Addition Pattern #1: The “Carbocation Pathway”

by James

in AlkeneCC, Alkenes, Organic Chemistry 1, Organic Reactions

MOC: What were some of your biggest roadblocks in learning organic chemistry?

OCI: Not learning the patterns. I think I wasn’t told that there were patterns.

-from this post

A Key Pattern For Alkene Addition Reactions

The last several posts have primarily dealt with one reaction: the addition of HCl to alkenes. As we’ve seen, the reaction proceeds through attack of the alkene [the nucleophile] upon a proton [the electrophile], leading to formation of a carbocation. The carbocation, being an electrophile, is then attacked by chloride ion to give the alkyl halide.

4-proposed mech

The major product will be that which proceeds through the most stable carbocation, giving rise to the regioselective formation of  “Markovnikov” products where the chloride adds to the most substituted carbon of the alkene.

Since the reaction proceeds through a carbocation, and nucleophiles may attack carbocations from either face of their empty p orbital, this reaction pathway has no inherent stereoselectivity. A mixture of syn and anti products will be formed [where possible, of course].*

Here’s the good news. If you understand how this reaction works, congratulations – you now understand how hydrobromination and hydroiodination of alkenes work as well!

h-br

h-I

These proceed through the exact same mechanism as we just described. So instead of having to learn three separate reactions, these are essentially three variations of the same reaction.

But wait! There’s more!

By learning this mechanism, you’ve also learned the key steps in the mechanism for the acid catalyzed addition of water to alkenes (“hydration”) and the acid-catalyzed addition of alcohols to alkenes. There’s just one extra step we have to add at the end to make it complete.

 hydration-etherification

Since our nucleophile is neutral, it will bear a positive charge after attacking the carbocation. This positive charge can be removed through deprotonation by a weak base. One little assumption here: we are using H2O (or ROH in the second case) as solvent, so there is a whopping excess around to act in this capacity. **

hydration mech

Why This Is A Big Deal 

All five of these reactions have the following features in common:

  • They proceed through a carbocation intermediate.
  • The most stable carbocation will be formed preferentially (giving rise to “Markovnikov” regioselectivity)
  • There is no inherent preference for syn or anti products (not stereoselective)

Learn One Mechanism, Learn Them All

Do you see the power of understanding mechanisms in organic chemistry? Reactions that go through a similar mechanism are providing similar outcomes. All that’s changing is the identity of the atoms. This is the power of understanding mechanisms in organic chemistry. It can help us identify patterns.

It’s a little like learning a song on piano or guitar and then adapting it to a different key. Learning the song the first time is hard, but changing the key is easy since the relationships are preseved. The important thing is to notice this pattern. 

In summary: we can group these five reactions into a family, that all proceed through the same key steps. They all share the same pattern of regioselectivity and stereoselectivity.

pattern-1a

There’s one last wrinkle with this family of reactions: rearrangements are possible. We’ll talk about that next.

NEXT POST: Rearrangements in Alkene Addition Reactions

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*Although the reaction mechanism has no inherent bias for syn or anti stereochemistry, there are many cases of reactions where this reaction will be stereoselective on account of the three-dimensional structure of the starting alkene.

**Even though the acidity of the protonated product and the protonated solvent are roughly equal, because solvent is present in high excess relative to our product, equilibrium will favor formation of the deprotonated product. In practice, the reaction will be subjected to a mildly basic workup that neutralizes the excess strong acid, giving us the neutral product in the end.

moc-elite-1

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{ 8 comments… read them below or add one }

Neal

I don’t understand how an oxygen atom can be positively charged if it so electronegative – can you explain?

Reply

james

Oxygen bears a positive formal charge because it is bound to three atoms. However you are correct that it is more electronegative and should therefore bear a higher electron density. Formal charge and electron density are not necessarily the same thing! http://www.masterorganicchemistry.com/2012/02/22/common-mistakes-formal-charges-can-mislead/

Reply

partha

you have assumed here that ROH and H20 are assumed as solvents and hence solvents are always in greater quantity to the other substances in the reaction ?

Reply

james

Yes, that’s a valid assumption.

Reply

Jose

Oh poop, this is such a great website. Glad I’ve found it, still have 2 more exams to go! Thank you for all this.

Reply

Devesh

WHoa… fascinating and extremely enlightening site.Just one caveat – lack of practise questions.Could you provide questions or perhaps create a “test yourself ” post at the end of every section ? It’ll be very helpful..

Reply

James Ashenhurst
Mukul

Why is energy of second transition state greater then that of intermediate in addition of HBr to alkene ?

Reply

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