Understanding Why Markovnikov’s Rule Works
Let’s assemble all the facts we know about the reactions of alkenes with an acid like HCl so far.
- Regiochemistry: as we saw in the last post, reactions of alkenes with acids like HCl follow Markovnikov’s rule: the major product formed is that where the hydrogen adds to the carbon containing the most hydrogens.
- Stereochemistry: as we saw in the stereochemistry post, this reaction provides a mixture of “syn” and “anti” products (when the reactant makes this possible).
- Rearrangements. The last post briefly touched on another issue. In some cases, the reaction of alkenes with acids like HCl can lead to rearrangements such as hydride shifts or alkyl shifts.
So how does this reaction work? Any mechanism we propose would have to be consistent with all of these facts.
Furthermore, we need to think about this reaction in terms of what we already know about electron flow. Electrons flow from areas of high electron density (“electron rich”, or “nucleophilic” areas) to areas of low electron density (“electron poor” or “electrophilic” areas). Remember how electrons are polarized in a molecule like H-Br ? Hydrogen is less electronegative, and therefore more electron poor. Bromine is more electronegative and therefore more electron rich. In an alkene, the relevant electrons we will consider are in the π bond, which form a kind of “π electron cloud” above and below the plane of the alkene.
Here’s the best hypothesis we have on how this reaction works so far.
In this reaction, electrons flow from the electron-rich carbon-carbon π bond to the electron poor hydrogen. [Step 1, arrow A] leading to breakage of the H-Cl bond (arrow B). [Recall that this isn’t such a bad state of affairs for Cl- as it is a weak base and therefore a good leaving group]. This forms an intermediate carbocation, which is then attacked by the chloride ion (Step 3, arrow C) leading to formation of the alkyl halide.
Why does this hypothesis fit with the data?
- Regioselectivity. As we saw in the last post, reactions of alkenes with acids like HCl follow Markovnikov’s rule: the major product formed is that where the hydrogen adds to the carbon containing the most hydrogens. This is consistent with the carbocation model, since carbocation stability increases as hydrogens are replaced with carbons.
- Stereochemistry. The observation of syn and anti products is also consistent with a carbocation intermediate. The second step of the reaction involves attack of the nucleophile (Cl- in this case) upon the empty p orbital of a free carbocation. Since this can occur from either direction (“top” or “bottom” faces of the carbocation) we should expect a mixture of syn and anti products here. And that’s what we see:
Note that in the above reaction, the two faces of this carbocation are not precisely equal. The bottom face is shielded by the methyl group adjacent to the carbocation, which occupies more space than the corresponding hydrogen on the top face. Therefore we should expect (and in fact do observe) more of the syn product relative to the anti product. However, both products are still observed.
- Rearrangements. Rearrangements can occur in situations where a hydride or alkyl shift can lead to a more stable carbocation. More on this in a subsequent post (this is getting long) but for similar examples, see the posts on rearrangements in substitution and elimination reactions.
A Reformulation of Markovnikov’s Rule
With these facts in mind, and having proposed this new hypothesis, we can now propose a rephrasing of Markovnikov’s rule. In its previous incarnation, Markovnikov’s rule sounds pretty arbitrary (H adds to the carbon with the most hydrogens? Why?). In this rephrasing, we can say exactly why these reactions proceed this way.
In the reaction of alkenes with hydrogen halides, the reaction will proceed through the most stable carbocation.
As we’ll see this will not only apply to the reaction of alkenes with hydrogen halides but also with acid in the presence of other nucleophiles (like water and alcohols).
One last note. If you look above, for the first time we’re using the arrow pushing formalism to show electrons flowing from a π bond to form a new sigma bond. In other words, it’s acting as a nucleophile! This arrow might look a little weird. In the next post we’ll explore this in a little more detail.
NEXT POST: Curved Arrows and Addition Reactions