Rearrangements in Alkene Addition Reactions

Carbocation Rearrangements In Alkene Addition Reactions

In exploring reactions that proceed along the carbocation pathway, every once in awhile you might see an example of an addition reaction that looks a little… strange. The alkene is gone, two new bonds have formed, but the positions of the new bonds is a little out of the ordinary. Like in this example!

If you tally up the bonds that form and the bonds that break, we notice that there is an extra set of C-H bond forming/breaking events.

If you’ve seen the previous articles in the substitution and elimination series, this should look familiar. It’s a telltale sign that a rearrangement has taken place.

Table of Contents

1. What Are Carbocation Rearrangements?
2. Hydride Shifts In Alkene Additions, Step 1: Attack Of Acid By The Nucleophile
3. The Key Rearrangement Step: Hydride Shift
4. Step Three: Attack Of Nucleophile On The Carbocation
5. Alkene Addition Reactions With Alkyl Shifts
6. Alkene Addition Reactions With Ring Expansion
7. Notes
8. (Advanced) References and Further Reading

1. What Are Carbocation Rearrangements?

[If you haven’t seen rearrangements before, read this. If you have, you can skip to the “walkthrough of an addition with rearrangement” section.] Rearrangements can accompany any reaction that proceeds through a carbocation, be it substitution (S_N1), elimination (E1) or, as we’ve just seen, addition.

Bearing less than a full octet of electrons, carbocations are unstable intermediates. Being electron-poor, the stability of a given carbocation greatly depends on the extent to which the atoms adjacent to it can donate electron density, either through resonance, “inductive effects”, or (although rarely taught) “hyperconjugation”.

Rearrangements occur when an entire bonding pair of electrons migrates to a carbocation from one of its neighbors. This will be favorable when a new, more stable carbocation is formed. The bonding pair in question may be attached to a hydrogen or alkyl group. Migrations of a hydrogen with its lone pair are called “hydride shifts”; migrations of a carbon atom with its lone pair are called “alkyl shifts”.

2. Hydride Shifts In Alkene Additions, Step 1: Attack Of Acid By The Nucleophile

The first step in this reaction we’ve seen before: attack of the alkene upon the electrophile (in this case, the H of H-Cl). The result is a carbocation.

Note that the carbocation that’s been formed is a secondary carbocation, and it’s adjacent to a tertiary carbon.

3. The Key Rearrangement Step: Hydride Shift

In this next step, the lone pair in the C-H bond migrates from the tertiary carbon to the secondary, forming a new (tertiary) carbocation. The driving force for this reaction is formation of the more stable carbocation.

Note how it’s just one arrow we’re drawing here! The same arrow shows C-H bond breakage and C-H bond forming.

4. Step Three: Attack Of Nucleophile On The Carbocation

We’ve also seen the third step before. Attack of the nucleophile (chloride ion) upon the new carbocation gives us our new alkyl halide!

5. Alkene Addition Reactions With Alkyl Shifts

Rearrangements can also occur with alkyl shifts, as seen in the example below. Note again that the rearrangement step is represented by just one curved arrow!

6. Alkene Addition Reactions With Ring Expansion

Finally, one of the cases that students often find very difficult is in recognizing reactions that occur with rings (ring expansion or ring contraction). Although perhaps difficult to see, in fact it proceeds through exactly the same mechanism as in the cases above. Note again that we’re depicting the rearrangement reaction with a single curved arrow. [Hint – if you’re doing this on your own, it might help to draw the ugly version first].

So why is it that the carbon from the ring migrates, and not the CH₃ as before? A fair question. Migration of the CH₃ would indeed produce a tertiary carbocation. However, migration of the CH₂ from the ring not only produces a tertiary carbon but incrases the size of the ring from 4-membered to 5-membered, which relieves considerable ring strain present in the cyclobutane ring (worth about 26 kcal/mol).

That about does it for the carbocation pathway of alkene addition reactions. In the next post we’ll go into the second (of three) major pathways for alkene addition mechanisms.

NEXT POST: Bromination of Alkenes – How Does It Work? 

Notes

(Advanced) References and Further Reading

1. Ueber die Beziehung der Pinenhaloïdhydrate zu den Haloïdanhydriden des Borneols
   Georg Wagner, W. Brickner
   Ber. 1899, 32 (2), 2302-2325
   DOI: 10.1002/cber.189903202168
   The oldest examples of these rearrangements are in the pinene series. Wagner showed these rearrangements occur in conversions of pinene to bornyl compounds.
2. Über die Gleichgewichts‐Isomerie zwischen Bornylchlorid, Isobornylchlorid und Camphen‐chlorhydrat
   Hans Meerwein and Konrad van Emster
   Ber. 1922, 55 (8), 2500-2528
   DOI: 10.1002/cber.19220550829
   Prof. Hans Meerwein extended Wagner’s work to non-terpene series, and came up with the crucial insight that it proceeded through a carbonium ion – a very controversial insight at the time! Crucial experiment was finding rate was dependent on solvent polarity. Rate order was SO₂ > MeNO₂ > MeCN > PhNO₂ > PhCN > PhOMe > PhBr > EtBr > PhCl > C₆H₆ > pet ether > ether. Furthermore, he found that certain acids considerably accelerated the rearrangement of camphene hydrochloride to isobornyl chloride. Alkyl migrations in carbocations are often called “Wagner-Meerwein” rearrangements after Georg Wagner and Prof. Hans Meerwein, who studied them more rigorously.
3. Mechanism of substitution at a saturated carbon atom. Part XXXII. The rôle of steric hindrance. (Section G) magnitude of steric effects, range of occurrence of steric and polar effects, and place of the Wagner Rearrangement in nucleophilic substitution and elimination
   I. Dostrovsky, E. D. Hughes, and C. K. Ingold
   J. Chem. Soc. 1946, 173-194
   DOI: 10.1039/JR9460000173
   Prof. Ingold formalized rules for carbocations:
   1) It is necessary for rearrangement that initial bond breakage result in an atom with an incomplete octet
   2) The system will only rearrange if the free energy change is in the right direction (i.e. the carbocation being rearranged to should be more stable, e.g. secondary -> tertiary).
   This tied together S_N1, E1, alkene addition with Wagner-Meerwein shifts in a unified framework.
4. Communications TO THE EDITOR
   The Journal of Organic Chemistry 1962, 27 (5), 1926-1932
   DOI: 10.1021/jo01052a098
   Rigid steroid and diterpenoid systems show addition reactions where multiple hydride and alkyl shifts can occur. One example is dihydroipimaric acid.
5. The constitution and stereochemistry of euphol
   H. R. Barton, J. F. McGhie, M. K. Pradhan, and S. A. Knight
   J. Chem. Soc. 1955, 876-886
   DOI: 10.1039/JR9550000876
   Euphenol, which is similar to lanosterol, also undergoes addition via a carbocation intermediate which can undergo successive hydride and alkyl shifts.
6. The Structures of the Triterpenes Friedelin and Cerin
   E. J. Corey and J. J. Ursprung
   Journal of the American Chemical Society 1956, 78 (19), 5041-5051
   DOI: 10.1021/ja01600a064
   The path from Friedelin to Cerin is a crazy series of fun rearrangements!
7. Total synthesis of (±)-isocomene and related studies
   Michael C. Pirrung
   Journal of the American Chemical Society 1981, 103 (1), 82-87
   DOI: 1021/ja00391a016
   The rearrangement of vinyl cyclobutane opening to cyclopentane is accompanied by relief of ring strain, and this paper shows that can be applied fruitfully in sesquiterpene synthesis.
8. The Addition of Hydrogen Bromide to Simple Alkenes
   Hilton M. Weiss
   Journal of Chemical Education 1995, 72 (9), 848
   DOI: 1021/ed072p848
   A simple experiment suitable for undergraduate organic chemistry laboratory courses that demonstrates that it is possible for the intermediate carbocation to rearrange and give different products.