Bromination of Alkenes

Bromination of Alkenes Gives anti Products

In a previous post we went through the key reactions of the carbocation pathway. It’s a family of reactions which proceed through 1) attack of an alkene upon an acid, forming a free carbocation, and 2) attack of a nucleophile upon the carbocation.

Although we saw that several key reactions of alkenes were consistent with this mechanism, it isn’t the case for  all. Take the bromination of alkenes, for instance.

Treatment of an alkene with bromine (Br₂) in a chlorinated solvent (CHCl₃, and CH₂Cl₂ are popular choices; CCl₄ is often cited in textbooks [Note 1]) leads to the formation of products containing two bromine atoms.

In this post we’ll describe the main observations that have been made about this reaction, and in the following post we’ll show the best theory we have for the mechanism.

Table of Contents

1. Bromination Of Alkenes Observation #1: Only anti Products Are Observed
2. Bromination of Alkenes Observation #2: The Reaction Is Stereospecific
3. Observation #3: Rearrangements Are Never Observed
4. Observation #4: Certain Solvents Can Affect The Reaction Products
5. Summary: Bromination Of Alkenes
6. Notes

1. Bromination Of Alkenes Observation #1: Only anti Products Are Observed

Possibly the most interesting feature of this reaction is that the products follow a very predictable stereochemical pattern. For instance, in the reaction of cyclohexene with Br₂, the two bromine atoms add to opposite faces of the alkene (“anti” stereochemistry). No “syn” products are observed. [Note 2]

2. Bromination of Alkenes Observation #2: The Reaction Is Stereospecific

What’s even more interesting is that the stereochemistry of the starting alkene is directly related to the stereochemistry of the product.

For instance if we treat  cis-2-butene [aka (Z)-2-butene] with Br₂, we get a mixture of enantiomers.

But if we treat trans-2-butene, we only get a single product (“meso” 2,3-dibromobutane), which is itself a diastereomer of (S,S)-2,3-dibromobutane and (R,R)-2,3-dibromobutane.

These starting materials, cis-But-2-ene and trans-But-2-ene, which differ only in the configuration of the double bond, lead to stereoisomeric products. This type of process is given the name, stereospecific.

What’s important about this? Two things.

First, given the product, it’s possible to work backwards to figure out which isomer of cis-But-2-ene was the starting material. (Expect to be tested on this).

Secondly, the fact that this happens means that the mechanism is  inconsistent with a free carbocation! If there was a free carbocation, the stereochemistry of the starting alkene wouldn’t matter, since attack can come from either face. [Indeed, we know from labelling experiments that the reaction of H-Cl with cis or trans 2-butene is not stereospecific].

3. Observation #3: Rearrangements Are Never Observed

Another reason this reaction is consistent with the absence of a free carbocation is because rearrangements are never observed. For example, in the case below, we’d expect to see rearrangement (a 1,2-alkyl shift, to be precise) if a free carbocation was formed.

Instead, note that the methyl groups stay in the same place.

4. Observation #4: Certain Solvents Can Affect The Reaction Products

Here’s another experimental observation. The solvent matters.

When we use water as the solvent for this reaction, we get the product below.

[Note – this is called a “bromohydrin” since we have incorporated both bromine and water] .

Note that the stereochemistry is still “anti”, as before.

What this means is that somehow our solvent has intercepted a reactive intermediate in this reaction to produce the product above. (Note – this also occurs when we use alcohols as solvents; in these cases, ethers are obtained).

What’s even more interesting is that the reaction is regioselective. That is, when we have an unsymmetrical alkene,  the major product is the one where water has added to the most substituted carbon of the alkene [most substituted = the sp² carbon of the alkene directly attached to the fewest hydrogen atoms]. Such so-called “Markovnikov” selectivity was also observed in the reactions that proceed along the “carbocation pathway”.

5. Summary: Bromination Of Alkenes

So what’s going on? How can we explain these observations?

•  Anti stereochemistry observed
• No carbocation intermediate (stereospecific, no rearrangements)
• Can be intercepted by nucleophilic solvent; attack occurs at most substituted carbon of the original alkene

What’s the mechanism for this process?

In the next post we’ll go through the best hypothesis we have for the mechanism of this reaction.

NEXT POST: Bromination of Alkenes – The Mechanism

Notes

Note 1. Off topic note: For some reason, textbooks continue to cite CCl₄ as a common solvent for these reactions. Back in the day, CCl₄ was a commodity chemical used for drycleaning (among other uses) and was a cheap, commonly available solvent. Since the  discovery of its role in depletion of the ozone layer, the Montreal convention on CFCs has heavily restricted the availability of CCl₄ to the point where legally obtaining CCl₄ has become  extremely difficult for labs in some countries.  (Although it can often be substituted for other solvents, there are cases where nothing else will do. During my PhD in Canada several of us hoarded old, near-empty bottles of  CCl₄ the same way one might guard a precious bottle of 18-year old Scotch. )

Note 2. There are exceptions to the rule that only anti products are observed. If bromination occurrs on an alkene adjacent to an aromatic ring, some products that appear to have been produced from syn bromination are observed. This is particularly true in any case where a long-lived, stable carbocation can be formed (such as a benzylic carbocation).