts

  • The stereochemical outcome of a reaction gives clues to the mechanism of that reaction
  • The addition of bromine to double bonds is via a cyclic bromonium ion that is formed in a concerted process

The Addition of Bromine to a Double Bond

Alkenes are important because they occur naturally in great quantity and we can use the reactive double bond to make more interesting molecules. The formation of epoxides is one example we have seen of how alkenes can be transformed into other molecules. The addition of bromine is another widely-used reaction. The jargon for this reaction is an electrophilic 1,2-addition. The bromine adds such that one bromine is added to one carbon and one to the other (a 1,2-addition) and the reagent we add (bromine) is seeking out the electrons of the double bond, meaning we refer to this as an electrophilic addition (unlike when a nucleophile adds to a carbonyl group, which is a nucleophilic addition).

Scheme 1: Addition of Bromine to an Alkene
Scheme 1: Addition of Bromine to an Alkene

Some Mechanistic Possibilities

How might this reaction be occurring? Perhaps a concerted mechanism like this:

Scheme 2: Possible Concerted Polar Addition of Bromine
Scheme 2: Possible Concerted Polar Addition of Bromine

...or a stepwise polar mechanism like this:

Scheme 3: Possible Stepwise Polar Addition of Bromine
Scheme 3: Possible Stepwise Polar Addition of Bromine

...or maybe something exotic like a stepwise radical mechanism like this:

Scheme 4: Possible Radical Addition of Bromine
Scheme 4: Possible Radical Addition of Bromine

On paper there is no easy way to say which is right, so we need to go to the lab, do the reaction carefully and see what happens. If you remember back to when we looked at the opening of epoxides, we found that we learned a lot about the reaction mechanism by using Z- and E- isomers of an alkene. Let's try that trick again.

Stereochemical Clues to the Addition Mechanism

Imagine we have some double bond with things on the end (R1 and R2). Let's start with the Z-isomer, and let's add some reagent Y2 to the double bond and let's say for the moment that the addition does not go via some carbocation or radical intermediate and think about what happens if the addition is cis or trans (i.e., if both atoms of Y2 add to the same face or to opposite faces.)

Scheme 5: Exhaustive Analysis of Reaction Outcomes for cis- and trans-Additions
Scheme 5: Exhaustive Analysis of Reaction Outcomes for cis- and trans-Additions

This rather complex diagram just shows that the outcomes from the different starting material isomers will be different for either cis or trans mechanisms, so we ought to be able to work out what's happened when we do the experiment. To make it simpler, imagine we go in the lab and pick the two isomers of stilbene, meaning that R1 and R2 are both phenyl groups and X = H. We do the reaction, and discover the following:

Scheme 6: The Actual Outcomes of the Bromination of Stilbene
Scheme 6: The Actual Outcomes of the Bromination of Stilbene




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