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Alkene Reactions

By James Ashenhurst

Rearrangements in Alkene Addition Reactions

Last updated: July 5th, 2019 |

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!

1-rearr

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

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 (SN1), 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.

2-rearr

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.

3-rearr

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!

4-rearr

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-alkylshift

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].

7-rearr

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? 

 

Comments

Comment section

23 thoughts on “Rearrangements in Alkene Addition Reactions

  1. So if you had HCl addition to just vinylcyclobutane (no methyl group as in the example above)… would you give the product as 1-chloro-2-methylcyclobutane or 1-chloro-1-methylcyclobutane?

    1. Do you mean cyclopentane instead of cyclobutane? If so, I’d think the 1-chloro-1-methylcyclopentane product would eventually result. Curious as to what experiment has to say.

  2. In case of addition of HBr to 3-methylcyclohexene..what will be the major product? I think it will be 1-bromo 1-methylcyclohexane but my teacher thinks otherwise, saying there will be no rearrangement. What do you think will happen? and why?

    1. Hi – addition of HBr to 3-methylcyclohexene, proposing a rearrangement is very reasonable, as it would mean that a secondary carbocation would be rearranging to a tertiary carbocation.

      Some profs are not very consistent when it comes to these things.

      James

  3. What if HCl is to be added across 1-Chloroethene ? Does the resulting carbocation stability and the position of the positive charge depend upon the fact that “for halogens inductive>mesomeric effect” ?

      1. Thanks for the link . What about 2-Chloro-2-Butene ? Which carbon will become the carbocation ? Here , would the C3 carbon be more stable as it doesnt have much effect from the -Cl ‘s inductive effect ?

  4. What about the stereochemistry of the ring expansion? How is the CH2 attack? Backside or frontside?! I mean, after the expansion, the CH3 (next to the Carbon with positive charge, out of the ring) will stay inside or outside the plane of the paper?

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