Addition pattern 2 – 3 membered rings
- “Initial Tails” and “Final Heads”
- 3 Ways To Make OH A Better Leaving Group
- A Simple Formula For 7 Important Aldehyde/Ketone Reactions
- Acids (Again!)
- Activating and Deactivating
- Actors In Every Acid Base Reaction
- Addition – Elimination
- Addition Pattern 1 – Carbocations
- Addition pattern 2 – 3 membered rings
- Addition Reactions
- Aldehydes And Ketones – Addition
- Alkene Pattern #3 – The “Concerted” Pathway
- Alkyl Rearrangements
- Alkynes – 3 Patterns
- Alkynes: Deprotonation and SN2
- Aromaticity: Lone Pairs
- Avoid These Resonance Mistakes
- Best Way To Form Amines
- Bulky Bases
- Carbocation Stability
- Carbocation Stability Revisited
- Carboxylic Acids are Acids
- Chair Flips
- Cis and Trans
- Conjugate Addition
- Curved Arrow Refresher
- Curved Arrows
- Determining Aromaticity
- Diels Alder Reaction – 1
- Dipoles: Polar vs. Covalent Bonding
- E2 Reactions
- Electronegativity Is Greed For Electrons
- Electrophilic Aromatic Substitution – Directing Groups
- Elimination Reactions
- Enantiocats and Diastereocats
- Epoxides – Basic and Acidic
- Evaluating Resonance Forms
- Figuring Out The Fischer
- Find That Which Is Hidden
- Formal Charge
- Frost Circles
- Gabriel Synthesis
- Hofmann Elimination
- How Acidity and Basicity Are Related
- How Are These Molecules Related?
- How Stereochemistry matters
- How To Stabilize Negative Charge
- How To Tell Enantiomers From Diastereomers
- Hybridization Shortcut
- Imines and Enamines
- Importance of Stereochemistry
- Intermolecular Forces
- Intro to Resonance
- Ketones on Acid
- Kinetic Thermodynamic
- Making Alcohols Into Good Leaving Groups
- Markovnikov’s rule
- Mechanisms Like Chords
- Mish Mashamine
- More On The E2
- Newman Projections
- Nucleophiles & Electrophiles
- Nucleophilic Aromatic Substitution
- Nucleophilic Aromatic Substitution 2
- Order of Operations!
- Oxidation And Reduction
- Oxidative Cleavage
- Pi Donation
- Pointers on Free Radical Reactions
- Protecting Groups
- Protecting Groups
- Proton Transfer
- Putting it together (1)
- Putting it together (2)
- Putting it together (3)
- Putting the Newman into ACTION
- Reaction Maps
- Recognizing Endo and Exo
- Redraw / Modify
- Robinson Annulation
- Robinson Annulation Mech
- Sigma and Pi Bonding
- SN1 vs SN2
- sn1/sn2 – Putting It Together
- sn1/sn2/e1/e2 – Exceptions
- sn1/sn2/e1/e2 – Nucleophile
- sn1/sn2/e1/e2 – Solvent
- sn1/sn2/e1/e2 – Substrate
- sn1/sn2/e1/e2 – Temperature
- Strong Acid Strong Base
- Strong And Weak Oxidants
- Strong and Weak Reductants
- Stronger Donor Wins
- Sugars (2)
- Synthesis (1) – “What’s Different?”
- Synthesis (2) – What Reactions?
- Synthesis (3) – Figuring Out The Order
- Synthesis Part 1
- Synthesis Study Buddy
- Synthesis: Walkthrough of A Sample Problem
- Synthesis: Working Backwards
- The 4 Actors In Every Acid-Base Reaction
- The Claisen Condensation
- The E1 Reaction
- The Inflection Point
- The Meso Trap
- The Michael Reaction
- The Nucleophile Adds Twice (to the ester)
- The One-Sentence Summary Of Chemistry
- The Second Most Important Carbonyl Mechanism
- The Single Swap Rule
- The SN1 Reaction
- The SN2 Reaction
- The Wittig Reaction
- Three Exam Tips
- Tips On Building Molecular Orbitals
- Top 10 Skills
- Try The Acid-Base Reaction First
- Two Key Reactions of Enolates
- What makes a good leaving group?
- What Makes A Good Nucleophile?
- What to expect in Org 2
- Work Backwards
- Zaitsev’s Rule
Let’s talk about the second key pattern for alkene addition reactions today. I don’t claim to have a snazzy name for it, although one of my first students always called reactions that went through this pathway “the three ring circus”.
Some additions to alkenes don’t go through a free carbocation. Instead, alkenes react with them to form 3-membered rings with a positive charge as an intermediate.
One example is halogens, like Cl2 and Br2. Add Br2 to an alkene, and you end up with formation of a 3 membered ring with a positive charge on the bromine. This is called a “bromonium” ion. (Similarly, the one with chlorine is called “chloronium”, iodine is “iodonium”, and so on. Carbocations tend to be “iums”)
Once you form the bromonium ion, the nucleophile [Br(-) in this case] then comes back and attacks one of the carbons, breaking open the 3 membered ring.
It turns out that the carbon it attacks is not random. Opposite charges attract – and the nucleophile will attack the carbon with the greatest amount of positive charge (not the bromine! formal charge is misleading here). This happens to be thecarbon which can best stabilize partial positive charge. In other words, the nucleophile attacks the most substituted carbon of the alkene.
Put yet another way, these additions follow Markovnikoff’s rule. In most cases this won’t matter. Exception below.
Since we’re not dealing with a free carbocation, the stereochemistry of these reactions is also important. The nucleophile HAS to attack the backside of the carbon-bromine bond (just like an SN2). This means that we get inversion at this carbon.
Bottom line: this pathway gives us products with anti stereochemistry. The two bonds we are forming add to opposite sides of what used to be the alkene.
There are quite a few reactions that follow this pathway. They are Cl2, Br2, I2, as well as all the reactions that involve these halogens in the presence of a nucleophilic solvent such as water or alcohols (e.g. Br2/H2O). It also applies to the reaction of alkenes with Hg(OAc)2 and water (or alcohols) followed by NaBH4.** Finally, if you end up talking about this, you can apply this same pattern to reactions that open epoxides under acidic conditions.
Here’s a summary.
Here’s an example where the reaction shows its selectivity for the Markovnikoff product: formation of halohydrins using Br2/H2O.
Next – the third major pathway.
Thanks for reading! James
P.S. The reaction of alkenes with Hg(OAc)2 and water followed by NaBH4 is initially “anti”, but NaBH4 removes the Hg in a way that randomizes the stereochemistry (i.e. it goes through a free radical on carbon). More details here.
The reaction is Markovnikoff however, as we’d expect.