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*) leads to the formation of products containing two bromine atoms.
Observation #1: Bromination Proceeds with anti stereochemistry
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.
Observation #2: The reaction is stereospecific
What’s even more mysterious 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). This property is called “stereospecificity” .
What’s important about this? It’s 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].
Observation #3: Rearrangements are never observed
It’s also consistent with the absence of a free carbocation 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.
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”.
How Do We Explain This?
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
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
* 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. )